Atomic structure

Atoms are the smallest particles of an element that can take part in chemical reactions, while molecules consist of two or more atoms chemically bonded together.

Nucleus

• Every atom contains a central nucleus made of protons and neutrons, surrounded by electrons arranged in shells (energy levels)

Protons

• Found in the nucleus, with a relative mass of 1 and a charge of +1

Neutrons

• Found in the nucleus, with a relative mass of 1 and no charge (0)

Electrons

• Found orbiting the nucleus in shells, with a negligible mass and a charge of −1

Atoms are electrically neutral because the number of protons equals the number of electrons.

Atomic number

• Gives the number of protons in an atom and defines which element it is

Mass number

• Gives the total number of protons and neutrons in the nucleus

Key Definition Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons, resulting in different mass numbers.

Because isotopes share the same electron arrangement, they have identical chemical properties but different physical masses.

Key Definition The relative atomic mass (Aᵣ) of an element is the weighted average mass of its isotopes, taking into account the percentage abundance of each isotope.

Calculating Aᵣ

1. Multiply each isotope's mass number by its percentage abundance
2. Add the products together
3. Divide the total by 100

The result is often not a whole number, because it reflects the proportional contribution of each naturally occurring isotope.

At a Glance