# Atoms and Isotopes

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

• At the nucleus, there are positively charged protons and neutral neutrons, each with a relative mass of 1.
• Surrounding the nucleus there are negatively charged electrons arranged in shells.

### Isotopes

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:

• Chemical reactions involve the outermost electrons.
• Neutrons have no effect on chemical reactivity.

### Ions

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.

$$N_{A} = 6.02 \times 10^{23}\,mol^{-1}$$
### 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:
$$n = \frac mM$$