Question

How many unpaired electrons are present in each of the following ground-state atoms? O Si K As

Answer 1

write the valance configurations, just like in (6) and remember that first you place an electron in each orbital and after that's done you start pairing them up.
a) O
2p(4)
Number of unpaired electrons is 2 because 3 are placed into the orbitals px, py, and pz and the fourth one is paired up with any of them. As a result you have to unpaired electrons.

b) Si
3p(2)
2 unpaired electrons because there is still an empty p orbital

c) K
4s(1)
1 unpaired electron

d) As
4p(3)
3 unpaired electrons. All 3p orbitals are singly occupied.