Here's why:
* Protons: These are positively charged particles found in the nucleus. The number of protons defines the element. Helium has 2 protons.
* Neutrons: These are neutral particles also found in the nucleus. They contribute to the atom's mass but not its charge.
* Electrons: These are negatively charged particles that orbit the nucleus. They are much lighter than protons and neutrons and are not included in the mass number.
For example:
* Helium-4 has a mass number of 4. This means it has 2 protons and 2 neutrons (4 - 2 = 2).
* Helium-3 has a mass number of 3. This means it has 2 protons and 1 neutron (3 - 2 = 1).
Therefore, the mass number is a simple way to represent the total number of particles in an atom's nucleus.
