- Protons and electrons
- Protons and neutrons
- Neutrons and electrons
- Isotopes and ions
The mass number (A), also called the nucleon number, represents the total count of protons and neutrons in an atom’s nucleus. While protons contribute positive charge and define the element, neutrons add mass without affecting charge, allowing for isotopes—atoms of the same element with different neutron numbers. For example, carbon-12 has 6 protons and 6 neutrons (mass number 12), while carbon-14 has 6 protons and 8 neutrons (mass number 14). Electrons are not included in mass number because they have negligible mass (approximately 1/1836 of a proton’s mass) and do not reside in the nucleus. The mass number differs from atomic mass, which is a weighted average of all naturally occurring isotopes expressed in atomic mass units. In nuclear notation, mass number appears as a superscript before the element symbol (e.g., ¹⁴C). Understanding mass numbers is crucial for nuclear chemistry, radioactive decay calculations, and applications like radiocarbon dating, where changes in mass number indicate nuclear transformations.
