An atom is electrically neutral, on loss or gain of electrons why does it become charged?

An atom is electrically neutral because of the same number of protons and electrons. But it becomes charged, to become a stable atom, loses or gains electrons. Hence,

Number of protons ≠ Number of electrons

If it loses electrons p > e; hence +ve charge is obtained.

If it gains electrons e > p; hence -ve charge is obtained.