Question

An atom contains 70 protons, 70 electrons, and 99 neutrons. What is the mass number?

Answer 1

`169` atomic mass units.

Explanation:

Mass number is number of protons + neutrons in the nucleus.

`therefore A=70+99=169am u`

Since it has atomic number `Z=70` it implies that this atom is an isotope of `Yb`, and is neutral charged.