In which atoms of the 2nd period there is one unpaired electron, two electrons.

In an atom, electrons are located at levels and sublevels. There are sublevels s, p, d, f. Elements of 2 periods have two levels, 1 level is completely filled, and 2 consists of an s-sublevel and a p-sublevel. The electrons first fill the s-sublevel, and only then the p-sublevel. There are 3 cells on the p-sublevel, electrons first fill each of the cells, and then begin to pair in them:
– Li 1s2 2s1
– Be 1s2 2s2
– B 1s2 2s2 2p1
– C 1s2 2s2 2p2
– N 1s2 2s2 2p3
– O 1s2 2s2 2p4
– F 1s2 2s2 2p5
– Ne 1s2 2s 2p6.
Therefore, for 1 unpaired electron in: Li, B, F. And 2 unpaired electrons in C.