Mass Number The mass number of an atom is the sum of the number of protons and the number of neutrons in the nucleus of an atom.

Mass number = number of protons + number of neutrons For example, you can calculate the mass number of the copper atom listed in Table 4: 29 protons plus 34 neutrons equals a mass number of 63.

Also, if you know the mass number and the atomic number of an atom, you can calculate the number of neutrons in the nucleus. The number of neutrons is equal to the mass number minus the atomic number. In fact, if you know two of the three numbers—mass number, atomic number, number of neutrons—you can always calculate the third.

The mass number of an atom is 35 and it has 16 protons. How many neutrons does this atom contain?

The atom contains
neutrons