Question

What happens to electrons when an ionic bond is formed between two atoms

Answer 1

There are three types of bonds
Covalent bond which is formed by mutual sharing of electrons between two atoms.
Coordinate (Dative covalent) bond is also formed due to sharing of electrons between two atoms. But, unlike covalent bonds here both the electrons are contributed by a single atom.
Ionic bonds involve complete transfer of electrons.

Complete step by step answer:
Bond formation is the process which can be understood from atomic numbers and the distribution of electrons in shells. According to rule the subshells are filled in sequence $s,{\text{ }}p,{\text{ }}d,{\text{ }}f$ . If we assume the shell number as $n$ then the number of electrons that can be occupied in that particular shell is given by $2{n^2}$ .
This will give rise to the concept of valency which means, the number of electrons in the outermost(valence) shell of a particular atom (or) the tendency of atoms to lose/accept the electrons in order to form the bond. Valency of donors is positive and valency of acceptor is negative.
Formation of bond due to complete transfer of electrons from one atom to another acceptor atom gives rise to atomic bond.
If we go by the definition given in books which says, positively charged ions form a bond with a negatively charged ions and one atom transfers electrons to another. Ionic bonds are generally formed between alkali and non-alkaline earth metals.
Consider the example of formation of salt chemical formula $NaCl$ .
Sodium ( $Na$ ) atomic number $11{\text{ }}\left( {2,{\text{ }}8,{\text{ }}1} \right)$has one electron in valence shell. It can donate one electron to acquire positive charge
$Na - {e^ - } = N{a^ + }$
Chlorine ( $Cl$ ) atomic number $17{\text{ }}\left( {2,{\text{ }}8,{\text{ }}7} \right)$ has seven electrons in valence shell it accepts one electron to get negatively charged
$Cl + {e^ - } = C{l^ - }$
Sodium cation and Chlorine anion then form an ionic bond as the electron is completely transferred to the valence shell of Chlorine.
$N{a^ + } + C{l^ - } = NaCl$

Note: We cannot say that every atom will accept or donate electrons in order to form ionic bonds. Or in every case the same type of bond will be formed. It depends on many factors and one of the important factors is energy required for removal of electrons.