Atomic Interactions and Generation of Radiation

Electrons are arranged at different energies

An atom has a tiny central nucleus surrounded by electrons. The electrons are not found at just any distance from the nucleus: they occupy particular arrangements associated with different energies. An electron farther from the nucleus generally has more energy than one closer to it.

Electromagnetic radiation can transfer energy between a source and an atom. What happens depends on the energy transferred and on which part of the atom or molecule interacts with the radiation.

Absorption changes an electron arrangement

When an atom absorbs electromagnetic radiation, one of its electrons may gain energy and move to a higher-energy arrangement, usually farther from the nucleus. The electron is then described as excited. The electron is still part of the atom, so excitation is not the same as ionisation.

An excited arrangement may not last. If the electron returns to a lower-energy arrangement, it loses energy. That energy can leave the atom as emitted electromagnetic radiation. The type of radiation produced depends on the size of the energy change.

This gives a useful two-way connection:

• during absorption, radiation transfers energy to the atom and an electron can move to a higher energy;
• during emission, an electron loses energy and the atom releases electromagnetic radiation.

When absorption causes ionisation

If the absorbed radiation transfers enough energy, an outer electron can be removed completely rather than merely moved to another arrangement. This is ionisation.

A neutral atom has equal numbers of positively charged protons and negatively charged electrons. If it loses an electron, it has more protons than electrons and becomes a positive ion. The nucleus has not changed, so the atom is still the same element.

High-energy ultraviolet radiation, X-rays and gamma rays have enough energy to ionise some atoms when absorbed. Ionisation is therefore different from the heating that often follows the absorption of lower-energy radiation, and it is also different from excitation, where the electron remains attached to the atom.

Different structures generate different radiations

Radiation can be generated or absorbed through changes in molecules, atoms and nuclei, covering a wide range of frequencies.

Visible light, ultraviolet and X-rays can be generated when electrons in atoms lose energy. An electron moves to a lower-energy arrangement and the lost energy is emitted as electromagnetic radiation. Larger energy changes are associated with higher-frequency radiation.

Gamma rays are emitted from atomic nuclei rather than from electron arrangements around the nucleus. This nuclear origin distinguishes gamma emission from the specified generation mechanism for X-rays, ultraviolet and visible light.

Infrared radiation is emitted and absorbed by molecules. This is why molecular substances can exchange energy with their surroundings through infrared radiation.

Ultraviolet radiation and atmospheric ozone

Ultraviolet radiation is also involved in reactions in the upper atmosphere. Oxygen absorbs ultraviolet radiation, leading to the production of ozone. Ozone itself absorbs ultraviolet radiation, so the ozone layer reduces the amount of harmful ultraviolet reaching living organisms on Earth.

This is an example of radiation absorption causing a chemical change as well as transferring energy. It also shows that absorption is not automatically harmful: the outcome depends on what absorbs the radiation and where the interaction occurs.

Electron changes

• Electrons in an atom are arranged at different distances and energies from the nucleus.
• Absorption: an electron gains energy and may move to a higher-energy arrangement.
• Emission: an electron loses energy and electromagnetic radiation is released.
• Excitation leaves the electron in the atom; ionisation removes an electron.

Ionisation

An atom that loses an outer electron becomes a positive ion. High-energy ultraviolet, X-rays and gamma rays can ionise some atoms when absorbed.

Recall the origins

• Gamma rays: emitted from atomic nuclei.
• X-rays, ultraviolet and visible light: generated when electrons in atoms lose energy.
• Infrared: emitted and absorbed by molecules.

Ozone link

Ultraviolet is absorbed by oxygen, producing ozone. Ozone also absorbs ultraviolet and therefore helps protect life on Earth.