* Around 500 B.C., Indian Philosopher Maharisi Kanad, postulated the theory if

 we go on dividing matter (padarth), we will obtain smallest particle beyond

 with further division can't be possible which is known as 'parmanu'

* Ancient Greek Philosophers - Democritus and Leucippus called these particles

 atom.

* Antoine L. Lovoisier laid the foundation of chemical sciences by establishing

 two important laws of chemical combination.

Law of chemical combination

• According to the law of conservation of mass, matter can neither be created

 nor destroyed in a chemical reaction. It remains conserved.

• Mass of reaction will be equal to the mass of products.

 Example: A (reactant) + B (reactant) → AB (product)

     Mass of A + mass of B = mass of AB

• A pure chemical compound contain the same element combined together in a fixed proportion by mass is given by the law of definite proportions.

• For example.., If we take water from river or from an ocean, both has oxygen and hydrogen in the same proportions.

✤ Dalton's Atomic Theory

i. All matter is made of very tiny particles called atoms which participate

  in chemical reaction.

ii. Atoms are indivisible particles, which cannot be created or destroyed in

  a chemical reaction.

iii. Atoms of a given element are identical in mass and chemical

  proportions.

iv. Atoms of different elements have different masses and chemical

  properties.

v. Atoms combine in the ratio of small whole numbers to form compound.

vi. The relative number and kinds of atoms are contained in a given

  compound.

 

• Atoms are building blocks of a matter.

• According to modern atomic theory, an atom is the smallest particle of an element which takes part in chemical reaction.

• Atoms one very small and which can't be seen even through a very powerful microscope.

• Atomic radius is measured nanometers. Nanometers 10-m.

The atomic symbol has three parts:-

• The symbol x: The usual element symbol

• The atomic number A : equal to the number of protons

• The mars number Z : equal to the total number of protons and neutrons in element

Atomic mass and atomic mass unit

• Atomic mass is the total of the masses of the electron, neutrons, and protons in an atom, an in a group of atoms the average mass.

• mass of an atomic particle is called the atomic mass.

• This is commonly expressed as per the international agreement in terms of a unified atomic mass unit (AMU)

• It can be best defined as 1/12 of the mass of a carbon- 12 atom in its ground state.

 

→ A molecule is defined as the smallest unit of an compound that contains the chemical properties of the compound.

• The atomicity of an element is the number of atoms in one molecule of the element.

• For example:- Hydrozen, nitrogen, Oxygen, chlorine, iodine, bromine all have two atoms in each of their molecules. So, the atomicity of hydrozen, nitrogen, Oxygen, chlorine, iodine, bromine is too each.

 The combining capacity of an element is known as its valency. Valency is used to find out how the atom of an element will combine with the atom of another element will combine with the atom of another

element to form a chemical compound. (Every atom wants to become stable, to do so it may lose, gain or share electrons.)

• If an atom contains of 1, 2 or 3 electrons in its valency shell then its valency is 1, 2, or 3 respectively.

• If an atom contains of 5, 6 or 7 electron in the outermost shell, then it will gain 3, 2 or 1 electron respectively and its valency will be 3, 2 or 1 respectively.

• If an atom has 4 electrons in the outermost shell then it will share this electron and hence its valency will be a.

• When two or more elements chemically combine in fixed by ratio by mass, the obtained product is known as a compound.

• Compounds are substances consisting of two or more different types of element in a fixed ratio of its atoms.

1. We first write symbols of elements which form compounds.

2. Below the symbol of each element, we should their valency.

3. Now cross over the valencies of combining atoms.

    Symbol H Cl

    Valency 1 1

        → Formula of the compound would be HCl

2. Formula of hydrogen sulphide

    Symbol H S

    Valency 1 2 → Formula: H₂S

3. Formula of a carbon tetrachloride

 Symbol C Cl

 Valency 4 1

     Formula: CCl₄

4 . Formula of magnesium

Symbol Mg Cl

Valency 2+ 1-

     Formula: MgCl₂

→ An ions may be defined as an atom or group of atoms having positive or negative charge.

→ Some positive charged ions: Na⁺, K⁺, Ca²⁺, Al³⁺

→ Some negatively changed ions : Cl⁻ (chloride ion), S²⁻ (Sulphite ion), OH⁻ (Hydroxide ion), SO₄²⁻ (Sulphate ion)

⇒ we can classify ion in two types

ions

Simple ions Compound ions

✤ Simple ions

Mg² (magnesium ion)

Na+ (sodium ion)

Cl- (chloride ion)

Al³ (Aluminium ion)

✤ Compound ion

Calcium oxide (CaO)

Magnesium sulphide (MgS2)

Sodium chloride (NaCl)

✤ Ionic compounds: chemical formula

• Ionic compounds are chemical compounds in which ion are held together by specialized bonds called ionic bonds.

• An ion compound always contains are equal amount of positive and negative charge. chemical formula of ionic compounds (polyatomic)

Sodium Carbonate = Na₂CO3

Aluminium sulphate = Al₂(So₄)₃

• Molecular mass of an element is an defined as the sum of masses of

 elements present in molecules.

• Molecular mass is obtained by multiplying the atomic mass of an element with the number of atoms in the molecule and then adding the masses of all elements in the molecule.

Example 3.1 (a) calculate the relative molecular mass of water (H2O).

Ans. Atomic mass of Hydrogen = 10

          Oxygen = 160

So, the molecular mass of water which contains two atom of hydrogen and One atom of oxygen is

      => 2 + 16 = 180

(b) calculate the molecular mass of HNO3.

Ans. The molecular mass of HNO3 = the atomic mass of H + the atomic

              mass of N + 3 X the atomic of O

         = 1 + 14 + 48 = 630