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Define chemical bond. Why do atoms combine (Kossel-Lewis approach) and how do atoms combine (modes of chemical combination)?

Chemical bond. A chemical bond may be defined as the attractive force or binding force which holds atoms, ions and molecules together.

Why do atoms combine (Kossel-Lewis approach)?
The noble gases are highly unreactive and stable. Except for He (1s²), all other noble gas elements have ns²np^b outermost electronic configurations. This indicates that the presence of 8 electrons (law of octet) in the outermost orbit must be related to the stability of the atom. If K-orbit is the outermost orbit, the presence of 2 electrons (law of duplet) causes stability.

Thus atoms of different elements combine with each other in order to complete their respective octets (i.e. 8 electrons in their outermost shell) or duplet (i.e. outermost shell having 2 electrons) in the case of H, Li and Be to attain stable inert gas configuration.

How do atoms combine (Modes of chemical combination):
Atoms combine together in two ways to acquire stable inert gas configuration:

(i) By complete transference of one or more electron from one atom to another: This process is referred to as electro-valency and the chemical bond formed is termed as an electrovalent bond or ionic bond.

(ii) By sharing of elements: This can occur in two ways:
(a) When the shared electrons are contributed by the two combining atoms equally, the bond formed is called the covalent bond.

(b) When these electrons are contributed entirely by one of the atoms but shared by both, the bond formed is known as coordinated bond or dative bond.