Chapter 3

Facts that matter

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Law of chemical combination:

Given by Lavoisier and Joseph L. Proust as follows.

1 Law of conservation of mass : Mass can neither be created nor destroyed in a chemical reaction. e.g.,A+B➡️C+D Rectants➡️ products mass of reactants = Mass of products

2 Law of constant proportion : In a chemical substance the elements are always present in definite proportions by mass.

E.g., in water, the ratio of the mass of hydrogen to the mass of oxygen is always 1:8 respectively .

These laws lacked explanation. Hence ,john Dalton gave his theory about the matter .he said that the smallest particle of called 'atom'

Dalton's atomic theory -

• Every matter is made up of very small or tinny particles called atoms .

• Atoms are not divisible and can not be created or a destroyed in chemical reaction.

• All atoms of given elements are in size, mass and chemical properties.

• Atoms of different elements are different in size,mass and chemical properties.

• Atoms combine in the ratio of a small whole number to form compounds.

• The relative number and kinds of atoms are constant in a given compound.

Atom - atoms are the smallest particles of an element which can take part in a chemical reaction.

Size of an atom: atomic radius is measured in nanometers.

1 nm = 1/10⁹

1m. = 10⁹nm

Atomic radii of hydrogen atom = 1×10¹⁰m.

Symbols of atoms:

(a) Symbols for some elements as proposed by Dalton

(b) Symbols of some common elements :

Molecule-

It is smallest particle of an element or a compound which can exist independently.

• Molecules of an element constitute the same types of atoms.

• Molecules may be mono-atomic, diatomic or poly-atomic.

• Molecules of compound join together in definite proportions and constitute a different type of atoms.

Atomicity- the number of atoms constituting a molecule is known as its atomicity.

Name of the element Atomicity Molecules Formula

Helium Monoatomic He

Neon '' Ne

Argon " Ar

Sodium " Na

Iron " Fe

Aluminium " Al

Hydrogen Di-Atomic H₂

Oxygen " O₂

Chlorine " Cl₂

Nitrogen " N₂

Phosphorus Poly-atomic Tetra P₄

Sulphur Poly-atomic Octa S₈

Ions - The charged particles (atoms) are called ions,they are formed by attaining positive charge or negative or negative charge on it .

Negatively charged ion is called anion ( CI-).

Positively charged ion is called cation (Na+).

Valency - The combining capacity of element is known as it valency. Valency is used to find out how the atom of an 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 atoms consists of 1,2,or 3, electrons in its Valence shell then is valency is 1,2 or 3, respectively,

• If an atoms consists of 5,6,or 7, electrons 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 atoms has 4 electrons in the outermost shell than it will share this electron and hence its valency will be 4.

• if an atom has 8 electrons in the outermost electron and hence its valency will be 0.

Name of the element symbol Valency Ions

Hydrogen H 1 H⁺

Helium He 0

Lithium Li 1 Li⁺

Beryllium Be 2 Be²⁺

Boron B 3 B³⁺

Carbon C 4 (Share Electrons)

Nitrogen N 3 N^3-

Oxygen O 2 O^2-

Fluorine F 1 F^-

Neon Ne 0

Sodium Na 1 Na⁺

Magnesium Mg 2 Mg²⁺

Aluminium Al 3 Al³

Some element show more than one valency,hence turned as variable valency.

E.g., iron shows valency II - Fe²⁺

And valency III - Fe³⁺

Copper shows valency I - Cu⁺

And valency II - Cu²⁺

Chemical formula :

Rules - 1 the valencies or charges on the ion must balance.

2 . Metals and non-metal compound should show the name or symbol of the metal first.

E.g.,. Na⁺ Cl^- ----> NaCl

3. If a compound consists of polyatomic ions.the ion is inclosed in a bracket before writing the number to indicate the ratio.

e.g., [SO₄]^2- -----> Polyatomic Radial

H¹⁺ SO₄^2- -----> H₂ SO₄

Chemical formula of some simple compounds-

(a) . Calcium hydroxide

Symbol Ca OH

Valency +2 -1

Criss cross

Formula. Ca(OH)₂

(b). Aluminium oxide-

Symbol Al O

Valency +3 -2

Formula. Al₂O₃

Molecular mass : it is the some of atomic masses of all the atoms in a molecule of the substance.it is expressed in atomic mass unit.

e.g., 2H⁺ + O₂ H₂O [H = 1 , O = 16]

1 x 2 + 16 = 18 u

Formula unit mass : it is the some of the atomic masses of all atoms in a formula unit of a compound.the constituent particles are ions.

e.g., Na⁺ + CL^- NaCl [Na = 23 , Cl = 35.5]

1 x 23 + 1 x 35.5 = 58.5 u

Mole concept :

Definition of mole - it is defined as one mole of any species (atoms,molecules,ions or particles)

Is that quantity in number of having a mass equal to its atomic or molecular mass in grams.

1 mole =6.022×10²³ in number

Molar mass= mass of 1 mall - is always expressed in grams and is also known as gram atomic mass.

Lu of hydrogen has- 1 atom of hydrogen 1kg of hydrogen has- 1 mole hydrogen= 6.022×10²³ atoms of hydrogen.

We hope the given CBSE/MP Board Class 9th Science Chapter 3 "Atoms & Molecules" Will help you.

Written By - Divya Bhadouriya & Himanshu Sharma