Answer:
The fluorine atom has 7 electrons in the outermost shell and its valency could be 7. But it is easier for fluorine to gain one electron instead of losing seven electrons. Hence, its valency is determined by subtracting seven electrons from the octet (8) and this gives you a valency of one (1) for fluorine.
and for chlorine valence electron is 7 and it needs to gain 1 electron from the outermost orbit to attain octet (8). Hence, the valency of chlorine is 1.
and they both are negative because of gaining of electron