Answer
$a)$ $Magnesium:$ Has 2 electrons on its outermost shell
$a)$ $Cobalt:$ Has 2 electrons in its outermost shell
$a)$ $Selenium$: Has 6 electrons on its outermost shell
Work Step by Step
$a)$ Magnesium would have two electron on its outermost shell and because it is coming from group 2 and has its electron configuration (1s² 2s²2p⁶ 3s²) ending with 2 electrons on 3s
$b)$ Cobalt would have two electrons on its outermost shell because has its electron configuration chain ( $1s^{2}2s^{2}2p^{6}3s^{2}3d^{7}4s^{2}$) ending with 2 electrons on 4s
$c)$ Selenium would have six electron on its outermost shell and because it is coming from group 6 and has its electron configuration ( $1s^{2}2s^{2}2p^{6}3s^{2}3d^{10}4s^{2}4p^{4}$ ) ending with 2 electrons on 4s and 4 electrons on 4p
