Question

The diagram below shows a helium atom and its

corresponding symbol.
Symbol: 4
HE
2
What does the top number in the symbol tell you
about the atom?​

Answer 1

This is the mass number A. It shows the number of nucleons (neutrons and protons in the nucleus). It is made up of the atomic number Z and the neutron number N:

A = Z + N

Using the example of helium:

Helium has a mass number A of 4 and the atomic number Z 2, so the atom has 2 neutrons in its nucleus.

A = Z + N

4 = 2 + 2