Atoms and Isotopes

Every atom is composed of sub-atomic particles called protons, neutrons and electrons:


Isotopes are atoms of the same element with the same number of protons but different number of neutrons. Isotopes all react in similar ways as:


Ions are atoms that have gained or lost electrons. They have either a positive or negative charge.

Relative Isotopic Mass

The relative isotopic mass is the mass of an isotope relative to 1/12th the mass of an atom of carbon-12.

Relative Atomic Mass $(A_{r})$

The relative atomic mass of an element is the weighted mean mass of an atom of an element, relative to one-twelfth (1/12) the mass of an atom of carbon-12. The weighted mean mass is used to allow for the contribution of each isotope in the overall mass.

Relative Formula Mass $(M_{r})$

The relative formula mass of a substance is the weighted mean mass of a formula unit relative to 1/12th the mass of an atom of carbon-12.

It can be calculated by adding up the relative atomic masses of all the elements in a compound:
$$ CaBr_{2} \\
\eqalign{&Ca = 40 \\ &Br_{2} = (79.9\times2) \\ &Relative\ formula\ mass = 40 + (79.9\times2) = 199.9} $$

Amount of Substance

Chemists use the mole to measure the amount of substance. The mole is based on a standard count of atoms called the Avogadro constant. A mole is the amount of any substance containing as many particles as there are carbon atoms in exactly $12g$ of carbon-12.

Avogadro's Constant

This is the number of atoms per mole of the carbon-12 isotope.
$$ N_{A} = 6.02 \times 10^{23}\,mol^{-1}$$

Molar Mass $(M)$

Molar mass, $M$ is the mass per mole of a substance. The units for molar mass are $gmol^{-1}$. The amount of substance $n$, mass $m$, and molar mass $M$ are linked by the equation:
$$ n = \frac mM $$

© Andrew Deniszczyc, 2023