Definition

1. Noun:
   • (Chemistry) The mass of an atom of a chemical element expressed in atomic mass units (amu or u). It is a weighted average of the masses of all the naturally occurring isotopes of that element, taking into account their relative abundance. It is approximately equal to the total number of protons and neutrons in the atom's nucleus.

Usage

• The term "atomic mass" is used in scientific contexts, particularly in chemistry and physics, to discuss the mass of atoms for calculations involving chemical reactions, stoichiometry, and the periodic table.
• It is typically expressed as a decimal number on the periodic table, contrasting with the whole number atomic number (number of protons) and mass number (sum of protons and neutrons in a specific isotope).

Examples

• Noun:
  • The atomic mass of carbon is approximately 12.01 amu.
  • To calculate the molecular mass of water, you add the atomic masses of two hydrogen atoms and one oxygen atom.
  • Elements are arranged in order of increasing atomic number, but their atomic mass generally increases across the periodic table.

Advanced Usage

• "Relative atomic mass": This is a synonym for atomic mass, emphasizing that it is a dimensionless quantity relative to one-twelfth the mass of a carbon-12 atom.
  • The relative atomic mass of chlorine is 35.45.
• "Standard atomic weight": This term is often used interchangeably with atomic mass, particularly by IUPAC (International Union of Pure and Applied Chemistry), to denote the expected range or value of atomic masses from normal terrestrial sources.

Variants and Related Words

• Atomic weight: A largely synonymous, older term that is still commonly used.
• Isotopic mass: The mass of a specific isotope of an element (e.g., the isotopic mass of carbon-12 is exactly 12 amu by definition).
• Mass number: The total number of protons and neutrons in the nucleus of a specific isotope, which is a whole number (e.g., the mass number of carbon-12 is 12).

Synonyms

• Relative atomic mass
• Standard atomic weight

Related Phrases

• Atomic mass unit (amu or u): The standard unit of measurement for atomic mass, defined as 1/12 the mass of a carbon-12 atom.
  • One proton has a mass of about 1.0073 atomic mass units.

Noun

1. (chemistry) the mass of an atom of a chemical element expressed in atomic mass units