A ppt to help the students of class 9th

1. ATOMS AND
MOLECULES
Presented by-
Tushar Ranjan
Class-9 A
Roll. No-38

2. Father of Modern Chemistry: Antoine L.
Lavoisier
 Antoine-Laurent de
Lavoisier (26 August
1743 – 8 May1794)was
a French
nobleman and chemist cen
tral 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.

3. Laws of Chemical Combinations
There are two laws of chemical
combination.
Law of Conservation of Mass.
Law of Constant Proportions.

4. 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”.

5. 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.

6. 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.

7. 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.

8. 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.

9. 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.

10. 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

11. 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.

12. Molecule
 A molecule is a group of two or more atoms that are chemically bonded
together, that is, tightly held together by attractive forces.

13.  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

14.  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.

15. 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’

16. 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.

17. 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.

18. 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.

19. 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.

20. 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.