Size and mass of atoms: Almost all of the mass of an atom is in the nucleus.

The idea

An atom is mostly empty space. The nucleus, a tiny central region containing protons and neutrons, carries almost all of the atom's mass because each proton and neutron has a relative mass of 1, while each electron has a relative mass of about 1/1836. The electrons occupy energy levels around the nucleus and contribute very little mass. For example, in a carbon-12 atom the nucleus contains 6 protons and 6 neutrons, giving a relative mass of 12, whereas its 6 electrons add only about 0.003. This explains why the mass number equals the number of protons plus neutrons and why relative atomic mass is dominated by nuclear particles.

What earns the marks

• State that the nucleus contains protons and neutrons, which are the particles that give an atom almost all of its mass.
• Compare relative masses: a proton and a neutron each have a relative mass of 1, whereas an electron has a relative mass of about 1/1836.
• Explain that electrons occupy energy levels around the nucleus and contribute a negligible fraction of the total atomic mass.
• Use the carbon-12 example: 6 protons plus 6 neutrons give a relative mass of 12, while 6 electrons add only about 0.003.
• Link the idea to mass number: mass number equals the number of protons plus the number of neutrons, so it reflects nuclear mass.

Common mistakes and how to put them right

• Thinking that electrons make a large contribution to atomic mass because they move quickly.

  Put it right by comparing relative masses, where an electron is about 1/1836 of a proton.

• Believing that the nucleus occupies most of the atom's volume.

  Put it right by stating that the nucleus is tiny compared with the whole atom, even though it contains almost all the mass.

• Confusing mass number with relative atomic mass.

  Put it right by noting that mass number counts protons and neutrons, while relative atomic mass is a weighted mean that also accounts for isotopes.

Exam tips

• Use the phrase 'almost all of the mass' rather than 'all of the mass', because electrons do contribute a very small amount.
• When explaining, quote relative masses: proton 1, neutron 1, electron about 1/1836.
• Link your answer to the structure of the atom: a small, dense nucleus surrounded by electrons in energy levels.