CBSE Class 9 Chemistry – 3. Atoms and Molecules (Notes)

Atoms:- Matter is made up of small particles are called atoms.

* The smallest building unit of all matter is called atom.

* Atom is the smallest particle of an element.

* The size is measured in unit of nm(nanometre)

* Hydrogen atom is the smallest of all. Its size is only 10^-10nm.

* Atoms are very small, they are smaller than anything we can imagine or compare with.

Dalton’s atomic theory

According to Dalton?s Atomic Theory,

* All matter is made up of small particles called atoms.

* Atom is indivisible.अदृश्य

* Atoms of an element are alike in all aspects.

* Atoms of different elements combine in fixed whole number ratios to form compounds.

* Atoms can neither be created nor destroyed.

Drawbacks of Dalton’s Atomic Theory 

* According to Dalton, an atom is indivisible but later on it was proved that atom can be subdivided into electrons, protons and neutrons.

* Atoms of an element can somehow differ from each other.

So, these drawbacks led to the failure of Dalton?s theory of an atom.

IUPAC and Atomic Symbols of different Elements

* IUPAC– International Union Of Pure & Applied Chemistry 

* The IUPAC approves the name of elements. According to it, there are 118 elements till Nov-2016.

* The first 94 occure naturally on earth and the remaining 24 are synthetic elements poroduced in nuclear reaction.

* Symbol: A short hand method of representing the full name of an element is called symbol.

* The element symbols were introduce by J.J.Berzelius.

* Many of the symbols are the first letter or first two letters or the first and third letters of the name of the elements.

* Symbols are taken from English, Greek or Latin names of elements.

* First letter of the symbol is always capital.

Ex: H for hydrogen, Na for sodium etc

Element	Symbol	Atomic Number(Z)  n(P)=n(E)	Atomic Weight(A) n(P)+n(N)	Valency
Hydrogen	H	1	1	1
Helium	He	2	4	0
Lithium	Li	3	7	1
Beryllium	Be	4	9	2
Boron	B	5	10	3
Carbon	C	6	12	4
Nitrogen	N	7	14	3
Oxygen	O	8	16	2
Fluorine	F	9	19	1
Neon	Ne	10	20	0
Sodium	Na	11	24	1
Magnesium	Mg	12	27	2
Aluminium	Al	13	28	3
Silicon	Si	14	31	4
Phosphorus	P	15	32	3
Sulfer	S	16	35	2
Chlorine	Cl	17	40	1
Argon	Ar	18	39	0
Potassium	K	19	39	1
Calcium	Ca	20	40	2
Chromium	Cr	24	52	2
Manganese	Mn	25	55	2, 4
Iron	Fe	26	55	2, 3, 4
Cobalt	Co	27	59	2, 3, 4
Nickel	Ni	28	58	3, 2
Copper	Cu	29	63	2, 1
Zinc	Zn	30	65	2

RADICALS	SYMBOL	VALENCY
Hydride	H	1-
Hydronium	H3O	1+
Hydroxide	OH	1-
Nitrate	NO3	1-
Nitrite	NO2	1-
Permanganate	MnO4	1-
Hydrogen Carbonate/ Bicarbonate	HCO3	1-
Hydrogen Sulfate/ Bisulfate	HSO4	1-
Hydrogen Sulfite/ Bisulfite	HSO3	1-
Ammonium	NH4	1+
Bromate	BrO3	1-
Bromic Acid	HBrO3	1-
Bromous Acid	HBrO2	1-
Bromite	BrO	1-
Chlorate	ClO3	1-
Chlorite	ClO2	1-
Oxide	O	2-
Peroxide	O2	2-
Sulfate	SO4	2-
Sulfite	SO3	2-
Silicate	SiO3	2-
Carbonate	CO3	2-
Chromate	CrO4	2-
Dichromate	Cr2O7	2-
Phosphate	PO4	3-
Phosphite	PO3	3-
Borate	BO3	3-

Element	Symbol	Latin Name
Antimony	Sb	Stibium
Copper	Cu	Cuprum
Gold	Au	Aurum
Iron	Fe	Ferrum
Lead	Pb	Plumbum
Mercury	Hg	Hydrargyrum
Potassium	K	Kalium
Silver	Ag	Argentum
Sodium	Na	Natrium
Tin	Sn	Stannum
Tungsten	W	Wolfram

Properties of Atoms

* Atoms are very small in size.

* Atoms of an element are identical in all respect.

Note – Atoms of two different elements are different.

Atomic Mass- Mass of an atom is called atomic mass. It is the number which tells that how many times an atom of an element is heavier than 1/12 of mass of one Carbon atom.

Mass of one atom of an element = n x 1/12 of the mass of one atom of carbon

Here, n is atomic mass.

Atomic mass = Mass of one atom of an element / n x 1/12 of the mass of one carbon atom

For example: If we say that the atomic mass of sodium is 23, it means that sodium is 23 times heavier than 1/2th of a carbon atom.

Unit of atomic mass is amu (atomic mass unit).

Note – Here,1/12 of the mass of one carbon atom is called Relative atomic mass. It doesn’t have unit.

Molecules- Molecules are formed by the combination of two or more atoms.

Ex- Two atoms of hydrogen (H₂₎ and one atom of oxygen (O₂) react with each other and form one molecule of water.

Atomicity- The number or kinds of atoms present in a molecule is called atomicity.

Ex- Phosphorous exists as 🡪 P4 (tetra atomic)
Sulphur exists as 🡪 S8 (polyatomic)
Nitrogen exists as 🡪 N2 (diatomic)

There are four types of molecules On the basis of number of atoms. 

1. Monoatomic: Molecules containing only one atom are called monoatomic.

Ex- He, Ne, Ar(argon) etc.

2. Diatomic: Molecules containing two atoms are called diatomic.

Ex- O₂, H₂, Br₂ (Bromine) etc.

3. Triatomic: Molecules containing three atoms are called triatomic.

Ex- O₃, CO₂, NO₂ (Nitrogen dioxide) etc.

4. Tetra atomic: Molecules containing four atoms are called tetra atomic.

Ex-  P₄, SO₃(sulphite) etc.

There are two types of molecules On the basis of types of atoms. 

1. Homo-atomic Molecule: Molecules formed by only one type of atoms are known as Homo-atomic molecule.

Ex-  H₂, N₂, P₄ etc.

2. Hetero-atomic Molecule: Molecules formed by different types of atoms are known as Hetero-atomic molecules.

Ex- CO₂, NO₂, CH₄, HCl etc.

Molecular Mass- The sum of atomic masses of all atoms present in a molecule is called molecular mass.

Ex- The molecular mass of CO₂

= 1×atomic mass of carbon+2×atomic mass of oxygen

= 12 + (2 × 16) = 44

Classification of Matter

1. Element-तत्व 

2. Compound- यौगिक 

3. Mixture- मिश्रण

Compound: When atoms of two or more different elements combine together in a definite proportion by mass and volume then the obtained is called compound.

Ex- H₂O is formed only when the ratio of H and O is 2:1 by volume and ratio by mass is 1:8

Ions: The atoms or group of atoms which have a net charge on them is called an ions.

Ex-  Na⁺, Cl^–, etc.

Classification of ions

1. Ions on the basis of charge:

Cation: Ions containing positive charge are called cations.

Ex- Na⁺, K⁺, Mg²⁺, etc

Anion: Ions containing negative charge are called anion.

Ex- Br^–, F^–, O^2-, etc.

2. Ions on the basis of number of ions:

Monoatomic Ions: Ions containing only one atom are said to be monoatomic.

Ex-  Na⁺, K⁺, Br^–, F^–, etc.

Polyatomic Ions: Ions containing more than one atom are said to be polyatomic.

Ex- CO₃^2-, NO₃^–, HCO₃^–, etc.

Valency: The combining capacity of an element is called valency.

* The outermost shell of any element is also called valence shell.

* The number of electrons present in outer shell is called valency electron.

Silver has 1 electron in its outermost shell. Silver donates one electron to complete its octet so valency of silver is +1.

Note –

* Generaly metals are said to be electropositive elements because they have the tendency to donate electrons.

Ex- Sodium has valency of +1, Calcium has valency of +2, etc.

* Non-metals are said to be electronegative elements because they have the tendency to accept electrons.

Ex- Chlorine has a valency of -1, Oxygen has a valency of -2, etc.

* There are also certain elements which possess more than one valency.

Ex- Iron shows two types of valencies-

Fe⁺² – Ferrous and Fe⁺³ – Ferric

Ex- Copper also shows two valencies-

Cu⁺¹ – Cuprous and Cu⁺² – Cupric

3. Classification of Ions on the basis of their Valency:

Monovalent Ions: Ions having the valency of 1 are called monovalent.

Ex- OH^–, NO₃^–, HCO₃^–, HSO₄^–, etc.

Divalent Ions: Ions having the valency of 2 are said to be divalent.

Ex- SO₄^2-, SO₃^2-, CO₃^2-, etc.

Trivalent Ions: Ions having the valency of 3 are said to be trivalent.

Ex- PO₄^3-, N^3- , etc.

Chemical formula-  The symbolic representation of compound is called chemical Formula.
Ex- Ammonium nitrates- NH₄NO₃

If we say that water is represented as H₂O, we get the following information from it-

* H₂O represents water.

* H₂O represents one molecule of water.

* It represents that 1molecule of H₂O contain 2 Hydrogen and 1 Oxygen atoms.

* It represents (1× 2+16) 18g of water.

* It represents that 1 molar mass of water has 6.022 x 10 23 molecules.

Writing Chemical Formulae of Compounds

Rule 1: Cross multiply the valencies of elements to form a compound.

A⁺² × B^-3 →  A₃B₂

Rule 2: If compound consist of metal and non-metal than metal is written first.

Ex- In calcium chloride (CaCl₂) and zinc sulphide (ZnS), calcium and zinc are metals, so they are written first, whereas chlorine and sulphur are non-metals.

Rule 3: If compound is formed with polyatomic ions then polyatomic ions are written in brackets.

Ex- In aluminium sulphate [Al₂(SO₄)₃], the polyatomic sulphate ion SO₄^2- is enclosed in a bracket before writing the subscript 3.

Here, the bracket with a subscript 3 indicates that three sulphate groups are joined to two aluminium atoms.

Note – Compounds made up of metal and non – metal are called salts. All the above examples are of salts.

Mole- The quantity of a substance is expressed in terms of mole. One mole is also defined as the amount of substance which contains 6.023 × 10²³ units of particle.

1 mole = 6.023 × 10²³ units (Avogadro’s Number N_A)

Ex- one mole of oxygen atoms represents 6.023 × 10²³ atoms of oxygen and 5 moles of oxygen atoms contain 5 × 6.023×10²³ atoms of oxygen.

Molar mass: The mass of 1 mole of substance is called molar mass. Atomic mass or molecular mass in gram is equal to molar mass.

Ex: Atomic mass of Fe is 56 amu

* Molar mass of Fe is 56 gram

* Molecular mass of H₂O is 18 (1× 2+16) amu

* Molar mass of H₂O is 18 gram

Number of mole- The ratio of mass of substance and mass of one mole is called number of mole of that substance.

Ex- Find the number of moles in 112 gm of iron?

Percentage of an element in a compound

A compound is composed of different elements and it tells that how much percentage of different elements is present in a compound.

Chemical reaction

The process by which some substances react to form a new substance is called chemical reaction.

Ex- Making of tea, changing milk to curd, making cheese from milk and lots more. These chemical reactions take place in our day to day life.

Part of Chemical reaction

1. Reactant अभिकारक

2. Product उत्पाद

The substances which react are called reactants and the new ones formed are called products.

Ex-  A  +  B  🡪  C + D
In this, A and B are reactants and C and D are called products.
Ex- When we make tea, we add sugar, tea, milk and water. We mix them and heat the mixture. The result is that we get a new substance that is tea.

Laws of chemical combination

There are certain sets of laws that are obeyed by all chemical reactions. These laws are given by Lavoisier & Joseph. Let us study them in detail.

Laws of Chemical Combination

Compounds are formed by chemical combination of reactants (atoms or molecules) in fixed proportion by weight or by volume. This is achieved by following certain laws known as Laws of chemical combination.

1. Law of Conservation of Mass:

The law of conservation of mass states-

 “Mass can neither be created nor destroyed in a chemical reaction”

Total mass of reactants = Total mass of products

Example:

2H₂ +  O₂   →  2H₂O

Total Mass of reactant = Total mass of product

Or, (2× mass of hydrogen + mass of oxygen) = 2×(mass of water)

Or, (2×2 + 2×16) = 2×18

Or, 36 = 36

2. Law of Constant Proportion:

Law of Constant Proportion states that-

“a chemical compound always contains exactly the same proportion of elements by mass”

This law is also known as Law of definite proportions. Joseph Louis Proust gave this law hence, this law is also known as Proust’s Law.

Example:

2H₂ + O₂  →  2 H₂O

(4 : 32)  =   (2 : 16)

1 : 8      =   1 : 8