Recall that in each atom its electrons are arranged at different distances…

The idea

Electrons in an atom occupy energy levels at different distances from the nucleus, with inner levels closer and lower in energy. An inner electron can absorb energy from electromagnetic radiation and become excited, rising to a higher energy level. The electron is unstable there, so it drops back and emits the energy difference as electromagnetic radiation, often seen as a characteristic line in a spectrum. Separately, an atom becomes a positive ion when it loses one or more outer electrons; this removal is called ionisation. Keep the two ideas distinct: excitation moves an electron between levels inside the atom, whereas ionisation removes an outer electron entirely.

For example: A gas discharge tube shows coloured light because excited electrons emit radiation as they fall back, while a spark or radiation can ionise gas atoms by stripping outer electrons.

What earns the marks

• States that electrons are arranged at different distances from the nucleus in distinct energy levels.
• Explains that an inner electron absorbs energy from electromagnetic radiation and is excited to a higher energy level.
• Explains that the excited electron emits radiation when it loses energy and returns to a lower level.
• States that an atom becomes an ion when it loses outer electrons.
• Names the loss of outer electrons as ionisation.
• Distinguishes excitation, which moves an electron between levels, from ionisation, which removes an outer electron.

Common mistakes and how to put them right

• Describes excitation as the removal of an electron.

  Put it right by stating that excitation raises an electron to a higher energy level within the atom.

• States that ionisation happens when an inner electron is excited.

  Put it right by linking ionisation to the loss of outer electrons.

• Claims that an electron emits radiation as it absorbs energy.

  Put it right by stating that emission occurs when the electron loses energy and falls to a lower level.

• Says all electrons sit at the same distance from the nucleus.

  Put it right by describing distinct energy levels at different distances.

Exam tips

• Link each process to its cause: absorption raises an electron, emission follows a fall to a lower level, and loss of an outer electron is ionisation.
• Use the words excited, energy level and ionisation precisely, since options often swap them.
• Check whether an option describes a change of level or a complete removal of an electron before choosing.