PSEB 9th Class Science Notes Chapter 3 Atoms and Molecules

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→ According to the latest IUPAC recommendations, a.m.u. has now been replaced by U (uniformed mass).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