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ICSE Chemistry · Chapter 1 · Lesson 8Checks 0 / 0

Periodic properties · Chapter 1

Electron affinity

Electron gain enthalpy: an atom’s pull on an incoming electron.

LESSON 8 · ICSE CLASS 10 · STUDY TIME 45–55 MINUTES

By the end, you can

  1. define electron affinity and write its equation;
  2. state its units and the ion formed;
  3. relate it to atomic size and nuclear charge;
  4. explain trends across periods and down groups;
  5. explain why Cl > F and S > O;
  6. connect electron affinity with non-metallic and oxidising character.
01e⁻gain

The opposite of ionisation energy

Ionisation energy removes an electron. Electron affinity adds one.

Ionisation energyElectron affinity
Electron is removedElectron is added
Positive ion formsNegative ion forms
Energy is suppliedEnergy is released
Atom loses electron Atom gains electron ↓ ↓ Positive ion forms Negative ion (anion) forms Energy must be supplied Energy is released

When a neutral atom gains an electron, it has more electrons than protons. It therefore becomes a negatively charged ion, called an anion.

02EAmeaning

Definition, equation and units

Textbook definition

Electron affinity is the amount of energy released when a neutral isolated gaseous atom is converted into a negatively charged gaseous ion by the addition of an electron.

It is also called electron gain enthalpy. “Isolated” means the atom is considered alone; “gaseous” is why the symbol (g) is essential.

X(g) + e⁻ → X⁻(g) + E.A.

The standard units in this chapter are eV/atom and kJ mol⁻¹. The chapter speaks of energy being released, even though its numerical table prints values with minus signs; follow the convention used in the question.

CHECK · 1 of 16

What particle is formed when a neutral atom gains an electron?

Correct: gaining an electron gives the atom more electrons than protons, so it becomes an anion.
03Clexample

Chlorine gains one electron

Chlorine has the configuration 2, 8, 7. One incoming electron completes its outermost shell.

Cl(g) + e⁻ → Cl⁻(g) + 349 kJ mol⁻¹ Cl: 2, 8, 7 Cl⁻: 2, 8, 8

This is why chlorine readily accepts one electron: the gain produces a stable complete outer shell.

CHECK · 2 of 16

Complete the expression: X(g) + e⁻ → ______ + E.A.

The product is X⁻(g), a negatively charged gaseous ion.
04↘factors

Why some atoms attract an electron more strongly

Two main factors govern electron affinity: atomic size and nuclear charge.

Smaller atomic size Greater nuclear charge ↓ ↓ Incoming electron nearer Stronger pull on e⁻ ↓ ↓ Greater electron affinity Greater electron affinity

Think of the positive nucleus as a magnet. If the incoming electron can come close, the attraction is stronger. In a larger atom the outer shell is farther away, so the pull is weaker.

CHECK · 3 of 16

Why does smaller atomic size generally increase electron affinity?

Correct: the key idea is stronger nucleus–electron attraction at the shorter distance.
05→period

Across a period: generally increases

From left to right, nuclear charge increases while atomic size generally decreases. Both changes make it easier for the atom to attract an incoming electron.

Across a period, left → right atomic size decreases + nuclear charge increases ↓ electron affinity generally increases

In the chapter’s broad pattern, Group 1 has low electron affinity and Group 17 (the halogens) has the highest. Halogens have seven valence electrons, so they need only one more for a complete outer shell.

Trend explorer

Tap an element in Period 3 to see the reasoning.

Choose an element above. The broad trend points towards chlorine; argon is different because its outer shell is complete.
CHECK · 4 of 16

Which family has the highest electron affinity?

Correct: halogens need one electron to complete their outermost shells.
06↓group

Down a group: generally decreases

Each step down adds an electron shell. The incoming electron must enter farther from the nucleus and feels weaker attraction.

Down a group ↓ Atomic size increases ↓ Incoming electron is farther from nucleus ↓ Electron affinity generally decreases

For Group 1, the source gives the broad order Li > Na > K > Rb for the amount of affinity.

CHECK · 5 of 16

Which has greater electron affinity: lithium or potassium?

Correct: electron affinity generally decreases down a group, so lithium is greater than potassium.
07!exceptions

Small-shell exceptions

A smaller atom usually attracts an incoming electron better—but fluorine and oxygen are exceptionally small. Their compact outer shells are crowded, so the incoming electron experiences strong electron–electron repulsion.

Unexpected comparisonWhy the lower element wins
Cl > FFluorine’s very small outer shell is crowded; chlorine has less repulsion.
S > OOxygen’s compact outer shell gives stronger repulsion; sulphur has more room.
Memory line

Chlorine beats fluorine; sulphur beats oxygen. In both comparisons, excessive crowding in the tiny atom reduces its electron affinity.

CHECK · 6 of 16

Why is chlorine’s electron affinity greater than fluorine’s?

Fluorine’s very small, crowded outer shell creates stronger electron–electron repulsion for the incoming electron.
CHECK · 7 of 16

Which comparison is an important exception to the usual down-group trend?

Correct: chlorine has greater electron affinity than fluorine, despite being below it in Group 17.
080noble

Noble gases and chemical character

Noble gases already have complete stable outer shells, so the chapter gives them zero electron-affinity values. They do not ordinarily tend to accept an extra electron.

Greater electron affinity ↓ Electron is gained more readily ↓ Greater non-metallic character and stronger oxidising nature

An oxidising agent accepts electrons from another substance. This is why elements with high electron affinity generally have stronger oxidising character.

CHECK · 8 of 16

Why are noble gases shown with zero electron affinity in the chapter’s table?

Correct: a complete outer shell has no normal tendency to accept another electron.
09⇄compare

Sort the periodic logic

For each statement, choose the explanation that fits. These are checks 9–12.

CHECK · 9 of 16
Across a period
CHECK · 10 of 16
Down a group
CHECK · 11 of 16
Cl > F
CHECK · 12 of 16
Noble gas
CHECK · 13 of 16

Which generally has greater electron affinity: a metal or a non-metal?

Correct: non-metals have a greater tendency to gain electrons.
CHECK · 14 of 16

Name the two units stated in this lesson.

The units are eV/atom and kJ mol⁻¹.
CHECK · 15 of 16

Which has greater electron affinity: oxygen or sulphur?

Correct: sulphur has the greater value; oxygen’s compact shell causes greater repulsion.
CHECK · 16 of 16

Complete: high electron affinity → easy electron gain → stronger ______ nature.

High electron affinity is associated with stronger oxidising nature.
10×mistakes

Catch the common mix-ups

Correction: The chapter defines electron affinity as the energy released when an electron is added to an isolated gaseous atom.
11✓practice

Worked examples and practice

Practice questions

Open answer key for all practice questions
Source-question supplement: remaining concept checks and ICSE practice
  1. Is energy released or supplied according to the textbook definition? Energy is released.
  2. Name the two main factors affecting electron affinity. Atomic size and nuclear charge.
  3. What is the general trend down a group? Electron affinity generally decreases from top to bottom.
  4. Which group has the lowest electron affinity in the chapter’s broad comparison? Group 1.
  5. Arrange Rb, K, Na, Li in increasing electron affinity. Rb < K < Na < Li.
  6. Arrange sodium, chlorine and argon in increasing electron affinity. Argon (zero in the chapter table) < sodium < chlorine.
  7. Complete: Cl(g) + ______ → Cl⁻(g) + ______. e⁻ and E.A.; the worked value is 349 kJ mol⁻¹.
  8. Give reasons: why does chlorine have greater electron affinity than fluorine, and sulphur than oxygen? Fluorine and oxygen have exceptionally small, crowded outer shells. The incoming electron experiences greater electron–electron repulsion than it does in chlorine or sulphur.
  9. Distinguish between ionisation energy and electron affinity. Ionisation energy is energy required to remove an electron and forms a positive ion; electron affinity is energy released when an electron is added and forms a negative ion.
  10. Why are halogens strongly non-metallic and oxidising? Their high electron affinity means they accept electrons readily.
12Σrevise

One-page revision

ELECTRON AFFINITY │ Energy released when a gaseous atom gains e⁻ │ X(g) + e⁻ → X⁻(g) + E.A. │ Units: eV/atom; kJ mol⁻¹ │ Depends on: atomic size, nuclear charge │ ├─ Across a period: generally increases ├─ Down a group: generally decreases ├─ Highest: Group 17; noble gases: zero in chapter table └─ Exceptions: Cl > F; S > O

“Add, anion, affinity”: adding an electron forms an anion. Across it attracts more; down the pull grows poor.