Atomic Number And Mass Number - Atomic Structure - AQA - BBC

In this guide

1. Revise
2. Audio
3. Test

Pages

1. Early ideas about atoms
2. Developing models of atoms
3. Structure of the atom
4. Atomic number and mass number
5. Isotopes
6. Calculating relative atomic mass

Atomic number and mass number

Atomic number

The number of protons in an atom of an element is its atomic number. Remember that:

• all atoms of a given element have the same number of protons
• atoms of different elements have different numbers of protons

An atom contains equal numbers of protons and electrons. Since protons and electrons have equal and opposite charges, this means that atoms have no overall electrical charge.

For example, the atomic number of sodium is 11. Every sodium atom has 11 protons and 11 electrons. It has 11 positive charges and 11 negative charges.

Mass number

Key factThe mass number of an atom is its total number of protons and neutrons.

Atoms of different elements usually have different mass numbers, but they can be the same. For example, the mass number of argon atoms and calcium atoms can both be 40.

Calculating numbers of subatomic particles

The symbol for an atom can be written to show its mass number at the top, and its atomic number at the bottom.

To calculate the numbers of subatomic particles in an atom, use its atomic number and mass number:

• number of protons = atomic number
• number of electrons = atomic number
• number of neutrons = mass number - atomic number

Question

The atomic number of a sodium atom is 11 and its mass number is 23. Calculate the number of protons, neutrons and electrons it contains.

Show answer

Number of protons = 11

Number of electrons = 11

Number of neutrons (mass number - atomic number) = 23 - 11 = 12

Next pageIsotopesPrevious pageStructure of the atom

More guides on this topic

• Atoms, elements and compounds - AQA
• Mixtures - AQA
• The periodic table - AQA
• Groups in the periodic table - AQA
• Transition metals - AQA
• Sample exam questions - atomic structure and the periodic table - AQA

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