The mass number of an atom is the total number of protons and neutrons present in its nucleus.
- Protons and neutrons are together called nucleons, and they make up nearly all of an atom’s mass.
- The mass number is represented by the symbol A.
- It is always a whole number because protons and neutrons are counted as whole particles.
- Unlike atomic number (which only counts protons), mass number gives an idea of the size and mass of the nucleus.
- Example:
- Carbon-12 has 6 protons and 6 neutrons → Mass number = 6 + 6 = 12.
- Oxygen-16 has 8 protons and 8 neutrons → Mass number = 16.
So, in short:
Mass number (A) = Number of protons + Number of neutrons.