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

49 neutrons in each nucleus.

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.

Again,

Mass number = Number of protons + Number of neutrons.

80 = 31 + Number of neutrons.

Number of neutrons = 80 - 31 = 49.

a compound is a molecule made of atoms from different elements.

a. by the combination of an unlimited number of elements