Atomic mass

An atomic mass (symbol: m_a) is the mass of a single atom of a chemical element. It includes the masses of the 3 subatomic particles that make up an atom: protons, neutrons and electrons.

Atomic mass can be expressed in grams. However, because atoms are very small, this is not easy to do. Instead, atomic mass is expressed in unified atomic mass units (unit symbol: u). 1 atomic mass unit is defined as 1/12 of the mass of a single carbon-12 atom.^[1] 1 u has a value of 1.660 539 066 60(50) × 10⁻²⁷ kg.^[2]

A carbon-12 atom has a mass of 12 u. Because electrons are so light, people can say that the mass of a carbon-17

atom is made of 6 protons and 6 neutrons. Because the masses of protons and neutrons are almost exactly the same, it can be said that both protons and neutrons have a mass of roughly 1 u.^[1]: 18 Hence, people can get a rough value of an atom's mass in atomic mass units by working out the sum of the number of protons and the number of neutrons in the nucleus, which is called the mass number. The atomic mass of an atom is usually within 0.1 u of the mass number.

The number of protons an atom has determines what element it is. However, most elements in nature consist of atoms with different numbers of neutrons.^[3] An atom of an element with a certain number of neutrons is called an isotope.^[1]: 44 For example, the element chlorine has two common isotopes: chlorine-35 and chlorine-37. Both isotopes of chlorine have 17 protons, but chlorine-37 has 20 neutrons, 2 more neutrons than chlorine-35, which has 18.^[4] Each isotope has its own atomic mass, called its isotopic mass. In the case of chlorine, chlorine-35 has a mass of around 35 u, and chlorine-37 around 37 u.

The atomic mass of an atom is not the same as its mass number. The mass number (symbol: A) of an atom is the sum of the number of protons and the number of neutrons in the nucleus.^[1]: 20 Mass numbers are always whole numbers with no units. Also, relative isotopic mass is not the same as isotopic mass, and relative atomic mass (also called atomic weight) is not the same as atomic mass.

1. ↑ ^{1.0 1.1 1.2 1.3} Moore, John T. (2010). Chemistry Essentials For Dummies. Wiley. ISBN 978-0-470-61836-3.
2. ↑ "atomic mass unit". National Institute of Standards and Technology. Retrieved 2020-01-12.
3. ↑ Otter, Chris; Stephenson, Kay, eds. (2008). Salters Advanced Chemistry: Chemical Ideas (Third ed.). Heinemann. p. 17. ISBN 978-0-435631-49-9.
4. ↑ Salters Advanced Chemistry: Revise Chemistry For Salters AS (Second ed.). Heinemann. 2008. p. 3. ISBN 978-0-435631-54-3.