ChemistryICSE Class 10 · Periodic Properties and Variations of Properties

Ionisation Potential, Electron Affinity and Electronegativity (ICSE Class 10)

Q: Why does ionisation potential increase across a period?

The nuclear charge rises, atomic size falls, and the outer electron is held more strongly.

Q: Why does ionisation potential decrease down a group?

The outer electron is farther from the nucleus and shielded by more shells.

Q: Do alkali metals have high or low ionisation potential?

Low, so they lose their single outer electron easily.

Q: Why are halogens good oxidising agents?

They have high electron affinity and electronegativity, so they gain electrons readily.

Updated 17 October 2026 · Fact-checked

Ionisation potential is the energy needed to remove an electron from an isolated gaseous atom. Electron affinity is the energy released when an isolated gaseous atom gains an electron. Electronegativity is the tendency of an atom in a molecule to attract the shared pair of electrons. All three increase across a period (as nuclear charge rises with the same shells) and decrease down a group (as shells are added).

Ionisation potential, electron affinity and electronegativity all increase across a period and decrease down a group.

Illustration drawn for QuestionBuddy.

Understand Ionisation Potential, Electron Affinity and Electronegativity

1. Ionisation potential (ionisation energy). The energy required to remove the most loosely held electron from an isolated gaseous atom to form a positive ion: $X (g) \to X^{+} (g) + e^{-}$ . It is measured in electron volts (eV) or kJ per mole.

Across a period: increases. Nuclear charge increases, size decreases and the electron is held more strongly. Example: Na < Mg < Al < Si < P < S < Cl < Ar. (There are small dips in real data, but the board expects the general rise.)
Down a group: decreases. Size increases, the outer electron is farther from the nucleus and shielded by more shells. Li > Na > K.
A low ionisation potential means that an atom becomes a cation easily: alkali metals are strongly metallic. The highest values in each period belong to the noble gases.

2. Electron affinity. The energy released when a neutral isolated gaseous atom accepts an electron to form a negative ion: $X (g) + e^{-} \to X^{-} (g)$ .

Across a period: increases (becomes larger), with the halogens having the highest values and the noble gases having zero.
Down a group: decreases in general (the incoming electron is farther from the nucleus). Note: fluorine is so small that its value is actually a little lower than that of chlorine, but board questions test the general trend.
A high electron affinity means that an atom forms an anion easily, as in Cl.

3. Electronegativity. The tendency of an atom in a molecule to attract the shared pair of electrons towards itself. It has no unit.

Across a period: increases. Down a group: decreases.
Fluorine is the most electronegative element. Elements with a large electronegativity difference between them form ionic compounds; with a small difference they form covalent (polar covalent) compounds.

4. Electron affinity versus electronegativity.

Feature	Electron affinity	Electronegativity
Applies to	an isolated gaseous atom	an atom in a molecule
Idea	energy released on gaining an electron	tendency to attract the shared pair
Measured as	energy (eV or kJ/mol)	a relative number without a unit

5. Alkali metals (Group 1) and halogens (Group 17), special reference.

Alkali metals Li, Na, K: ionisation potential falls and metallic character rises down the group, so reactivity rises (K is more reactive than Na, and Na more than Li).
Halogens F, Cl: electronegativity and non-metallic character fall down the group, so fluorine is the most reactive halogen and chlorine is less reactive.
Across Period 3, Na (lowest IP) is the most reactive metal and Cl (high EA, high electronegativity) is a reactive non-metal.

Ionisation Potential, Electron Affinity and Electronegativity: formulas and key facts

Ionisation: $X (g) \to X^{+} (g) + e^{-}$ (energy absorbed)
Electron gain: $X (g) + e^{-} \to X^{-} (g)$ (energy released)
Trend summary: Ionisation potential increases across, decreases down
Electron affinity increases across, decreases down
Electronegativity increases across, decreases down
Extremes: Highest ionisation potential noble gases
Highest electronegativity fluorine
Zero electron affinity noble gases

Ionisation potential	increases across, decreases down
Electron affinity	increases across, decreases down
Electronegativity	increases across, decreases down

Highest ionisation potential	noble gases
Highest electronegativity	fluorine
Zero electron affinity	noble gases

How to solve Ionisation Potential, Electron Affinity and Electronegativity questions

Use a two-step answer: direction, then reason.

1Name the property and recall its definition word for word (isolated gaseous atom; energy absorbed or released).
2Locate the elements in the table (period and group) from their electron arrangements.
3State the trend: across a period all three properties increase; down a group they decrease.
4Give the reason in terms of nuclear charge and shells: more charge with the same shells increases the grip; extra shells reduce it.
5For comparison questions, remember that the highest ionisation potential in a period is for the noble gas, and fluorine is the most electronegative element.

Ionisation Potential, Electron Affinity and Electronegativity in real life

Each concept, met in an everyday situation. Read the situation, try the problem, then check the solution. The one-line hook is what to remember.

Why a "fizzy" metal and a "pungent" gas sit at opposite ends of a period

Suppose in a Period 3 tray a student places a small piece of sodium in water and it moves rapidly across the surface, while chlorine gas from a jar takes electrons from a metal strongly.

Use ionisation potential and electron affinity to explain why sodium readily loses an electron and chlorine readily gains one.

Try it first, then show the solutionSolution

Sodium has a low ionisation potential: its single outer electron is far from the pull of the nucleus and is easy to remove.
Chlorine has a high electron affinity (and electronegativity): its nuclear charge pulls strongly on an incoming electron, which completes its octet.

Answer: Sodium easily forms Na⁺ (low ionisation potential); chlorine easily forms Cl⁻ (high electron affinity).

Memory hook: Low IP: gives; high EA: takes.

Tricks to remember Ionisation Potential, Electron Affinity and Electronegativity

Short memory aids. Say each one aloud once, then use it on a question the same day.

Three properties, one direction
IP, EA and electronegativity all behave the same way: rise across a period, fall down a group.
Use it when: MCQs on trends.
IP needs energy, EA releases it
Removing an electron costs energy (IP). Adding an electron usually gives energy back (EA).
Use it when: Definitions.
F wins, noble gases lead IP
Fluorine tops electronegativity; noble gases top ionisation potential in their periods and have zero EA.
Use it when: Choosing the maximum or minimum.

Ionisation Potential, Electron Affinity and Electronegativity: solved examples

Ordered from board level to HOTS. Try each one before you open the solution.

Example 1: Define · 2 marks · Short answerBoard level

Define (a) ionisation potential, (b) electron affinity.

Try it first, then show the solutionSolution

(a) The energy required to remove an electron from an isolated gaseous atom.
(b) The energy released when an isolated gaseous atom gains an electron.

Answer: (a) Energy needed to remove an electron from an isolated gaseous atom. (b) Energy released when an isolated gaseous atom gains an electron.

Example 2: Pick the maximum · 3 marks · Short answerBoard level

Among Li, Be, F and Ne (atomic numbers 3, 4, 9, 10), which has (a) the highest ionisation potential, (b) the lowest, (c) zero electron affinity?

Try it first, then show the solutionSolution

All are in Period 2; ionisation potential rises across the period up to the noble gas.
(a) Ne. (b) Li. (c) Ne, as the noble gas has a full outer shell and accepts no electron.

Answer: (a) Ne (b) Li (c) Ne.

Example 3: Why potassium is more reactive · 3 marks · Short answerHarder board level

Explain why potassium is a stronger reducing agent than lithium using ionisation potential.

Try it first, then show the solutionSolution

Potassium has four shells and lithium has two, so the outer electron of potassium is much farther from the nucleus and more shielded.
Hence potassium has a lower ionisation potential.
It loses its outer electron more easily, so it is a stronger reducing agent.

Answer: K has the lower ionisation potential because its outer electron is farther away and more shielded.

Example 4: Electron affinity versus electronegativity · 4 marks · Long answerHarder board level

State three differences between electron affinity and electronegativity, and name the most electronegative element.

Try it first, then show the solutionSolution

Electron affinity is for an isolated gaseous atom; electronegativity is for an atom in a molecule.
Electron affinity is the energy released on gaining an electron; electronegativity is the tendency to attract the shared pair.
Electron affinity is measured in energy units; electronegativity has no unit.
The most electronegative element is fluorine.

Answer: As listed; fluorine is the most electronegative.

Example 5: Data reasoning · 5 marks · Long answerHOTS / competency

Elements A, B and C lie in the same period with atomic numbers 11, 14 and 17. Arrange them in the increasing order of (a) ionisation potential, (b) electronegativity, and (c) state which will form the most stable negative ion. Give reasons.

Try it first, then show the solutionSolution

A is sodium (2,8,1), B is silicon (2,8,4), C is chlorine (2,8,7), all of Period 3.
(a) and (b): both increase across the period, so A < B < C.
(c) C, because its high electron affinity and nuclear charge pull in one electron to complete the octet.
Reason: same shells, nuclear charge rises from +11 to +17.

Answer: (a) A < B < C (b) A < B < C (c) C.

Where students lose marks in Ionisation Potential, Electron Affinity and Electronegativity

The slips examiners see most often, and what to write instead.

Mistake: Leaving out "isolated gaseous atom"
The definition of ionisation potential and electron affinity needs the gaseous isolated atom.
Wrong working
Energy to remove an electron from an atom.
Right working
Energy required to remove an electron from an isolated gaseous atom.
Do this: Learn the definition as a sentence.
Usually costs: 1 mark.
Mistake: Confusing electronegativity with electron affinity
Both increase across a period, so they are mixed up in answers.
Do this: Electron affinity is an energy for a free atom; electronegativity is the pull on a shared pair in a molecule.
Usually costs: 1 mark.
Mistake: Stating the trend without a reason
The question says "explain", but no mention of nuclear charge or shells is made.
Do this: Add the nuclear charge and shell argument.
Usually costs: 1 mark.
Mistake: Saying the noble gas has the highest electron affinity
It is zero, as the outer shell is complete.
Do this: Halogens have the highest electron affinities; noble gases have zero.
Usually costs: 1 mark.

Exam tips

Write the units (eV or kJ per mole) for ionisation potential and electron affinity, and remember electronegativity has none.
Use "across a period" and "down a group" in every trend answer.
Name fluorine as the most electronegative element and the noble gas as the highest IP in a period.
Support the trend with the nuclear charge and shell argument, never with orbitals.

Practice questions on Ionisation Potential, Electron Affinity and Electronegativity

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

Q1 · 1 mark · MCQBoard level
The element with the highest electronegativity is:
(a) oxygen
(b) chlorine
(c) fluorine
(d) nitrogen
Hint
It is at the top right of the table.
Answer
(c) fluorine.
Q2 · 1 mark · MCQBoard level
Ionisation potential generally ______ down a group and ______ across a period from left to right.
(a) increases, decreases
(b) decreases, increases
(c) increases, increases
(d) decreases, decreases
Hint
Size and shells.
Answer
(b) decreases, increases.
Q3 · 2 marks · Short answerBoard level
What is electronegativity? What is its trend down a group?
Hint
Shared pair.
Answer
The tendency of an atom to attract the shared pair of electrons; it decreases down a group.
Q4 · 2 marks · Short answerHarder board level
Arrange Li, Na and K in increasing order of ionisation potential and state the reason.
Hint
Bigger atom means easier removal.
Answer
K < Na < Li; the outer electron is farther from the nucleus and more shielded as shells increase.
Q5 · 3 marks · Short answerHarder board level
Why does a noble gas have zero electron affinity and the highest ionisation potential in its period?
Hint
Full outer shell.
Answer
Its outer shell is complete and stable, so it does not accept an electron and holds its electrons very strongly.
Q6 · 1 mark · Assertion-reasonHarder board level
Assertion (A): Chlorine has a higher electron affinity than sodium.
Reason (R): Chlorine has a higher nuclear charge and a smaller atomic size than sodium in the same period.
(a) Both A and R are true and R is the correct explanation of A.
(b) Both A and R are true but R is not the correct explanation of A.
(c) A is true but R is false.
(d) A is false but R is true.
Hint
Compare the pull on an added electron.
Answer
(a).
Q7 · 4 marks · Long answerHarder board level
Element X (2,8,1) and element Y (2,8,7) combine. (a) Identify which will lose and which will gain an electron, using the terms ionisation potential and electron affinity. (b) Name the type of bond. (c) Write the formula.
Hint
Na and Cl.
Answer
(a) X has a low ionisation potential and loses; Y has a high electron affinity and gains. (b) Electrovalent. (c) XY (NaCl).
Q8 · 4 marks · Long answerHOTS / competency
The halogen fluorine reacts more vigorously than chlorine, and potassium reacts more vigorously than sodium. Explain both observations using the periodic trends of electronegativity and ionisation potential.
Hint
Both depend on the number of shells.
Answer
Fluorine is smaller and more electronegative; it attracts electrons more strongly than chlorine. Potassium has a lower ionisation potential, since its outer electron is farther and shielded by more shells, so it loses it more readily than sodium.

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1.Unit of electronegativity?Show answer
It has no unit.
2.Trend of ionisation potential down a group?Show answer
It decreases.
3.Which has the lowest ionisation potential in Period 3?Show answer
Sodium.
4.Electron affinity of a noble gas?Show answer
Zero.
5.Most electronegative element?Show answer
Fluorine.

Ionisation Potential, Electron Affinity and Electronegativity: frequently asked questions

Why does ionisation potential increase across a period?

The nuclear charge rises, atomic size falls, and the outer electron is held more strongly.

Why does ionisation potential decrease down a group?

The outer electron is farther from the nucleus and shielded by more shells.

Do alkali metals have high or low ionisation potential?

Low, so they lose their single outer electron easily.

Why are halogens good oxidising agents?

They have high electron affinity and electronegativity, so they gain electrons readily.