Notes Class 9 Science Chapter 3 Atoms and Molecules

Notes For All Chapters Science Class 9 CBSE

 Laws of Chemical Combination :-

The chemical reaction between two or more substances giving rise to products is governed by certain laws. These laws are called Laws of Chemical Combination’.

Law of Chemical Combination :-

• Law of Conservation of Mass
• Law of Constant Proportion
• Law of Multiple Proportion
• Law of Reciprocal Proportion
• Gay Lussac’s law of combining volume

 Law of Conservation of Mass :-

•  According to this law, “Mass can neither be created nor destroyed.” In a chemical reaction, this law can be understood in the following way:-
•  During a chemical reaction total mass of reactants will be equal to total mass of products.”
• For example, ( Reactant ) A + B → AB ( Product )

Law of Constant Proportion :-

According to this law, “A pure chemical compound always contain the same elements combined together in the same proportion by mass irrespective of the fact from where the sample has been taken or from which procedure has it been produced.”

Dalton’s Atomic Theory :-

Based upon laws of chemical combination, Dalton’s Atomic Theory provided an explanation for the Law of Conservation of Mass and Law of Constant Composition.

Postulates of Dalton’s atomic theory :-

• All matter is made up of very tiny particles called ‘Atoms’.
• Atom are indivisible particles, which can’t be created or destroyed in a chemical reaction. (Proves ‘Law of Conservation of Mass’)
• Atoms of an element have identical mass and chemical properties.
• Atoms of different elements have different mass and chemical properties.
• Atom combine in the ratio of small whole numbers to form compounds. (proves ‘Law of Constant Proportion’)
  The relative number and kinds of atoms are constant in a given compound.

Atom :-

• According to modern atomic theory, an atom is the smallest particle of an element which takes part in chemical reaction such that during the chemical reaction, the atom maintain its identity, throughout the chemical or physical change.
• Atoms are very small and hence can’t be seen even through very powerful microscope.

Measurement of Atomic radius :-

1 nm = 10⁹m

Atomic radius of smallest atom in hydrogen is 0.37 x 10-¹⁰m or 0.037 nm.

Atomic Mass :-

• The mass of an atom of an element is called its atomic mass.
• In 1961, IUPAC have accepted ‘atomic mass unit’ (u) to express atomic and molecular mass of elements and compounds.

Note :- IUPAC (International Union of Pure & Applied Chemistry)

Atomic Mass Unit :-

The atomic mass unit is defined as the quantity of mass equal to 1/12 of mass of an atom of carbon-12.

1 amu or u = 1/12 × Mass of an atom of C
1u = 1.66 × 10²⁷kg

How do atoms exist ?

Atoms of most of the elements are very reactive and does not exist in free state.

Only the atoms of noble gases (such as He, Ne, Ar, Kr, Xe and Rn) are chemically unreactive and can exist in the free state as single atom.

Atoms of all other elements combine together to form molecules or ions.

Molecule :-

A molecule is a group of two or more atoms which are chemically bonded with each other.

A molecule is the smallest particle of matter (except element) which is capable of an independent existence and show all properties of that substance.

 E.g., ‘H₂O’ is the smallest particle of water which shows all the properties of water.

Categorized of Molecule :-

A molecule may have atom of same or different elements, depending upon this, molecule can be categorized into two categories :-

Homoatomic molecules (containing of same element) Eg., O₂, N₂, O₃, S₈, P₄ etc.

Heteroatomic molecules or compounds (containing atoms of different elements) Eg., H₂O , NO₂, SO₂ etc.

Atomicity :-

The number of atoms present in one molecule of an element is called its atomicity.

Chemical formulae :-

It is the symbolic representation of the composition of a compound.

Characteristics of chemical formulae :-

• The valencies or charges on ion must balance.
• When a compound is formed of metal and non-metal, symbol of metal comes first. E.g., CaO, NaCl, CuO.
• When polyatomic ions are used, the ions are enclosed in brackets before writing the number to show the ratio. E.g., Ca(OH)₂ , (NH₄)₂ SO₄

Rules for writing chemical formulae :-

• we first write symbols of elements which form compound.
• Below the symbol of each element, we should write their valency.
• Now cross over the valencies of combining atoms.
• with first atom, we write the valency of second atom (as a subscript).
• with second atom, we write the valency of first atom (subscript).

Molecular Mass :-

It is the sum of atomic masses of all the atoms in a molecule of that substance.

e.g.,

Molecular mass of H₂O = 2 x Atomic mass of Hydrogen + 1 x Atomic mass of Oxygen
So, Molecular mass of H₂O = 2 × 1 + 1 × 16 = 18 u

Formula Unit Mass :-

It is the sum of atomic mass of ions and atoms present in formula for a compound.

e.g

In NaCl,
Na = 23 a.m.u ,
C1 = 35.5 a.m.u.
So, Formula unit mass = 1 × 23 + 1 × 35.5 = 58.5 u

Ions :-

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

Some positively charged ions :- Na⁺, K⁺, Ca²⁺, AI³⁺

Some negatively charged ions :- Cl- (chloride ion), S²⁻ (sulphide ion), OH⁻ (hydroxide ion), so₄²⁻ (sulphate ion)

Molar Mass :-

The molar mass of a substance is the mass of 1 mole of that substance.

It is equal to the 6.022 × 10²³ atoms of that element/substance.