The mass number of an element on the periodic table represents the total number of protons and neutrons in the nucleus of an atom of that element.

Here's a breakdown:

* Protons: Positively charged particles found in the nucleus. The number of protons determines the element's atomic number.

* Neutrons: Neutrally charged particles also found in the nucleus.

* Electrons: Negatively charged particles that orbit the nucleus.

Key Points:

* Mass number is an approximation: It's not an exact measurement of the atomic mass because it doesn't account for the mass of electrons or the binding energy within the nucleus.

* Isotopes: Elements can have different numbers of neutrons, creating isotopes. Isotopes of the same element have the same number of protons but different mass numbers.

Example:

Carbon-12 has a mass number of 12. This means it has 6 protons and 6 neutrons (12 - 6 = 6).