Atoms and Molecules

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Atoms and Molecules


It is the smallest particle of an element which does not always exist independently.

For example atoms of hydrogen do not exist independently but instead always occur in pairs.

Two atoms of hydrogen form a molecule of hydrogen which exists independently. Atom is the smallest particle of an element that can take part in a chemical change.


It is the number of atoms present in a molecule of an element.

Atomicity of hydrogen, oxygen, nitrogen and chlorine is 2, for ozone it is 3 and for phosphorus it is 4.


It is the smallest part of an element or compound that can normally exist separately.

Atomic Weight (or Atomic Mass)

The atomic mass of an element is the number of times its atom is heavier the 1/12th of the mass of carbon atom.

The unit used to measure atomic mass is called atomic mass unit i.e., amu

In this unit atomic mass of hydrogen is 1 amu and that of oxygen is 16 amu.

∴ Atomic Weight of an Element

= Average Weight of One Atom of an Element / Weight of One Atom of C12 x 12

Atomic Mass of Some Common Elements

ElementsAtomic MassElementsAtomic Mass

Molecular Weight

The molecular weight of an element or compound is the number of times its molecule is heavier than 1/12th of mass of carbon atom.

∴ Molecular Weight of an Element or Compound

= Average Weight of One Atom of the Element or Compound / Weight of One Atom of C12 x 12


(or Mol) One mole is equal to the number of atoms present in 12 gram C12 i.e.,

1 mole = 6.022 x 1023 amu

Here 6.022 x 1023 is known as Avogadro's number

Relation between gram and amu = 1 gram = 6.022 x 1023 amu

Equivalent Weight

It is the number of the parts of a substance by weight, which combine with or displace directly or indirectly 1.008 part by weight of H, 8 parts of weight of O or 35.5 parts by weight of Cl.

Physical and Chemical Changes in Chemistry

  1. Physical Change : A physical change is a change of state only and is not accompanied by an alteration in the chemical composition, weight or chemical properties of any substance e.g., melting of ice, magnetizing a needle etc.

  2. Chemical Change : A chemical change gives rise to a new substance having composition and properties altogether different from those of the original substances, e.g., radioactive decay, rusting of iron, photosynthesis etc. Energy is absorbed or released during a chemical change.

Physical and Chemical Properties

  1. Physical Properties : The properties which do not depend on reaction with any other substance, e.g., colour, melting point, boiling point, density etc.

  2. Chemical Properties : The chemical properties of a substance are those that describe its reactions with other substances.

Empirical Formula

The empirical formula gives the simple whole number ratio between the number of atoms of the different elements forming the compound, e.g., H2O tells that there are twice as many H atoms as O atoms.

Molecular Formula

It indicates the number of atoms in a molecule of a molecular substance.

Elements as well as compounds can be represented by a molecular formula, e.g., H2O2, Cl2, N2 etc.

Atoms and Molecules

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