a. 1s2 , 2s2 , 2p6 , 3s2 , 3p6 , 3d5 , 4s2
b. XCl2 – ion X2+(electrons in 4s only)
XO2 – ion X+4 (2 electrons in 4s and 2 electrons in 3d)
X2 O7 – ion X7+ (2 electrons in 4s and 5 electrons in 3d)
XO2 , X2 O7 d – subshell electrons are taking part in chemical reaction during the formation of X207
c. X2+ – 1s2 , 2s2 , 2p6 , 3s2 , 3p6 , 3d5
X4+ – 1s2 , 2s2 , 2p6 , 3s2 , 3p6 , 3d3
X – 1s2 , 2s2 , 2p6 , 3s2 , 3p6