Atomic Mass Units and Atomic Mass

Vocabulary flashcards covering fundamental terms from the lecture on atomic and unified atomic mass units, isotopes, and average atomic mass.

Atomic Mass Unit (AMU)

Historical unit for expressing atomic-scale masses; set so that a proton or neutron is ≈1 AMU; equals 1.66054 × 10⁻²⁷ kg.

Unified Atomic Mass Unit (u)

Modern standard mass unit for atoms and molecules; symbol u; exactly 1.66054 × 10⁻²⁷ kg.

Proton

Positively charged nuclear particle with a mass of about 1.007 u (≈1 u).

Neutron

Neutral nuclear particle slightly heavier than a proton with a mass of about 1.008 u.

Electron

Negatively charged particle outside the nucleus; its mass (~0.0005 u) is roughly 1⁄2000 that of a proton.

Atomic Number (Z)

Number of protons in an atom’s nucleus; uniquely identifies an element (e.g., Z = 1 for hydrogen).

Isotope

Atoms of the same element that contain identical proton counts but different numbers of neutrons.

Average Atomic Mass

Weighted average of the masses of an element’s naturally occurring isotopes, expressed in unified atomic mass units.

Relative Atomic Mass

Unitless form of average atomic mass shown on many periodic tables; compares an atom’s mass to 1 u without stating the unit.

Atomic Weight (obsolete term)

Older term for average atomic mass; technically incorrect because it refers to mass, not weight.

Weighted Average

Calculation where each isotope’s mass is multiplied by its natural abundance, then summed to obtain the average atomic mass.

Periodic Table of Elements

Chart organizing elements by atomic number and displaying their relative/average atomic masses.

Protium

Most abundant hydrogen isotope (≈99.98 %); has 1 proton, 0 neutrons, 1 electron; mass ≈1 u.

Subatomic Scale

Extremely small size domain below the atomic level involving particles like protons, neutrons, and electrons.