In first group of periodic table alkali metals are present with one electron in their outer most shell. Halogens are present in the seventeenth group of a periodic table with seven electrons in their outer most shell. According to Octet rule, atoms with less than eight electrons in their outer most shell are unstable. Atoms combine with other atoms to complete their outer most shell and to become stable. Alkali metals need seven electrons to complete their orbits. It is difficult to gain seven electrons so, they lose their one electron and complete their outer most shell. Halogens have seven electrons, it is easy for them to gain an electron from other atoms. In this way, halogens complete their outer most shell by gaining an electron.
