Define the mass number of an atom.

The mass number of an atom is the total number of protons and neutrons in the nucleus of an atom. It is an integer value and it is always greater than the atomic number (number of protons) of an atom. The mass number gives the approximate atomic mass of a particular atom. It is denoted by the symbol 'A'.

Here are the steps to define the mass number of an atom:

1. Identify the atom: The first step is to identify the atom you are dealing with.

2. Find the number of protons: The number of protons in an atom is equal to the atomic number of the atom. This information can be found on the periodic table.

3. Find the number of neutrons: The number of neutrons in an atom can be found by subtracting the atomic number from the mass number.

4. Add the number of protons and neutrons: The mass number of an atom is the sum of the number of protons and neutrons in the atom.

For example, for a carbon atom (C), the atomic number is 6 (which means it has 6 protons), and if its mass number is 12, it means it has 6 neutrons (12-6). So, the mass number of this carbon atom is 12 (6 protons + 6 neutrons).