Exercise 1 solution

Group rule (Valence electron article): for groups 1-12 the group number equals the
number of valence electrons; for groups 13-18 the units digit of the group number.

Potassium, group 1: 1 valence electron. It is a metal, so it loses that electron
(Ion article: metals lose electrons to form positive ions). Result: K+, a cation,
charge +1. Fewer electrons than protons (by one).

Fluorine, group 17: units digit 7, so 7 valence electrons. It needs one more for a
full outer shell of eight (octet rule), and it is a non-metal, so it gains one
electron. Result: F-, an anion, charge -1. More electrons than protons (by one).
(The Ion article itself writes F- as an example of a singly charged ion.)

Protons and electrons: I use atomic numbers from general knowledge (check):
potassium 19, fluorine 9. Neither number comes from a page I read for this chapter.
K+ : 19 protons, 19 - 1 = 18 electrons, net charge 19 - 18 = +1.
F- : 9 protons, 9 + 1 = 10 electrons, net charge 9 - 10 = -1.
The charges agree with the group-based prediction. The method is the same as for
sodium and chlorine in the chapter.