Foundations — Electron gain enthalpy - electron affinity — trends, anomalies (e.g. Cl - F)

Before you can read the parent note, you need to be fluent in a handful of pictures and symbols. This page builds every single one from nothing, in an order where each rung stands on the one below it.

1. The atom picture — nucleus, shells, electrons

Everything starts here. If this picture is fuzzy, nothing else lands.

• Picture: concentric rings around a bright centre. Inner rings are close and tightly held; outer rings are loose and far.
• Why the topic needs it: electron gain enthalpy is the story of one new electron approaching this picture from outside and trying to settle into a ring. No atom picture → no story.

See Atomic Radius for how "how far out is the last shell" is measured, and Periodic Trends Overview for how these pictures line up in the periodic table.

2. Gaseous, and the arrow — reading a chemical process

Put together, the parent's core line reads:

$X(g)+e^{-}⟶X^{-}(g)$

Why "gaseous" matters: if the atom were in a liquid or solid, neighbours would tug on the electron too, muddying the measurement. We insist on $(g)$ so we measure only the atom-versus-electron interaction — nothing else.

3. The ion and its superscript — $X^{-}$, $X^{2-}$

• Picture: the same atom, but now the outer ring has one (or two) more electron dots than protons in the centre — so the whole thing is out of balance, tilted negative.
• Why the topic needs it: worked example 3 adds a second electron ($O^{-}\to O^{2-}$). To see why that costs energy you must first read $O^{-}$ as "already negative" — a picture that will repel the next electron.

4. Energy released vs. energy supplied — the sign of a change

This is the concept students trip on most, so we build it slowly with a picture.

When the incoming electron drops into an atom's orbital and the atom becomes more comfortable (lower energy), that drop is exothermic — energy leaves.

5. Enthalpy and its symbol — ${\Delta }_{eg}H$

Now we can name the measured number.

Related energy quantities you'll meet next door: Ionization Enthalpy (energy to remove an electron — the reverse spirit) and Electronegativity (an in-a-bond version of "loves electrons").

6. Electron affinity $A_e$ — the same thing, flipped sign

Every case check:

7. Effective nuclear charge $Z_{eff}$ — the felt pull

• Picture: the far electron "sees" a nucleus partly hidden behind a cloud of inner electrons — so it feels a weakened central pull.
• Why the topic needs it: a bigger $Z_{eff}$ pulls the new electron in harder → energy dropped is larger → ${\Delta }_{eg}H$ more negative. This is the attraction half of the tug-of-war.

Full treatment lives in Effective Nuclear Charge (Z_eff).

8. Electron–electron repulsion & crowding — the other half

• Picture: a tiny ring packed with dots; the incoming dot has nowhere comfortable to sit and gets shoved.
• Why the topic needs it: this crowding is the entire reason for the star anomaly Cl > F. Fluorine's shell is small and crowded, so repulsion cancels a lot of the attraction; chlorine's is roomier, so the electron settles easily and more energy is released.

9. Shells, subshells and "half-filled / full" — where the electron goes

• Picture: boxes for the room; five filled, one empty slot inviting the new electron.
• Why the topic needs it: whether the incoming electron finds a gap in the current shell (energy released) or must start a whole new far-out shell (energy costly, like neon) decides the sign. And a half-filled ($p^3$) or full ($p^6$, $s^2$) arrangement is unusually stable and resists newcomers — the exceptions in the "across a period" trend.

The extra-stability idea is developed in Half-filled and Fully-filled Stability.

How the foundations feed the topic

Trace any arrow into $K$ (the parent topic the topic itself) and you're standing on a foundation this page just built.

Equipment checklist

Cover the right side and answer aloud — if any stalls, reread that section.