Electron affinity describes the energy change when a neutral atom in the gas phase gains an electron to form a negative ion, called an anion. It matters because it helps explain why some elements readily form negative ions while others do not. High electron affinity is especially important for nonmetals such as halogens, which strongly attract extra electrons.
The idea connects atomic structure, periodic trends, and chemical bonding.
Key Facts
- Electron affinity reaction: X(g) + e- -> X-(g) + energy for many nonmetals.
- Electron affinity is the energy change when 1 mole of gaseous atoms gains 1 mole of electrons.
- A more negative electron affinity value means more energy is released when the electron is added.
- Electron affinity generally becomes more negative from left to right across a period.
- Electron affinity generally becomes less negative down a group because the added electron is farther from the nucleus.
- Ionization energy removes an electron: X(g) -> X+(g) + e-, while electron affinity adds an electron: X(g) + e- -> X-(g).
Vocabulary
- Electron affinity
- Electron affinity is the energy change that occurs when a gaseous neutral atom gains an electron.
- Anion
- An anion is a negatively charged ion formed when an atom or group of atoms gains one or more electrons.
- Ionization energy
- Ionization energy is the energy required to remove an electron from a gaseous atom or ion.
- Effective nuclear charge
- Effective nuclear charge is the net positive pull felt by an electron after accounting for shielding by other electrons.
- Shielding
- Shielding is the reduction of nuclear attraction on outer electrons caused by inner electron shells.
Common Mistakes to Avoid
- Confusing electron affinity with ionization energy. Electron affinity involves adding an electron, while ionization energy involves removing an electron.
- Assuming every atom releases energy when it gains an electron. Some atoms resist gaining electrons because the added electron enters an unfavorable orbital or increases repulsion too much.
- Saying electron affinity always increases smoothly across the periodic table. There are exceptions due to filled and half-filled subshell stability, such as noble gases and some group 2 or group 15 elements.
- Forgetting that electron affinity is defined for gaseous atoms. Values for atoms in solids, liquids, or solutions include other energy effects and are not the same quantity.
Practice Questions
- 1 Write the electron affinity equation for chlorine forming chloride, including physical states and energy on the correct side for an energy-releasing process.
- 2 An atom has an electron affinity of -349 kJ/mol. How much energy is released when 2.00 mol of gaseous atoms each gain one electron?
- 3 Compare fluorine, chlorine, and bromine. Which is expected to have a very favorable electron affinity, and why does atomic size affect the trend down the group?