(a) 1(-1). It has electronic configuration = 2,8, 7.
Therefore , it requires one more electron to become stable.
(b) Non-metal. Because it will show the tendency to gain electrons.
© It belongs to third period and it has a smaller size as compared to other members of this period because size of atom decreases in a period due to increase in the nuclear charge.
Note: It belongs to group 17, which is of non-metals and in this group it is bigger than fluorine but smaller than other member.