Atomic mass number
Atomic Mass Number
The Atomic Mass Number (pronunciation: /əˈtɒmɪk mæs ˈnʌmbər/), also known as the mass number, is a term used in Nuclear Physics and Chemistry to describe the total number of protons and neutrons in an atomic nucleus.
Etymology
The term "Atomic Mass Number" is derived from the English words "Atomic", referring to atoms, "Mass", referring to the amount of matter in an object, and "Number", referring to a mathematical value. The term was first used in the early 20th century when scientists began to understand the structure of the atom.
Definition
The Atomic Mass Number is denoted by the symbol 'A'. It is a dimensionless quantity, meaning it has no units. It is always a whole number because protons and neutrons are discrete, indivisible entities. The Atomic Mass Number is different from the Atomic Number, which only counts the number of protons in an atom's nucleus.
Calculation
The Atomic Mass Number is calculated by adding the number of protons and neutrons in an atom's nucleus. For example, the Atomic Mass Number of Hydrogen is 1 (1 proton + 0 neutrons), while the Atomic Mass Number of Helium is 4 (2 protons + 2 neutrons).
Related Terms
- Isotope: Atoms of the same element that have different numbers of neutrons, and therefore different atomic mass numbers.
- Atomic Weight: The average mass of atoms of an element, calculated using the relative abundance of isotopes in a naturally-occurring element.
- Nucleon: A term referring to either a proton or a neutron, the particles that make up an atomic nucleus.
External links
- Medical encyclopedia article on Atomic mass number
- Wikipedia's article - Atomic mass number
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