Electron Affinity
Definition and meaning of Electron Affinity in chemistry.
Electron affinity is the energy change that happens when a neutral gaseous atom gains an electron. This addition creates a negative ion and usually releases a specific amount of energy. Chemists report this released energy as a negative value because the atom loses energy to its surroundings.
In more detail
A neutral atom must often release energy to become a stable negative ion. This released energy shows how strongly the atom wants to grab an extra electron. Electron affinity generally becomes more negative as you move from left to right across a periodic table row.
The increasing positive charge in the nucleus pulls the added electron in much more strongly. The trend changes as you move down a column on the periodic table. Electron affinity becomes less negative because the atom gets physically larger.
The added electron sits further from the nucleus and feels more pushback from existing inner electrons. Chemists call this pushback electron shielding or electron repulsion. There are a few important exceptions to these general periodic trends.
Atoms in groups 2, 15, and 18 have completely filled or exactly half-filled electron subshells. These stable arrangements make adding another electron much less favorable. In these specific cases, the atom might actually require an input of energy to accept an electron.
We call this an endothermic electron affinity. Understanding electron affinity helps chemists predict overall elemental reactivity. It perfectly explains why nonmetal elements like halogens so easily form negative ions in chemical bonds.
Key facts
| Field | Physical Chemistry |
|---|---|
| Typical units | kJ/mol or eV |
| Example value (Chlorine) | -349 kJ/mol |
| Periodic trend (rows) | More negative moving left to right |
| Periodic trend (columns) | Less negative moving top to bottom |
| Related concept | Electronegativity |
Chlorine has a highly negative electron affinity of about -349 kJ/mol. The chemical equation is Cl(g) + e- → Cl-(g). This reaction releases exactly 349 kilojoules of energy for every mole of chlorine atoms. This large energy release reflects the strong pull of the chlorine nucleus. The atom easily grabs an incoming electron to complete its outer valence shell.
Frequently asked questions
Is electron affinity always a negative number?
No. Most elements release energy when gaining an electron, which gives a negative value. Noble gases require energy to accept an electron, giving them a positive electron affinity.
How is electron affinity different from electronegativity?
Electron affinity is a measurable energy change for a single atom gaining one electron. Electronegativity is a relative scale describing how an atom attracts shared electrons within a chemical bond.
Why does nitrogen have a positive electron affinity?
Nitrogen has an exactly half-filled p subshell, which is highly stable. Adding another electron disrupts this stability and requires an input of energy.