Back to: Inorganic Chemistry 100 Level
Welcome to class!
It’s wonderful to have you here again. Let me start with a simple picture. Imagine you are at a party, and someone offers you a gift bag. If you love surprises, you grab it quickly with a smile. But if you don’t like surprises, you may reject it or take it reluctantly. In the world of atoms, when a neutral atom is offered an extra electron, the way it accepts (or resists) that “gift” is described by electron affinity.
Electron Affinity
What is Electron Affinity?
Electron affinity is the amount of energy released (or sometimes absorbed) when a neutral atom in the gaseous state gains an extra electron to form a negative ion.
For example:
Cl(g) + e⁻ → Cl⁻(g) ; ΔH = –349 kJ/mol
Here, chlorine releases 349 kJ/mol when it gains an electron, showing it has a high electron affinity.
Understanding Electron Affinity
If energy is released when the atom accepts the electron, the process is exothermic (most common).
If energy is absorbed, the process is endothermic (rare, occurs in some atoms like noble gases).
High electron affinity means the atom strongly “desires” extra electrons (like halogens).
Low electron affinity means the atom has little or no interest in gaining electrons (like noble gases).
Factors Affecting Electron Affinity
Nuclear charge: More protons increase the attraction for incoming electrons.
Atomic size: Smaller atoms have stronger nuclear pull, so they attract electrons more easily.
Electron configuration: Atoms close to achieving a stable octet (like halogens) have high electron affinity. Atoms already stable (like noble gases) have low or no affinity.
Shielding effect: More inner electrons reduce the pull of the nucleus, lowering electron affinity.
Trends in the Periodic Table
Across a period (left to right): Electron affinity generally increases. Why? Nuclear charge increases and atoms want to complete their outer shell. Example: chlorine has higher electron affinity than sodium.
Down a group (top to bottom): Electron affinity decreases. Why? Atoms get larger, incoming electrons are farther from the nucleus, and shielding increases. Example: fluorine has higher electron affinity than iodine.
Examples in Everyday Life
Halogens (fluorine, chlorine, bromine, iodine) have high electron affinities, which explains why they readily form negative ions and are used in disinfectants and salts.
Noble gases, like helium and neon, have almost zero electron affinity, which is why they are chemically unreactive and used in bulbs and balloons.
Summary
- Electron affinity = energy change when a gaseous atom gains an electron.
- It is usually exothermic (energy released).
- Across a period: electron affinity increases.
- Down a group: electron affinity decreases.
- High in halogens, low in noble gases.
Evaluation
- Define electron affinity in your own words.
- Why does chlorine have a higher electron affinity than sodium?
- Which has higher electron affinity: fluorine or iodine? Why?
Fantastic work today! You’ve just added another tool to your Chemistry toolkit—understanding how atoms “receive gifts” of electrons. Keep this confidence alive, because with Afrilearn, Chemistry is not just a subject—it’s a story of how the world works, and you are learning to tell it beautifully.