Atomic Mass, The Mole & Avogadro – Lecture Vocabulary

Key vocabulary covering atomic mass units, isotopes, molar mass, the mole concept, Avogadro’s number, and example compound masses used in lecture calculations.

Atomic Mass Unit (amu or u)

A tiny mass standard equal to 1⁄12 the mass of one ¹²C atom; 1 amu ≈ 1.66 × 10⁻²⁷ kg.

Atomic Number (Z)

The number of protons in the nucleus of an atom; identifies the element.

Mass Number (A)

The total count of protons plus neutrons in an individual nucleus.

Proton

A positively charged sub-atomic particle found in the nucleus; mass ≈ 1 amu.

Neutron

An electrically neutral nuclear particle with a mass ≈ 1 amu.

Electron

A negatively charged particle located outside the nucleus; mass ≈ 1⁄1836 of a proton.

Isotope

Atoms of the same element (same Z) that differ in mass number because they contain different numbers of neutrons.

Average Atomic Mass

The weighted average of the masses of all naturally occurring isotopes of an element, expressed in amu and shown on the periodic table.

Relative Atomic Mass

Another term for average atomic mass; compares an element’s mass to the ¹²C standard.

Molar Mass

The mass of one mole of a substance; numerically equal to the substance’s formula mass expressed in g mol⁻¹.

Mole

The SI counting unit that contains exactly 6.022 140 76 × 10²³ specified entities (atoms, molecules, ions, etc.).

Avogadro’s Number

6.022 × 10²³; the number of particles present in exactly one mole of any substance.

Formula Unit

The simplest whole-number ratio of ions represented in an ionic compound (e.g., SrF₂).

Sample-to-Mole Conversion

Using molar mass as a conversion factor to change a sample’s mass into an amount in moles (g → mol).

Mole-to-Particles Conversion

Multiplying the amount in moles by Avogadro’s number to find the number of individual particles.

Glucose (C₆H₁₂O₆) Molar Mass

Calculated as 180.16 g mol⁻¹ (≈180.75 g mol⁻¹ in the notes) by summing the masses of 6 C, 12 H, and 6 O atoms.

SrF₂ Molar Mass

125.62 g mol⁻¹, obtained from one Sr (87.62 g mol⁻¹) plus two F (2 × 18.998 g mol⁻¹).

Avogadro-Sized Analogy

The mole is to particles what a ‘dozen’ is to eggs—simply a large fixed count of items.