Atoms and Molecules | Class 9

Laws of chemical combination

Two important laws of chemical combination are:

1. Law of conservation of mass

2. Law of constant proportion / Law of Definite proportion

Law of conservation of mass

It states that mass neither be created nor be destroyed during a chemical reaction. This means that in any chemical reaction the total mass of the reactants is equal to the total mass of the product, there is no change in mass during the chemical reaction. 

Law of constant proportion /  Law of Definite proportion

It states that in a chemical substance or compound the elements are always present in definite proportion or ratio by mass. 

Dalton's Atomic Theory

1. Every matter is made up of very small particle called Atom.

2. Atoms are indivisible particles which can neither be created nor be destroyed in a chemical reaction.

3. Atoms of given element are identical in mass as well as in chemical properties whereas atoms of different element have different mass and different chemical properties.

4. Atom combine in the ratio of small whole number to form compounds.

Limitations of Dalton Atomic Theory

    1. Atom is no longer considered as the smallest indivisible particle it has been established that it is made up of electron, proton and neutron.
    2. Atom of the same element may have slightly different masses (isotopes).
    3. Atom of different element may have the same masses (isobar).

    4. Substances made up of same kind of atom may have different properties (allotropes).

Atoms

Atom are the smallest particle of an element which may have or may not have independent existence but take part in a chemical reaction. These are the building blocks of all matter.

e.g. atom of Hydrogen, Oxygen, Nitrogen,etc are not capable of independent existence whereas atoms of Helium, Neon, etc are capable of existence independently.

Atom are very small and their radius is measured in nanometres. Hydrogen atom is the smallest atom and it radius is 0.1 nm.

Atomic mass

It is defined as the number of time a given atom is heavier than 112th of mass of one atom of carbon-12. It is the average mass of the atom as compared to the 112th mass of the one carbon atom.

Atomic mass unit (amu)

 It is defined as the mass of 112th of the mass of one atom of carbon 12  isotope .

Molecule

Molecule is a group of two or more atoms that are chemically bonded together. Atoms of the same element or of different elements can join together to form molecules. Molecule is capable of independent existence. 

Molecules can be divided into two categories:

        1. Molecule of elements

        2. Molecules of compounds

Molecules of Element

The molecules of an element contains same type of atom.

e.g. a molecule of oxygen O2 consists of two atom of Oxygen,  Ozone O3 consists of three atom of Oxygen.

Molecules of Compound

Atoms of different elements join together in definite proportion to form molecules of compound.

e.g. H2O water molecule is made up of hydrogen and oxygen element in the ratio of 1:8 by mass

Atomicity

It is defined as the number of atoms present in a molecule.  On the basis of atomicity molecule can be classified as:

Monoatomic, Diatomic, Triatomic, Tetra-atomic and Polyatomic.

Ions

When atoms, group of atoms or molecules lose or gain electrons they become charge these charge species are known Ions.

Atoms in a solution generally exist in the form of ions there can be negatively or positively charged, thus can be categorised into two groups:

Cations

Cations are the positively charged ions. e.g. Na+, Ca2+, K+, etc. 

Anions

Anions are negatively charged ions. e.g. Cl-, Br-, O2-, etc.

Valency

The combining power or capacity of an element is called its valency. Valency can be used to find out how the atom of an element will combine with the atom of another element to form a chemical compound. The valency of an Ion is equal to the charge on the ion. 

Molecular mass 

The molecular mass of a substance is the sum of the atomic masses of all the elements in a molecule of a substance. e.g. the molecular mass of water H2O is 18 u,  which can be calculated as:

 atomic mass of hydrogen = 1 u 

 atomic mass of oxygen = 16 u

 H2O = 2 X 1 + 1 X 16   =  18 u

Mole concept

1 mole is the amount of a substance which contains as many particles as in 12 gram of C-12.
1 mol = 6.022 X1023 particles  =   Avogadro constant (Na).
Mass of 6.022 X 1023 particle  =  molar mass in grams.
Moles  =  (Given mass) / (molar mass)  =  (Given number of particles) / (Avogadro's constant).

Molar mass and Moles 

The mass of 1 mole of a substance is equal to its relative atomic or molecular mass in gram. 

Percentage Composition of Element

It is the percentage of the mass contributed by the element to the total mass of the compound.