Atoms and Molecules: Points to Remember
Two or more forms of an element having the same atomic number, but different mass number are called Isotopes.
Example: (3517Cl, 3717Cl)
Atoms of different elements having the same mass number, but different atomic numbers are called Isobars.
Example: (4018Ar, 4020Ca)
Atoms of different elements having the same number of neutrons, but different atomic number and different mass number are called Isotones.
Example: (136C, 147N)
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 can be calculated using the formula:
The relationship between molecular mass and vapour density is: