How many neutrons does element X have if its atomic number is 31 and its mass number is 80?

49 neutrons in each nucleus.

Explanation

For each nucleus:

Mass number = Number of protons + Number of neutrons.

The atomic number of a nucleus is the same as its number of protons. The atomic number of the nucleus here is 31. There are 31 protons in each nucleus.

Mass number = 80; Number of protons = Atomic number = 31; The number of neutrons is to be found.

Again,

Mass number = Number of protons + Number of neutrons.

80 = 31 + Number of neutrons.

Number of neutrons = 80 - 31 = 49.