Points to Remember

Atoms and Molecules (Science)

Points to Remember

·                 Two or more forms of an element having the same atomic number, but different mass number are called Isotopes (17Cl35, 17Cl37).

·                 Atoms of different elements having the same mass number, but different atomic numbers are called Isobars (18Ar40, 20Ca40).

·                 Atoms of different elements having the same number of neutrons, but different atomic number and different mass number are called Isotones ( 6C13, 7N14 ).

·          Relative atomic mass of an element is the ratio between the mass of one atom of the element to 1/12th of the mass of the atom of carbon -12.

·          Average atomic mass of an element is calculated by adding the masses of its isotopes, each multiplied by their natural abundance on the Earth.

·          Relative molecular mass of a molecule is the ratio between the mass of one molecule of the substance to 1/12th of the mass of the atom of carbon – 12.

·          The Avogadro’s law states that “equal volumes of all gases under similar conditions of temperature and pressure contain equal number of molecules”.

·          The vapour density is defined as “the ratio between the masses of equal volumes of a gas (or a vapour) and hydrogen under the same condition”.

·          Atomicity of a monoatomic element = Molecular mass / Atomic Mass.

·                 Molecular mass = 2 × Vapour density.