A, B and C are three elements with atomic numbers 17,18 and 20 respectively.
(1) Write theirelectronoic configuration.
(2) which one of them is metal and which one a non-metal?

Explanation

(1) Electronic configuration of `""_(17)A:1s^(2)2s^(2)2p^(6)3s^(2)3p^(5)`
Electronic configuration of `""_(18)B:1s^(2)2s^(2)2p^(6)3s^(2)3p^(6)`
Electronic configuration of `""_(20)C:1s^(2)2s^(2)2p^(6)3s^(2)3p^(6)4s^(2)`.
(2) Element C is a metal as it can easily form a dipositive ion by the loss of two electrons from 4s-orbital. element A is a non metal as it can achieve inert gas configuration by accepting one electron in 3p-subshell.