20. an atom has a mass of 50 and 20 protons. how many
neutrons does it have?

30  

Explanation:

The mass number (A) is the sum of the numbers of protons and neutrons .

A = p + n

If p = 20,  

n = A - p = 50 - 20 = 30

periodic trends is an atom that gets larger as the number of electronic shells increase, the radius of atoms increases as you go down a certain group in the periodic table of elements. in general, the size of an atom will decrease as you move from left to right of a certain period.