Bhagya

This PSEB 9th Class Science Notes Chapter 9 Force and Laws of Motion will help you in revision during exams.

PSEB 9th Class Science Notes Chapter 9 Force and Laws of Motion

→ Gallileo and Isac Newton put forth the scientific basis with regard to the motion of the objects.

→ The concept of force is based on the activity of pull, push or hit/kick.

→ By the application of force, the size and shape of an object can be changed.

→ Force is of two types:

• Balanced force
• Unbalanced force

→ An unbalanced force acting on an object produces motion in the object.

→ The frictional force acts against the direction of motion of an object.

→ The first law of motion is also called the Laws of Inertia.

→ The tendency of the body to remain in the state of rest or of uniform motion is called Inertia.

→ Everybody opposes the change in its state of motion.

→ The moment of inertia of the train is more than that of the cart so that it does not get displaced on being pushed. In other words, the heavier bodies have more inertia.

→ The inertia of an object is the measure of its mass.

→ The momentum ‘p’ of an object is measured by the product of its mass ‘m’ and velocity ‘v’. i.e. p = m × v

→ The momentum has both magnitude and velocity. Its direction is the same as that of velocity.

→ The S.I. unit of momentum is kg ms.

→ Force changes the momentum of an object.

→ To reduce the effect of friction the surface is either made plane or the surface is painted with a lubricant.

→ S.I. unit of force is the newton.

→ For the motion of the object due to the force acting on it, Newton gave three fundamental laws of motion which hold good for all types of motion.

→ According to the first law, everybody tends to remain in its state of rest or of uniform motion unless some external force acts on it.

→ According to the second law, the rate of change of momentum of a body is directly proportional to the unbalanced force acting on it and in its direction.

→ According to the third law, when one body applies force on another body, the second body also applies instantaneous force on the first body. These two forces are equal in magnitude but opposite in direction.

→ By applying force the motion can be given to a body at rest and the body in motion can be brought to rest or a change can be brought about in the same direction.

→ Force: It is an external cause that produces or tends to produce a change in its state of rest or of uniform motion.

→ One Newton Force: It is the force that produces an acceleration of 1 ms-2 in a body of mass 1 kg. It is denoted by ‘N’.

→ Balanced Forces: If a number of forces acting on an object do not produce any change in its state then these forces are called Balanced Forces. Their net result is zero force.

→ Unbalanced Forces: If the result of various forces acting on an object is not zero, then these forces are called unbalanced forces.

→ Frictional Force: It is the opposing force that comes into play when a body moves over the surface of another body.

→ Inertia: It is the property of the bodies due to which they cannot change their position of rest or of uniform motion unless some external force is applied.

→ The inertia of Rest: It is the property by virtue of which a body at rest will continue to remain at rest unless some external force is applied to bring about that change.

→ The inertia of Motion: It is the property of a body by virtue of which the body in motion, will continue to move with the same uniform speed unless some external force is applied to bring about that change.

→ Momentum: The product of mass and velocity possessed by a body is called its momentum.

→ Law of Conservation of Momentum: If no external force acts on a system of particles then the total momentum of the system remains conserved.

→ Newton’s Second Law of Motion: Tine rate of change of momentum of a body is directly proportional to the applied force and takes place in the direction of the force.

→ Newton’s Third Law of Motion: Every action has an equal and opposite reaction.

→ Newton’s First Law of Motion: If a body is at rest, it will continue to remain at rest. And if a body is moving with a uniform velocity in a straight line it will continue to do so unless some external force is applied to bring about that change in its state.

This PSEB 9th Class Science Notes Chapter 8 Motion will help you in revision during exams.

PSEB 9th Class Science Notes Chapter 8 Motion

→ A body is said to be in motion when it changes its position with time.

→ At any instant, a body may appear to be in motion to one person but the same object may appear at rest to another person.

→ To describe the position of an object we are required to fix the directive point which is called the origin.

→ The motion of some objects can be controlled while that of some other objects remains uncontrolled and irregular.

→ When an object moves along a straight line path then its motion is called linear motion.

→ Those quantities which can be described completely by their magnitude only are called physical quantities.

→ The shortest distance measured between initial and final position is called displacement of the body.

→ When the final position of the body in motion coincides with its initial position then the value of displacement becomes equal to zero.

→ The displacement of an object may be zero but its distance will not be zero.

→ An odometer is a device that measures the distance covered by automobiles.

→ The rate of change of motion of an object is called its speed. The unit of speed is ms^-1.

→ The average speed of an object is obtained by dividing the distance covered by the body by the time taken to cover that distance.
∴ Average speed = Total distance covered/Time taken to cover that distance

→ The rate of change of motion of a body in a particular direction is called its velocity i.e. speed in a particular direction is called velocity.

→ Time interval is measured by digital wristwatch or stopwatch.

→ Speed of sound in air = 340 m/s.

→ Speed of light in air = 3 × 10⁸ m/s.

→ The rate of change of velocity of a body is called its acceleration

• Acceleration = Change in velocity/Time taken
• S.I. unit of acceleration is ms^-2.

→ If the velocity of an object changes by the same rate then its mean (average) velocity, is obtained by taking the arithmetic mean of initial velocity and final velocity.
∴ Average Velocity = \(\frac{\text { Initial velocity + Final velocity }}{2}\)

→ If an object covers equal distances in equal intervals of time however small the interval maybe then its motion is called uniform motion.

→ If a moving object covers unequal distances in equal intervals of time then the motion of the object is called non-uniform motion.

→ Rest: A body is said to be at rest if it does not change its position with respect to its surroundings.

→ Motion: A body is said to be in motion if it changes its position with time with respect to its surroundings.

→ Distance: It is the length of the actual path travelled by a body between its initial position and final position.

→ Displacement: Tire change in position of an object in a given direction is known as displacement. It is measured by the shortest distance moved by a body from the initial position to the final position.

→ Uniform Motion: If an object covers equal distances in equal intervals of time however small the time intervals may be, then the motion of the object is said to be uniform motion.

→ Non-Uniform Motion: If an object covers unequal distances in equal intervals of time then the object is said to be in non-uniform motion.

→ Speed: The distance travelled by a body in a unit of time
Speed = Distance traveled/Total time taken

→ Average Speed: It is the total distance travelled bv an object divided by the time taken to travel that distance.
Average speed = Total distance travelled/Total time taken

→ Uniform Velocity: If a body covers equal distances in equal intervals of time, however small these intervals maybe, in a particular direction its velocity is said to be uniform velocity.

→ Variable Velocity: If a body travels unequal distances in equal intervals of time or changes its direction or both then its velocity is said to be variable velocity.

→ Uniform Circular Motion: When a body moves along a circular path with uniform speed, its motion is called uniform circular motion. Tire motion is uniformly accelerated due to continuous change of direction.

→ Angular Velocity: It is the angle swept by the radius per unit time
ω = \(\frac{\text {θ}}{t}\) rad/s

→ Velocity: It is the displacement produced per unit time.
or
It is the distance travelled per unit time in a given direction.
Velocity = Displacement/Time

→ Uniform Speed: When an object covers equal distances in equal intervals of time, however small these intervals may be, it is said to move with uniform speed.

→ Non-Uniform Speed: When an object covers unequal distances in equal intervals of time, it is said to move with non-uniform speed.

This PSEB 9th Class Science Notes Chapter 7 Diversity in Living Organisms will help you in revision during exams.

PSEB 9th Class Science Notes Chapter 7 Diversity in Living Organisms

→ There are about 10 million living organisms present on earth and only 1/3rd of these organisms have been identified.

→ All organisms differ from each other in their structure.

→ The classification of organisms is arranging organisms into groups or sets on the basis of similarities and differences which also exhibit their relationships.

→ Each organism is different from all others to a lesser or greater extent.

→ The science of classification is called taxonomy.

→ Bacteria are microscopic of a few micrometres in size whereas blue whale and red Wood trees of California are of size 30 metres and 100 metres respectively.

→ Present-day diversity in living beings is the product of 3.5 billion years of organic evolution.

→ Carolus Linnaeus is the Father of Taxonomy.

→ Classification makes the study of a wide variety of organisms easy.

→ Classification helps us explore the diversity of life forms.

→ Linnaeus classified organisms on the basis of similarities and differences.

→ Binomial nomenclature was given by Linnaeus. Each organism is assigned two names, generic name, name of the genus, and specific name, of species.

→ In two-kingdom systems of classification, kingdom Plantae and kingdom Animalia were classified.

→ Plants are autotrophic, prepare their own food and plant cells have cell walls made up of cellulose. Divided into two groups crypto game and phanerogamae.

→ Animals lack chlorophyll and are heterotrophic in nutrition.

→ R. H. Whittaker (1969) proposed five kingdom systems of classification.

→ The five kingdoms are Monera, Protista, Fungi, Plantae, and Animalia.

→ Monerans lack well-defined nuclei and membrane-bound cell organelles.

→ This group includes bacteria, blue-green algae (cyanobacteria), and Mycoplasma.

→ Protists include unicellular eukaryotic organisms. Some may have specialized structures for moving such as cilia, flagella, or pseudopodia.

→ Agar, diatoms, and protozoans are examples.

→ Plantae and Animalia are further divided into subdivisions on the basis of increasing complexity.

→ Charles Darwin (1959) described the idea of evolution in his book, ‘The Origin of Species’.

→ ‘Primitive’ or ‘lower’ organisms have ancient body designs.

→ Advanced or higher organisms acquired their particular designs relatively recently.

→ Complexity in design increases over evolutionary time.

→ Biodiversity refers to a variety of life forms found in a particular region.

→ The warm and humid tropical regions of the earth are rich in diversity of plant and animal life. The region is called the region of megadiversity.

→ Ernst Haeckel (1894), Robert Whittaker (1959), and Carl Woese (1977) have tried to classify all living organisms into broad categories, called Kingdoms.

→ Aristotle classified animals according to their habitat (land or water).

→ Basis of classification

• Nature of cell Prokaryotic or eukaryotic cell.
• The number of cells Single-cell or multicellular.
• Method of preparing food
• Plants carry out photosynthesis thus autotrophs.
• Fungi absorb food, thus absorptive.
• Animals have an ingestive (holozoic) modes of feeding.

Classification of Kingdom Animalia:

→ Protozoa are included in Protista, include single-celled, aquatic, free-living or parasitic organisms.
Examples: Amoeba, Paramecium, Euglena.

→ Phylum Porifera era includes pore-bearing organisms called sponges which are the simplest multicellular animals.

→ Sponges are vase-like, rounded, sac-like, or branched. Euplectella, Sycon, Spongilla, Bath sponge are examples.

→ Phylum Cnidaria is a diploblastic radially symmetrical animal with two-layered body walls.
Examples: Hydra, Jellyfish, Sea anemone, and Corals.

→ In Phylum Platyhelminthes, flatworms have been included which are mostly parasitic.
Examples: Dugesia, Liver fluke, Tapeworm.

→ Round or threadworms are included in Phylum Aschelminthes which are triploblastic, unsegmented, and of bilateral symmetry. Example: Ascaris.

→ Annelids are triploblastic, bilaterally symmetrically elongated with segmented bodies.
Examples: Earthworm, Leech, Nereis.

→ Animals with jointed legs have been placed in Phylum Arthropoda. These animals’ body is covered with chitinous cuticle.
Examples: It is the largest phylum and includes prawns, insects, spiders, scorpions, etc.

→ In Phylum Mollusca, the body is divided into three regions i.e. head, a dorsal visceral mass, and ventral foot. These are soft-bodied animals.
Examples: Pila, Unio.

→ Phylum Echinodermata includes starfishes, brittle stars, and sea urchins. These are spiny-skinned.
Examples: Starfish, Sea lily

→ In Phylum Hemichordata, the body is divided into three parts i.e. proboscis, collar, and trunk.
Examples: Balanoglossus.

→ Phylum Chordata represents the most advanced group of kingdom Animalia.

→ Protochordate group includes Herdmania and Amphioxus.

→ Chordates bear

• a solid rod-like structure called the notochord, on the dorsal side above the gut
• dorsal hollow nervous system
• pharynx, perforated by gill slits

→ Systematics: It is the study of the diversity of organisms and all their comparative and evolutionary relationship.

→ Taxonomy: It deals with the identification, nomenclature, and classification of different types of organisms.

→ Species: A group of living organisms of similar individuals capable of exchanging genes and interbreeding. The species is ranked below a genus.
Example: Homo sapiens.

→ Fertilization: Fusion of male gamete and female gamete is called fertilization.

→ Classification: It is the arrangement of organisms into groups on the basis of similarities and differences.

→ Binomial Nomenclature: Every organism is given a scientific name that has two parts, the first is the name of the genus (generic name) and the second is the name of the species (specific name).

→ Dicotyledonous: Plant having seeds with two cotyledons.

→ Ovule: A structure in the ovary of a seed plant that develops into a seed following fertilization.

→ Gametophyte: The haploid generation producing gametes in plants.

→ Annuals: A plant that completes its life cycle in one growing season.

→ Mycelium: A mass of filament or hyphae, composing the vegetative part of many fungi.

→ Hyphae: One of the filaments composing mycelium.

→ Saprophyte: A plant that lives on decaying organic matter.

→ Notochord: An elongated dorsal cord which is the primitive axial skeleton of chordates.

→ Aerobe: An organism that can grow and live in the presence of oxygen.

→ Nocturnal: Active during night e.g. cockroach.

→ Polygamy: When one male lives in the company of many females e.g. Struthio.

→ Producers: The first trophic level in a food chain. Producers are those organisms that can prepare food from inorganic materials i.e. green plants.

This PSEB 9th Class Science Notes Chapter 6 Tissues will help you in revision during exams.

PSEB 9th Class Science Notes Chapter 6 Tissues

→ A cell is the structural and functional unit of life, while the group of cells coordinating to perform a specific function is called tissue.

→ The scientific study of tissues is called histology.

→ A cluster of cells called a tissue is arranged and designed so as to give the highest possible efficiency as tissues have provided division of labor in multicellular organisms.

→ A Group of tissues is called an organ.

→ A Group of organs constitute an organ system.

→ Due to improved organisation, higher efficiency, multicellular organisms have higher survival value.

→ Plant Tissues:

• Most of the plant cells are dead which provides mechanical strength and needs less maintenance.
• In plants, there is some tissue that continue dividing.
• On the basis of the power of division of cells, plant tissues are of two types: Meristematic tissues and permanent tissues.
• Meristematic tissue has the power of division throughout life, so helps in the growth of the plant but in some specific regions.
• Meristematic tissue is of three types: apical, lateral, and intercalary.
• Meristem is of two types depending upon the power of division: primary meristem and secondary meristem.
• Simple Permanent tissue has lost the division power and is of three types: Parenchyma, Collenchyma, and Sclerenchyma.
• Tissue may be simple or complex.
• Xylem and phloem are complex tissues.

→ Animal Tissues:

• On the basis of their functions, animal tissues are of four types: Epithelial, connective, muscular, and nervous tissue.
  
• The muscles of the heart show rhythmic contraction and relaxation throughout life.
• The functional combination of nerve and muscle tissue is fundamental to most animals.
• This combination enables the animals to move rapidly in response to stimuli.

→ Tissue: A group of similar or dissimilar cells alongwith intercellular substances coordinating to perform a specific function. Blood, phloem, and muscles are examples of tissues.

→ Meristematic tissue: The immature cells which are in a state of division and growth with no intercellular spaces.

→ Meristem: A group of cells capable of dividing to form new cells.

→ Companion cells: These are narrow, elongated, and nucleated cells that are connected to sieve tube cells.

→ Phloem Parenchyma: Parenchyma tissue associated with phloem.

→ Tendon: A band of white fibres surrounded by connective tissue sheath which joins muscles to bone.

→ Sarcolemma: Surface covering of striated muscle fibres.

→ Sarcoplasm: Cytoplasm of a muscle fibre.

→ Sarcomere: A structural and functional unit of muscle fibre.

→ Chlorenchyma: The type of parenchyma cells having chloroplast and carrying out photosynthesis.

→ Actin: It is a type of protein present in muscle fibres.

→ Bone marrow: A hemopoietic tissue inside the marrow cavity of the bones.

→ Cartilage: An elastic skeletal tissue that acts as a shock absorber.

→ Neurilemma: It is a covering around nerve fibres formed of Schwann cells.

→ Ligament: A band of yellow fibres surrounded by connective tissue sheath which joins bone to bone.

→ Osteoblasts and Osteocytes: They are bone-forming and bone cells respectively.

→ Schwann Cells: Cells around nerve fibres that form neurilemma.

→ Blood platelets: Blood corpuscles that help in blood clotting at an injury.

→ Axon: An efferent nerve process of a neuron.

→ I-band: It is a thin band of multiple fibres formed of actin protein.

→ Epithelial tissue: An animal covering and protective tissue.

→ Histology: Microscopic study of tissues.

→ Xylem: A compound tissue formed of tracheids, vessels, parenchyma, and fibres, and helps in the conduction of water and minerals in the plants.

→ Phloem: A compound tissue formed of sieve tube cells, companion cells, parenchyma, and fibres, and helps in the conduction of food in the plants.

→ Tracheids: These are elongated dead cells with large cavities and possess highly lignified cell walls.

→ Sieve tubes: These are tubular cells with perforated walls.

→ Vessels: These are composed of many cells joined end to end with their perforated walls to give a tube-like appearance.

This PSEB 9th Class Science Notes Chapter 5 The Fundamental Unit of Life will help you in revision during exams.

PSEB 9th Class Science Notes Chapter 5 The Fundamental Unit of Life

→ All living organisms around us are complex structural compartments called cells.

→ A.V. Leeuwenhoek (1674) first studied the living cell. He examined bacteria, sperms, and erythrocytes (RBC).

→ The biosphere is the highest level of organisation of living organisms.

→ Level of organisation is Atoms → Elements → Cell → Tissue → Organ → Organ System → Living Organisms.

→ Robert Hooke (1665) examined dead cells,

→ Rober Brown (1831) observed the nucleus in the centre of cell.

→ Huxley regarded protoplasm as the ‘Physical basis of life’.

→ The cytoplasm is the fluid content of cells present between the nucleus and plasma membrane.

→ It contains metabolites and organelles.

→ Organelles are special components of cells performing specific functions.

→ Cells are of two types i.e. prokaryotic cell and eukaryotic cell.

→ Organisms may be single-celled e.g. Amoeba, CMamydomonas (an algal plant), Paramecium or they are multicellular.

→ Higher plants and animals are made up of a large number of cells.

→ A true nucleus is present, It is generally single-spherical and central in position.

→ The nucleus is separated from the cytoplasm by a double-layered membrane called nuclear membrane. It controls the functioning of cells.

→ The nucleus contains chromosomes composed of DNA and protein.

→ A functional segment of DNA is called a gene.

→ All living organisms start their life cycle from a single cell.

→ Cell size varies from 0.2 – 0.5 micron to 30 micron (one micron = 1/1000 mm ). Nerve cells may be as long as a few metres.

→ A plant cell is bounded by a protective cell wall.

→ A plasma membrane is a living membrane.

→ Mitochondria are rod-shaped, double-membranous, light-microscopic, eukaryotic structures. Inner membranes have cristae and exosomes.

→ Functions: These act as powerhouses or ATP mills as they are sites for cellular respiration and release energy.

→ The centrosome is an animal structure and is formed of two microtubular centrioles, each being formed of 9 triplet microtubules showing a 9 + 0 arrangement.

→ Function: These help in cell division.

→ Basal bodies give rise to cilia or flagella centrioles form.

→ Contractile vacuoles are present in freshwater protozoans.

→ The cell is a Latin word for a little room’.

→ The electron microscope was discovered in 1942.

→ Water obeys the laws of diffusion.

→ The nucleus plays a central role in cellular reproduction, the process by which a cell divides and forms two new cells.

→ Plastids are the largest-sized eukaryotic structures of plant cells. These are of three types: Leucoplasts (colourless and store the food).

→ The primary function of the leucoplast is storage. Chloroplasts (green coloured and are sites of photosynthesis, so-called kitchens of cells)

→ Each chloroplast is a double-membranous structure having grana in its inner chamber.

→ Each granum is formed of many chlorophyll-containing thylakoids present in stacks.

→ Functions: Chloroplasts are sites for photosynthesis.

→ Chromoplasts (coloured and help in pollination of dowers and dispersal of seeds and fruits).

→ The endoplasmic reticulum is an electron-microscopic interconnected network of cisternae, vacuoles, and tubules.

→ It is of two types: RER (cisternae are studded with ribosomes and involved in protein synthesis) and SER (tubules are without ribosomes).

→ Functions:

• It is a passageway for intracellular and intercellular transport of materials.
• It gives internal support to the cell.
• SER is involved in the synthesis of lipids and steroids.
• RER is concerned with protein synthesis.

→ Lysosomes are electron-microscopic single membrane-bound vesicular structures of animal cells and contain hydrolytic enzymes.

→ Functions:

• These are centres of intracellular digestion and act as both digestive bags and suicidal bags.
• They destroy foreign substances.
• They remove cellular debris.

→ Living organisms are composed of one or a large number of cells. The cell is the structural and functional unit of life.

→ A large number of build-up and breakdown reactions take place in the cell.

→ Life is passed on from one generation to the next generation in the form of cells.

→ Robert Hooke (1665) first discovered cells on the basis of compartments observed in a thin section of the bark of a tree.

→ Prokaryotic cells lack a well-organized nuclear membrane and membrane-bound organelles. They have 70 S type of ribosomes.

→ Eukaryotic cells have a proper nucleus and membrane-bound organelles. 80 S ribosomes are present.

→ The Golgi body is formed of stacked cisternae with swollen ends, vacuoles, and vesicles.

→ Functions:

• It is involved in cell secretions such as mucous, enzymes, and hormones.
• It helps in the storage of secretory products.

→ Cell inclusions include reserve food in the form of glycogen granules or lipid droplets or starch grains.

→ Vacuoles and fluid-filled membrane-bound spaces each containing cell sap within a tonoplast, wastes, gases, secretions, etc.

→ Functions: They help in the storage of food, water, and other wastes.

→ Ribosomes are composed of RNA and proteins, granular electron-microscopic particles without membrane.

→ Functions: These act as protein factories.

→ Prokaryotes: The simple organisms called Monerans without a proper nucleus. e.g. Bacteria, Blue-green algae.

→ Eukaryotes: Organisms with true nucleus (plant and animal cells).

→ Organelles: Special living components of cells each performing a definite function.

→ Leucoplasts: Colourless plastids.

→ Centriole: Star-shaped structure present near the nucleus in an animal cell. It forms a spindle during cell division.

→ Genes: They are present on chromosomes and act as carriers of characters from parents to offspring.

→ Lysosome: These are electro-microscopic structures bounded by a single membrane. They are full of digestive enzymes. They are called ‘suicidal bags.’

→ DNA (Deoxyribose nucleic acid): It controls cellular functions and also acts as genetic material.

→ RNA (Ribose nucleic acid): It plays important role in protein synthesis.

→ Cyclosis: Streaming movements of cytoplasm.

→ Autolysis: Self-digestion of the cell by its lysosomal enzymes.

→ Autophagy: Digestion of its own cell organelles or reserve food by the lysosome.

→ Cristae: Infolds of the inner mitochondrial membrane.

→ Chloroplast: Chlorophyll-containing green coloured photosynthetic plastids.

→ Chromoplast: A pigmented plastid.

→ Tonoplast: Vacuolar membrane present around cell sap.

→ Camillo Golgi discovered Golgi bodies and shared Noble Prize in 1906 with Santiago Ramony Cajal for their work on the structure of the nervous system.

→ Nucleoid: A primitive nucleus of prokaryotes, not covered by a nuclear membrane.

This PSEB 9th Class Science Notes Chapter 4 Structure of the Atom will help you in revision during exams.

PSEB 9th Class Science Notes Chapter 4 Structure of the Atom

→ It was known by 1900 that the atom was not a simple, indivisible particle but contains at least one sub-atomic particle i.e., electron identified by J.J. Thomson.

→ Even before the electron was identified by J.J. Thomson, E. Goldstein in 1886 discovered the presence of new radiations in a discharge tube called canal rays.

→ The canal rays were positively charged radiations which led to the discovery of the proton.

→ Proton has a charge, equal in magnitude but opposite in sign to that of electron and a mass approximately 2000 times as that of the electron.

→ Generally, the electron is represented by ‘e’ and a proton as ‘p’.

→ The mass of a proton is taken as one unit and charge +1.

→ The mass of an electron is considered to be negligible and its charge -1.

→ α-particles are doubly-charged helium ions having mass 4u \(\left({ }^{4}{ }_{2} \mathrm{He}^{2+}\right)\)

→ E. Rutherford discovered the nucleus of an atom on the basis of an α-ray scattering experiment.

→ E. Rutherford was awarded the Nobel prize in chemistry for his famous work discovery of radioactivity and the discovery of the nucleus of the atom.

→ On the basis of his experiment, Rutherford put forward the nuclear model of an atom.

→ According to Rutherford’s model, there is a positively charged centre of an atom called a nucleus and contains nearly all the mass and all the positive charge.

→ The electrons revolve around the nucleus in circular paths. The size of the nucleus is very small as compared to the size of the atom.

→ Neils Bohr’s model of the atom was more successful.

→ He suggested that only certain special orbits known as discrete orbits of electrons are allowed inside the atom. While revolving electrons do not radiate energy.

→ J. Chadwick discovered another sub-atomic particle that had no charge but mass nearly equal to a proton. This particle is called the neutron.

→ The orbits or shells in an atom are designated as K, L, M, N ………… shells starting from the nucleus side.

→ J.J. Thomson suggested that an atom is a uniform sphere of positive electricity in which electrons are embedded it.

→ The total positive charge is equal to the total negative charge and the atom, on the whole, is electrically neutral.

→ Electron is a negatively charged particle with 1.602 × 10^-19 coulomb negative charge (-1 unit) and mass 9.1089 × 10^-19 kg (negligible mass). It is represented by the symbol ‘e’. It is a fundamental particle of an atom.

→ Proton is a positively charged particle with a 1.602 × 10^-19 coulomb positive charge (+1 unit) and mass 1.672 × 10^-27 kg, it is represented by the symbol ‘p’. It is a fundamental particle of an atom.

→ Neutron is a neutral particle with no charge and mass equal to 1.678 × 10^-27 kg. It is represented by the symbol ‘n’. It is a fundamental particle of an atom.

→ The nucleus is the small, positively charged, and heavy central portion in an atom that contains in it protons and neutrons.

→ Nucleons. The neutrons and protons present in the nucleus of an atom are collectively known as nucleons.

→ An atomic number of an element represents the number of protons in an atom. It is denoted by the symbol Z.

→ Shells of an atom are designated as K, L, M, N, etc. These are also called energy levels.

→ The valence shell of an atom represents the outermost shell where electrons are present and the electrons are called valence electrons.

→ A mass number of an element is the sum of the number of protons and neutrons present in the nucleus of the atom. It is denoted by A.

→ The valence shell of an atom represents the outermost shell where electrons are present and the electrons are called valence electrons.

→ Valency. It is the combining capacity of an atom of the element.

→ Isotopes are the atoms of the same element having the same atomic number but different mass numbers.

→ Isobars are the atoms of the different elements having the same mass number but different atomic numbers.

This PSEB 9th Class Science Notes Chapter 3 Atoms and Molecules will help you in revision during exams.

PSEB 9th Class Science Notes Chapter 3 Atoms and Molecules

→ The idea of divisibility of matter was considered long back in India around 500 R.C.

→ Another Indian Philosopher, Pabuda Katyayama said these particles normally exist in a combined form which gives us various forms of matter.

→ Greek Philosophers-Democritus and Leucippus suggested that if we go on the dividing matter, a stage will come when particles obtained cannot be divided further.

→ Democritus called these indivisible particles atoms.

→ By the end of the 18th century, scientists recognized the difference between elements and compounds and how and why elements combine, and what happens when they combine.

→ Antoine L.Lavoisier gave two important laws of chemical combination given below.

→ According to the law of conservation of mass, mass can neither be created nor destroyed in a chemical reaction.

→ John Dalton provided the basic theory about the nature of matter.

→ Dalton picked up the idea of divisibility of matter and it was based upon laws of chemical combination.

→ Dalton gave Dalton’s atomic theory.

→ According to Dalton’s atomic theory, all matter, whether an element, a compound, or a mixture is composed of small particles called atoms, which are indivisible.

→ Dalton’s theory provided an explanation for the law of conservation of mass and the law of definite proportions.

→ All matter is made up of very tiny particles called atoms which cannot be seen with the naked eye.

→ The elements are represented with the help of symbols.

→ The symbol represents one atom of the element.

→ Berzelius suggested that the symbols of elements be made up from one or two letters of the name of the element.

→ The atomic mass of an element is the average relative mass of one atom of the element as compared to the mass of one atom of carbon-12, taken as 12 a.m.u. (atomic mass unit).

→ According to the latest IUPAC recommendations, a.m.u. has now been replaced by U (uniformed mass).

→ A molecule is in general a group of two or more atoms that are chemically bonded together.

→ A molecule can be defined as the smallest particle of an element or a compound that can exist independently and shows all the properties of that substance.

→ The molecules of an element are constituted by the same type of atoms.

→ The number of atoms constituting a molecule is known as its atomicity.

→ Atoms of different elements join together in definite proportions to form molecules of compounds.

→ Compounds composed of metals and non-metals contain charged species called ions. An ion is a charged particle and can be negatively or positively charged.

→ A negatively charged ion is called an anion and the positively charged ion is called a cation.

→ A group of atoms carrying a charge is known as a polyatomic ion.

→ The chemical formula of a compound is a symbolic representation of its composition.

→ The chemical formulae of different compounds are based upon their valencies.

→ The combining capacity of an element is known as its valency.

→ The molecular mass of a substance is the sum of the atomic masses of all the atoms in a molecule of the substance.

→ The formula unit mass of a substance is a sum of the atomic masses of all the atoms in a formula unit of a substance.

→ One mole of any species (atoms, molecules, ions, or particles) is that quantity in number having mass equal to its atomic or molecular mass in grams.

→ One mole of any substance represents 6.022 × 10²³ particles (atoms, molecules, or ions) of it.

→ This number is an experimentally obtained value and is called Avogadro’s number or constant.

→ The mass of 1 mole of a substance is equal to its relative atomic or molecular mass in grams.

• 1 mole = 6.022 × 10²³ particle = Relative mass in grams.

→ An Indian philosopher Maharishi Kanad, postulated that if we go on dividing matter (padarth), we shall get smaller and smaller particles.

→ Ultimately, a stage will reach when further division will not be possible. He named these particles ‘Parmanu’.

→ The radius of an atom is expressed in nanometres (nm)

• 1 nm = 10^-9 m.
• 10⁹ nm = 1 m.

→ Atom: It is the smallest or ultimate particle of an element that takes part in chemical reactions. It may or may not exist independently.

→ Molecule: It is the smallest or ultimate particle of a substance (element or compound) that can exist freely. It shows all the properties of a substance.

→ Law of conservation of mass: It states that matter (or mass) can neither be created nor destroyed during any known physical or chemical change.

→ Law of definite proportions (or Law of constant compositions): It states that a pure chemical compound is always found to be made up of the same elements combined together in the same fixed ratio by mass.

→ Symbol: It is the shorthand representation of an element. It is made up of one or two letters from the name of the element.

→ Relative atomic mass (RAM) or Atomic mass: It is the average relative mass of one atom of the element as compared to one atom of carbon-12 taken at 12 a.m.u.

→ Relative molecular mass (RMM) or Molecular mass: It is the average relative mass of one molecule of a substance as compared to one atom of carbon-12 taken at 12 a.m.u.

→ Ion: It is an atom or group of atoms carrying some charge and can exist freely in solution.

→ Polyatomic ion: It is an ion having more than one atom.

→ The chemical formula of a molecular compound is determined by the valency of each element.

→ The chemical formula of an ionic compound is determined by the charge on each ion.

→ Mole: It is the amount of substance that contains the same number of particles (atoms/ions/molecules/formula units.) etc. as there are atoms in 12 g of carbon-12.

→ Avogadro’s number or constant is the number of atoms in 12 g of carbon-12. It is denoted by N₀ and its value is 6.022 × 10²³, N₀ = 6.022 × 10²³.

→ The molar mass of a substance is the mass of one mole of a substance.

→ Variable valency: When an element shows more than one valency, it is said to have variable valency.

→ Radical: It is an atom or group of atoms having positive or negative charges and behaving as a single unit in chemical reactions. e.g. Na⁺, \(\mathrm{SO}_{4}^{2-}, \mathrm{NO}_{3}^{-}, \mathrm{NH}_{4}^{+}\) etc.

→ Simple radical: A radical which is made up of only one kind of atom is called a simple radical, e.g. Na⁺, Cl^– etc.

→ Compound radical: A radical which is made up of more than one kind of atom is called a compound radical, e.g. \(\mathrm{NH}_{4}^{+}, \mathrm{SO}_{4}^{2-}, \mathrm{CO}_{3}^{2-}\), etc.

→ Atomic mass unit (AMU or u): It is 1/12th of the mass of one atom of carbon (C^12- isotope).

→ S.T.P. stands for standard temperature (0°C or 273K) and pressure (1 atmosphere).

→ Molar volume: It is the volume occupied by one mole of a gas and its value at S.T.P./ N.T.P. is 22.4 litres.

→ Valency: It is the combining capacity of an atom of an element and is numerically equal to the number of hydrogen atoms or number of chlorine atoms or double the number of oxygen atoms with which one atom of the element can combine.

Atomic masses of some common elements in a.m.u. or u.

Element	Symbol	Atomic Mass	Element	Symbol	Atomic Mass
Aluminium	A1	27	Lead	Pb	208
Argon	Ar	40	Lithium	Li	7
Beryllium	Be	9	Magnesium	Mg	24
Boron	B	10.8	Neon	Ne	20
Bromine	Br	80	Mercury	Hg	201
Calcium	Ca	40	Nitrogen	N	14
Carbon	C	12	Oxygen	O	16
Chlorine	Cl	35.5	Phosphorus	P	31
Copper	Cu	63.5	Potassium	K	39
Fluorine	F	19	Silicon	Si	28
Helium	He	4	Silver	Ag	108
Hydrogen	H	1	Sodium	Na	23
Iodine	I	127	Sulphur	S	32
Iron	Fe	56	Zinc	Zn	65

Formulae of a few common compounds

Name of the compound	Formula	Elements Present
Hydrogen	H2	Hydrogen
Nitrogen	N2	Nitrogen
Ammonia	NH3	Nitrogen, Hydrogen
Carbon dioxide	CO2	Carbon, Oxygen
Water	H2O	Hydrogen, Oxygen
Sulphur dioxide	SO2	Sulphur, Oxygen
Nitric acid	HNO3	Hydrogen, Nitrogen, Oxygen
Hydrochloric acid	HCl	Hydrogen, Chlorine
Sulphuric acid	H2SO4	Hydrogen, Sulphur, Oxygen
Calcium carbonate	CaC03	Calcium, Carbon, Oxygen
Silver nitrate	AgN03	Silver, Nitrogen, Oxygen
Caustic soda	NaOH	Sodium, Oxygen, Hydrogen
Caustic potash	KOH	Potassium, Oxygen, Hydrogen
Baking soda	NaHC03	Sodium, Hydrogen, Carbon, Oxygen
Phosphoric acid	H3PO4	Hydrogen, Phosphorus, Oxygen
Carbonic add	H2CO3	Hydrogen, Carbon, Oxygen
Nitrous acid	HNO2	Hydrogen, Nitrogen, Oxygen
Marble	CaC03	Calcium, Carbon, Oxygen
Phosphine	PH3	Phosphorus, Hydrogen
Hydrogen sulphide	H2S	Hydrogen, Sulphur
Urea	NH2CONH2	Nitrogen, Hydrogen, Carbon, Oxygen
Butane	C4H10	Carbon, Hydrogen
Benzene	C6H6	Carbon, Hydrogen
Acetic acid	CH3COOH	Carbon, Hydrogen, Oxygen
Methane	CH4	Carbon, Hydrogen
Soda ash	Na2C03	Sodium, Carbon, and Oxygen

Symbols or formulae of some common ions or radicals.
(A) Positive ions or Cations or Electropositive radicals:
(a) Cations having 1+ charges:

Name	Symbol or Formula
Hydrogen	H+
Lithium	Li+
Sodium	Na+
Potassium	K+
Ammonium	NH4+
Silver	Ag+
Cuprous	Cu+
Aurous	Au+
Mercurous	Hg+

(b) Cations having 2+ charges:

Name	Symbol or Formula
Magnesium	Mg2+
Barium	Ba2+
Calcium	Ca2+
Strontium	Sr2+
Cobalt	Co2+
Nickel	Ni2+
Manganous	Mn2+
Zinc	Zn2+
Cadmium	Cd2+
Ferrous	Fe2+
Cupric	Cu2+
Mercuric	Hg2+
Plumbous	Pb2+
Stannous	Sn2+
Platinous	Pt2+

(c) Cations having 3+ charges:

Name	Symbol or Formula
Ferric	Fe3+
Chromium	Cr3+
Aluminium	Al3+
Auric	Au3+

(d) Cations having 4+ charges:

Name	Symbol or Formula
Plumbic	Pb4+
Stannic	Sn4+
Platinic	Pt4+
Manganic	Mn4+

(B) Negative ions or Anions or Electronegative radicals:
(a) Anions having 1- charges

Name	Symbol or Formula
Flydride	H–
Fluoride	F–
Chloride	Cl–
Bromide	Br–
Iodide	I–
Cyanide	CN–
Hypochlorite	CIO–
Chlorate	ClO3–
Perchlorate	ClO3–
Bicarbonate	HCO3–

Name	Symbol or Formula
Bisulphite	HSO3–
Bisulphate	HSO4–
Bisphide	HS–
Hydroxide	OH–
Meta-aluminate	AlO2–
Nitrite	NO2–
Nitrate	NO3–
Acetate	CH3COO–
Permanganate	MnO4–
Sulphocyanide	SCN–

(b) Anions having 2- charges:

Name	Symbol or Formula
Carbonate	CO32-
Sulphite	SO32-
Sulphide	S2-
Sulphate	SO42-
Zincate	ZnO22-
Oxide	O2-
Manganate	MnO42-
Chromate	CrO42-
Dichromate	Cr2O72-
Oxalate	Cr2O42-
Peroxide	O22-
Silicate	SiO32-

(c) Anions having 3- charges:

Name	Symbol or Formula
Nitride	N3-
Phosphide	P3-
Phosphite	PO33-
Phosphate	PO43-
Arsenite	AsO33-
Arsenate	AsO43-
Ferricyanide	[Fe(CN)6]3-
Borate	BO33-

(d) Anions having 4- charges:

Name	Symbol or Formula
Carbide	C4-
Ferrocyanide	[Fe(CN)6]3-
Pyrophosphate	P2O74-