Electron Affinity (EA)
Electron Affinity (EA)
Electron affinity (EA) is a fundamental chemical property that measures the energy change when an electron is added to a neutral atom in the gaseous state to form a negative ion. It is an indication of the tendency of an atom to accept an electron.
Definition
Electron affinity is defined as the amount of energy released or absorbed when an electron is added to a neutral atom to form a negatively charged ion. It is usually expressed in units of kilojoules per mole (kJ/mol) or electronvolts (eV).
Formula
The electron affinity can be represented by the following equation:
$$ EA = E_{initial} - E_{final} $$
Where:
- $EA$ is the electron affinity,
- $E_{initial}$ is the energy of the neutral atom,
- $E_{final}$ is the energy of the negatively charged ion.
A positive EA value indicates that energy is released when the electron is added, which means the process is exothermic. Conversely, a negative EA value means that energy is absorbed, and the process is endothermic.
Trends in the Periodic Table
Electron affinity varies across the periodic table. Generally, it increases across a period from left to right and decreases down a group. However, there are exceptions to these trends.
Across a Period
| Element | EA (kJ/mol) |
|---|---|
| Li | -60 |
| Be | 0 |
| B | -27 |
| C | -122 |
| N | 0 |
| O | -141 |
| F | -328 |
As you move from left to right across a period, the nuclear charge increases, which leads to a greater attraction for the added electron, and thus, a higher electron affinity.
Down a Group
| Element | EA (kJ/mol) |
|---|---|
| Li | -60 |
| Na | -53 |
| K | -48 |
| Rb | -47 |
| Cs | -46 |
Moving down a group, the atomic radius increases, and the added electron is further from the nucleus, resulting in a lower electron affinity.
Factors Affecting Electron Affinity
Several factors can affect the magnitude of electron affinity:
- Atomic Size: Larger atoms have lower electron affinity because the added electron is further from the nucleus and experiences less electrostatic attraction.
- Nuclear Charge: A higher effective nuclear charge increases electron affinity as the nucleus can attract the added electron more strongly.
- Electron Configuration: Atoms with a half-filled or completely filled valence shell have lower electron affinity because adding an electron would result in a less stable electron configuration.
Examples
- Chlorine (Cl) has a high electron affinity of -349 kJ/mol because it has seven valence electrons and by gaining one more electron, it achieves a stable noble gas configuration.
- Noble Gases: Noble gases like Neon (Ne) and Argon (Ar) have very low or positive electron affinities because their valence shells are already full, and they do not tend to gain electrons.
Differences and Important Points
| Property | Electron Affinity (EA) |
|---|---|
| Definition | Energy change when an electron is added to a neutral atom. |
| Trend Across Period | Generally increases from left to right. |
| Trend Down Group | Generally decreases down a group. |
| Units | Usually expressed in kJ/mol or eV. |
| Significance | Indicates how readily an atom can gain an electron. |
| Factors Affecting EA | Atomic size, nuclear charge, electron configuration. |
| Example of High EA | Chlorine (Cl) due to its near completion of the valence shell. |
| Example of Low/Zero EA | Noble gases (e.g., Ne, Ar) because they already have a complete valence electron shell. |
In summary, electron affinity is a crucial property that helps predict the reactivity of elements, especially in the formation of ionic compounds. It is an important concept in both inorganic and organic chemistry and is often considered alongside ionization energy and electronegativity when discussing the periodic trends of elements.