How many unpaired electrons are present in the ground state of an atom from each of the following groups? Enter the numbers in order, from (a) to (d) in the answer boxes. All 4 numbers must be correct.
A) 5A(15)
B) 4A(14)
C) 7A(17)
D) 8A(18)
1 Answer

OK, working through the main groups left to right the outer shell structures will be
Group I A s1 = one unpaired electron
Group 2A s2 = 2 paired electrons
Group 3A s2p1 = 2 paired + 1 unpaired
Group 4A s2p2 = 2 paired + 2 unpaired
Group 5A s2p3 = 2 paired + 3 unpaired
Group 6A s2p4 = 4 paired + 2 unpaired
Group 7A s2p5 = 6 paired + 1 unpaired
Group 8A s2p6 = 8 paired
So
5A has 3 unpaired electrons
4A has 2 unpaired electrons
7A has 1 unpaired electron
8A has no unpaired electrons
Hope this is clear. Electrons fill the three p orbitals singly before any pairing takes place which explains the outer shell structure for groups 4A, 5A and 6A