Electrons occupy set orbits around the nucleus
Definition
Electron orbit
One of the allowed set distances from the nucleus at which an electron can exist.
- Electrons occupy orbits at set distances from the nucleus rather than any distance between them.
- An electron in an orbit farther from the nucleus has more energy than one in an orbit closer to the nucleus.
- A change of orbit requires a fixed energy transfer equal to the energy difference between the two orbits.
Absorption moves an electron farther out
Definition
Absorption
The transfer of energy from electromagnetic radiation to a material when the radiation is taken in rather than reflected or transmitted.
- When an atom absorbs electromagnetic radiation, energy is transferred to an electron.
- The electron gains energy and moves to an allowed orbit farther from the nucleus.
- The electron can make the change only when the absorbed radiation supplies the correct energy difference.
Emission moves an electron closer in
Definition
Emission
The transfer of energy away from an atom as electromagnetic radiation when an electron moves to an orbit closer to the nucleus.
- When an electron moves to an allowed orbit closer to the nucleus, it loses energy.
- The atom emits the energy difference as electromagnetic radiation.
- The electron remains in the atom after emission; it has changed orbit rather than being used up.
Example
Following an electron transition
- An electron absorbs 4.0×10−19 J4.0\times10^{-19}\,\text{J}4.0×10−19J and moves to a higher-energy orbit.
- If it returns directly to its original orbit, the atom emits 4.0×10−19 J4.0\times10^{-19}\,\text{J}4.0×10−19J as electromagnetic radiation.
- If it returns in smaller steps, it emits radiation in more than one energy amount whose total is 4.0×10−19 J4.0\times10^{-19}\,\text{J}4.0×10−19J.
Common Mistake
- Do not reverse the orbit changes; absorption moves an electron farther from the nucleus, while emission moves it closer.
- Do not say that the electron is emitted when the atom emits radiation; the electron stays in the atom.
Losing outer electrons forms a positive ion
Definition
Positive ion
An atom that has lost one or more electrons and therefore has more protons than electrons.
- Outer electrons can be removed from an atom, but the number of protons in the nucleus does not change.
- Losing a negatively charged electron leaves an excess of positive charge.
- If a neutral atom loses one electron, its relative charge becomes +1+1+1, and losing two electrons gives +2+2+2.
- Ion formation changes the number of electrons, not the identity of the element, because the proton number remains fixed.
Exam technique
Linking orbit changes to energy
- For absorption, state that the electron gains energy and moves farther from the nucleus.
- For emission, state that the electron moves closer to the nucleus and electromagnetic radiation carries energy away.
- For a positive ion, state that the atom loses outer electrons and is left with more protons than electrons.
Self review
- What is meant by a set electron orbit?
- What happens to an electron during absorption?
- What happens to an electron during emission?
- Why must an electron absorb a particular amount of energy to change orbit?
- Why does losing an outer electron form a positive ion?
