Please answer me this question with both parts

18.  Explain the terms  a] Lone pair of electrons   b] Coordinate bond. Explain diagrammatically the lone pair effect of:

a] The nitrogen atom of the ammonia molecule leading to the formation of  ammonium ions ${\left[N{H}_4\right]}^{+}$
b] The oxygen atom of the H₂O molecule leading to formation of hydronium ${\left[H_{3}O\right]}^{+}$ & hydroxyl ions ${\left[OH\right]}^{-}$

1) Lone pairs are defined as the electron pairs present in the valence shell of an atom that remain paired and do not participate in bond formation. Number of lone pairs of an electron can be calculated, using the octet rule and valence shell electronic configuration of the element. Let us take an example of water (H₂O), and try to find the lone pair of electrons on O in water.

The valence shell electronic configuration of O is: 2s²2p⁴. (6 electrons). Thus in an oxygen atom, the s-orbital has an electron pair and one p-orbital has an electron pair. The other two p-orbitals are occupied by one electron each. So, it requires only 2 electrons to complete its octet. For that it shares 2 electrons with 2 H-atom. So out, of 6 electrons, only 2 electrons are used in forming the covalent bond. The two electron pairs present in s-orbital and a p-orbital do not participate in bond formation. Thus, O in water molecule has 2 lone pair of electrons.

2) 

A coordinate bond is a type of covalent bond formed between two species in which shared pair of electrons is denoted by one species only but shared by both species. It is also known as dative bond. 

For example:

NH ₃ shares the pair of electrons present on Nitrogen with BF ₃ , which is electron deficient.

Hence when in a bond between two species both the electrons are donated by one atom (unlike covalent bond in which both the atoms donate one electron each), the bond will be coordinate bond. The coordinate bond is denoted by an arrow from the donor atom of the electron donating species to the acceptor atom of the electron deficient species.

3) 

1. A single lone pair can be found with atoms in the nitrogen group such as nitrogen in ammonia.

Ammonium ions, NH₄ ⁺, are formed by the transfer of a hydrogen ion to the lone pair of electrons on the ammonia molecule.

The arrow points from the atom donating the lone pair to the atom accepting it.

2. 

In water, oxygen atom has two lone pairs of electrons. So it can donate lone pair of electron to hydrogen atom and form hydronium ion.

So, lone pair of electron plays an important role in formation of compounds.