Atoms and Molecules Ncert 9th

ATOMS AND

MOLECULES

Presented by-

Tushar Ranjan

Class-9 A

Roll. No-38

Father of Modern Chemistry: Antoine L.

Lavoisier

Antoine-Laurent de Lavoisier (26 August 1743 – 8 May1794)was a French nobleman and chemist central to the 18th-century Chemical Revolution and a large influence on both the histories of chemistry and biology. Antoine L. Lavoisier laid the foundation of chemical sciences by establishing two important laws of chemical combination.

Laws of Chemical Combinations

There are two laws of chemical

combination.

Law of Conservation of Mass.

Law of Constant Proportions.

Law of Conservation of Mass

Antoine L. Lavoisier, a French scientist,

established the theory of Law of

Conservation of Mass.

The law of conservation of mass states,

“Mass can neither be created nor

destroyed in a chemical reaction”.

Law of Constant Proportions

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.

John Dalton

John Dalton (6 September

1766 – 27 July 1844) was an

English chemist, meteorolo

gist and physicist. He is

best known for his

pioneering work in the

development of

modern atomic theory, and

his research into colour

blindness.

Main points of Dalton's

atomic theory.

Elements are made of extremely small particles called

atoms.

Atoms of a given element are identical in size, mass,

and other properties;

Atoms of different elements differ in size, mass, and

other properties.

Atoms cannot be subdivided, created, or destroyed.

Atoms of different elements combine in simple whole-

number ratios to form chemical compounds.

In chemical reactions, atoms are combined, separated,

or rearranged.

What is an Atom?

Atom can be defined as the smallest particle of matter

that takes part in a chemical reaction.

How big are Atoms?Atoms are very small, they are smaller than anything

that we can imagine or compare with.

Characters of atoms:

Atom is the smallest particle of matter.

All elements are made of tiny particles called atom.

Atoms are very small in size and cannot be seen through naked eyes.

Atom does not exist in free-state in nature. But atom takes part in a chemical reaction.

The properties of a matter depend upon the characteristics of atoms.

Atoms are the building block of an element similar to a brick which combine together to make a building.

The size of atoms is indicated by its radius.

In ancient time atoms was considered indivisible.

Symbols of Atoms of Different

Elements

Symbols of some elements are formed

from the first letter of the name and a letter,

appearing later in the name.

E.g.: (i) chlorine: Cl, (ii) zinc: Zn etc

Some have been taken from the

names of elements in Latin, German or Greek.

E.g.: s Iron is Fe from ferrum,

Sodium is Na from natrium,

Potassium is K from kalium

Atomic Mass

Mass of atom is called atomic mass. Since, atoms are very small

consequently actual mass of an atom is very small. For example the

actual mass of one atom of hydrogen is equal to 1.673 x 10-24 g. This is

equal to 0.000000000000000000000001673 gram. To deal with such small

number is very difficult. Thus for convenience relative atomic mass is

used.

Molecule

A molecule is a group of two or more atoms that are chemically bonded

together, that is, tightly held together by attractive forces.

Mono-atomic: When molecule is formed by single atom

only, it is called mono-atomic molecule. Generally noble

gases form mono-atomic molecules. For example: Helium

(He), Neon (Ne),

Di-atomic: When molecule is formed by the combination

of two atoms of it is called diatomic molecule. For

example: Hydrogen (H2), Oxygen (O2), Nitrogen (N2),

Chlorine (Cl2), etc.

Tri-atomic: When molecule is formed by the combination

of three atoms it is called tri-atomic molecule. For

example: molecule of ozone (O3)

Atomicity of Molecules

Tetra-atomic:- When molecule is formed by the

combination of four atoms it is called tetra-

atomic molecule. For example: Phosphorous

molecule (P4)

Polyatomic:- When molecule is formed by the

combination of more than two atoms, it is

called polyatomic molecule. For example:

Sulphur molecule (S8)

All metals are mono-atomic.

All gases are di-atomic.

Molecules of different

Elements

The molecules of an element are constituted by the

same type of atoms.

e.g.: O + O→ O2 (Oxygen Gas) ‘di-atomic’

O + O + O→ O3 (Ozone) ‘tri-atomic’

4P→ P4 (Phosphorous) ‘tetra-atomic’

8S→ S8 (Sulphur) ‘poly-atomic’

Ions

Particles carrying positive or negative charges are called

ions.

Types of ions:

1. Cations: Positively charged ions E.g.: Al3+, Ca2+

2. Anions: Negatively charged ions E.g.: Cl−, Br−

All metals form cations by loosing electrons.

All non-metals form anions by gaining electrons.

Chemical Formulae

The chemical formula of a

compound is a symbolic

representation of its composition.

e.g.: H20 (Water), CaO (Calcium

Oxide), NH3 (Ammonia) , K2SO4

(Potassium sulphate) etc.

Concept of writing chemical

formulae

• The valencies or charges on the ion must balance.

• When a compound consists of a metal and a non-metal,

the name or symbol of the metal is written first. For

example: calcium oxide (CaO), sodium chloride (NaCl),

iron sulphide (FeS), copper oxide (CuO) etc., where

oxygen, chlorine, sulphur are non-metals and are

written on the right, whereas calcium, sodium, iron and

copper are metals, and are written on the left.

• In compounds formed with polyatomic ions, the ion is

enclosed in a bracket before writing the number to

indicate the ratio.

Mole Concept

One mole of any species (atoms, molecules, ions or

particles) is that quantity in number having a mass

equal to its atomic or molecular mass in grams.

1 mole (of anything) = 6.022×1023 in number

Mass of 1 mole of a particular substance is always fixed.

Avogadro’s Number

It was named after the Italian scientist named Amedeo

Avogadro.

It is denoted by.

Its value is 6.022×1023

Ao is equal to 1 mole.