State and explain the following.(i) Atom (ii) Molecule (iii) Cation (iv) Anion (v) Atomic Mass unit.

Correct Answer    

(i) Atom is defined as the smallest particle of an element that can take part in a chemical reaction and which may or may not be capable of free existence. Atoms of most of the elements are very reactive and do not exist in the free state, instead, they exist in combination with the atoms of the same element or another element. For example, H2, N2, O2, HCl, NH3, etc.

(ii) A molecule is the smallest particle of an element or a compound which can exist freely and possesses all the properties of that substance. Atoms of the same element or of different elements can join together to form molecules.

(iii) A positively charged ion is known as the cation. For example, sodium ion (Na+), magnesium ion (Mg++) etc. A cation is formed by the loss of one or more electrons by an atoms. This can be represented as follows :

sodium ion (cation)

(iv) A negatively charged ion is known as anion. For example chloride ion (Cl−), oxide ion (O2 −) are anions as they are negatively charged. An anion is formed by the gain of one or more electrons.

Chlorine atom       chloride ion (anion)

              (v) “One atomic mass unit is a mass unit equal to exactly one twelfth (1/12th) the mass of one atom of carbon-12. The relative atomic masses of all elements have been found with respect to an atom of carbon -12”. Thus, 1/12th of the mass of an atom of carbon -12 isotope represents one unit of mass on the atomic scale. It is represented simply by ‘u’ which stands for unified mass.