Question

If an atom has an atomic number of 15 and an atomic mass of 31, how many electrons are contained in the outer shell?

Answer 1

6 valence electrons

Explanation:

The atomic number of 15 means the element is Phosphorous (P)

Now,
P is in group 16 on the periodic table (the group # is usually right on top of each column).

A little trick for finding the number of valence electrons is to simply look at its group number.

So, Phosphorous has 6 electrons in its outer shell (you just ignore the 1 in front for this).

Some other examples:

Carbon (C) is in group 14, it has 4 valence electrons
Magnesium (Mg) is in group 2, it has 2 valence electrons.