What determines the mass number of an element?

Question

What determines the mass number of an element?

Answer 1

Hi,

Good Question!  Here you need to think about the structure of an atom and recall that electrons circle around the central part of the atom called the nucleus.  Now, in regards to the nucleus, remember that it's comprised of protons and neutrons which carry most of the mass.  Since we don't consider the electrons to have any physical mass, we therefore don't account for them in the total mass of the atom.

Hence, the mass number is accounting for the mass of the protons and the mass of the neutrons in the nucleus.

I hope that helps you out!  Please let me know if you have any other questions!

Answer 2

Number of protons and neutrons

Answer 3

The mass number is the sum of the number of protons and the averagae number of neutrons.

Recall that isotopes are atoms of the same element (same number of protons) with differeing numbers of neutrons.  The number of neutrons does not affect the chemical identity of the atom.