CBSE – 9th Standard Science

LAWS OF CHEMICAL COMBINATION

According to Dalton’s atomic theory, all matter, whether an element, a compound or a mixture is composed of small particles called atoms.

• The relative atomic mass of the atom of an element is defined as the average mass of the atom.
• The number of atoms constituting a molecule is known as its atomicity.

• A molecule can be defined as the smallest particle of an element or a compound that is capable of an independent existence and shows all the properties of that substance.
• Atoms of the same element or of different elements can join together to form molecules.

Compounds composed of metals and non-metals contain charged species. The charged species are known as ions

MOLECULAR MASS

CHEMICAL FORMULAE

• It is therefore the relative mass of a molecule expressed in atomic mass units (u).
• The number of particles (atoms, molecules or ions) present in 1 mole of any substance is fixed, with a value of 6.022 x 10²³. This number is called the Avogadro Constant or Avogadro Number.
• Avogadro Number is named in honour of the Italian scientist, Amedeo Avogadro.
• The mass of 1 mole of a substance is equal to its relative atomic or molecular mass in grams.

• Molar mass of atoms is also known as gram atomic mass.

MOLE CONCEPT

ISOTOPES

The atoms of same element, having same atomic number but different mass numbers. e.g. hydrogen ₁¹H, deuterium ₁ ²H and tritium ₁ ³H