Question

why a cation contains less electrons than proton ?

Answer 1

Cation

Consider a sodium atom. Now a sodium atom will lose an electron to gain nobel gas electronic configuration. Normal Sodium configuration is 2,8,1 and it loses its valence electron forming a unipositive cation. The electron is lost from the outer most shell, so, basically an electron lost is a shell lost. It reduces the size.

In case of some higher metal, say, aluminium; it loses one electron from its 2,8,3 configuration and forms a unipositive cation. The entire shell is not lost however the nuclear attraction for the remaining 2 electrons in the valence shell pulls the shell closer. This results in the smaller cation than a normal atom.

Anion

Consider a chlorine atom. It has a deficiency of one electron in its outermost shell to attain nobel gas configuration. It gains an electron forming a uninegative anion. The added electron increases the repulsion between the inner shell and the valence shell. The nuclear attraction remains same. So, the shell size increases. Consequently, the size of anion increase

Answer 2

Let us know what's a cation.
Cation is a positively charged ion, that gets attracted to cathode. It is formed by loss of electrons from the outermost or valence shell or orbit or energy level.

In a neutral atom, the number of protons is equal to the number of electrons. Since cation is formed by loss electrons, after formation the number of protons exceeds the number of electrons.