Question

an element has an atomic number 12.How many electrons will be present K,L,and M energy shells of its atom?

Answer 1

2,8,2 and 2 will valency and element is magnesium

Answer 2

atomic no.12 represents magnesium 
 the configuration of magnesium is 2 , 8 , 2
hence 
K shell have 2 electron
L shell have 8 electron
M shell have 2 electron