Which of the following largely determines an atom's atomic weight?

A) the number of neutrons
B) the number of electrons
C) the number of electrons in the outer shell
D) the number of protons and neutrons

Atomic Weight

The average mass of atoms of an element, calculated using the relative abundance of isotopes in a naturally-occurring element.

Neutrons

Subatomic particles found in the nucleus of an atom, with no electric charge and a mass slightly greater than that of protons.

Protons

Protons are positively charged subatomic particles found in the nucleus of an atom.