ChemistryISC Class 11 · Classification of Elements and Periodicity in Properties

Electron Gain Enthalpy, Electronegativity, Oxidation States and the Nature of Oxides

Updated 17 October 2026 · Fact-checked

Electron gain enthalpy measures the energy change when an atom gains an electron (more negative for halogens); electronegativity measures the tendency to attract shared electrons (F is highest). Across a period, oxides change from basic (Na₂O) through amphoteric (Al₂O₃) to acidic (SO₃, Cl₂O₇).

Moving left to right across the period, electronegativity rises and the oxides change from basic to amphoteric (Al₂O₃) to acidic.

Illustration drawn for QuestionBuddy.

Understand Electron Gain Enthalpy, Electronegativity and Nature of Oxides

Electron gain enthalpy. ΔegH is the enthalpy change when an isolated gaseous atom gains an electron to form an anion: X(g) + e⁻ → X⁻(g). It is negative for most elements (energy is released), most negative for halogens, and positive for noble gases and for second electron gain (O⁻ → O²⁻). Across a period it becomes more negative; down a group it generally becomes less negative, but the second-period elements O and F have smaller values than S and Cl because of strong electron-electron repulsion in their compact 2p subshell. Cl has the most negative value of all.

Electronegativity. The tendency of an atom in a molecule to attract the shared electron pair to itself. The Pauling scale gives F = 4.0, O = 3.5, N = 3.0 and Cl = 3.0, C = 2.5, H = 2.1, Na = 0.9. It increases across a period and decreases down a group. A large difference in electronegativity (roughly above 1.7) indicates an ionic bond; a small difference gives a polar covalent bond and zero a pure covalent bond.

Valency and oxidation states. For s- and p-block elements, the maximum oxidation state generally equals the group number (for groups 1 and 2, and groups 13 to 17 as the group number minus 10) — for example Cl is +7 in Cl₂O₇ and S is +6 in SO₃. Lower oxidation states in p-block elements arise by loss of only the p electrons (inert pair effect in heavier members).

Anomalous behaviour of the second period. Li, Be, B, C, N, O and F differ from their group members because of small size, high electronegativity, high charge/radius ratio and lack of d orbitals (so no expansion of the octet).

Diagonal relationship. Li resembles Mg, Be resembles Al and B resembles Si because the pairs have similar size, charge/radius ratio and electronegativity.

Nature of oxides. Metal oxides on the left are basic (Na₂O + H₂O → 2NaOH); non-metal oxides on the right are acidic (SO₃ + H₂O → H₂SO₄); Al₂O₃ is amphoteric (reacts with both acids and bases), and neutral oxides such as CO and N₂O do not react with either.

Electron Gain Enthalpy, Electronegativity and Nature of Oxides: formulas and key facts

Electron gain: X(g) + e⁻ → X⁻(g) + energy | ΔegH is negative; Second electron gain is positive.
Electronegativity (Pauling): $F 4.0∣ O 3.5∣ N 3.0, C l 3.0∣ C 2.5∣ H 2.1∣ N a 0.9$; A rough guide to bond polarity.
Bond type guide: ΔEN > 1.7 ionic
0.4 to 1.7 polar covalent
~0 covalent; Approximate rule of thumb: 1.7 corresponds to about 50% ionic character. Borderline cases such as HF (1.9) are still treated as polar covalent with high ionic character.
Nature of oxides: $N a_{2} O (ba s i c) ∣ A l_{2} O_{3} (am p h o t er i c) ∣ S O_{3}, C l_{2} O_{7} (a c i d i c)$; Basic to acidic across period 3.

ΔEN > 1.7	ionic
0.4 to 1.7	polar covalent
~0	covalent

Electron Gain Enthalpy, Electronegativity and Nature of Oxides 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.

Neutralising an amphoteric oxide

Suppose a technician tests aluminium oxide with both hydrochloric acid and sodium hydroxide solution.

Write the two equations and state what the results show.

Try it first, then show the solutionSolution

With acid: Al₂O₃ + 6HCl → 2AlCl₃ + 3H₂O.
With base: Al₂O₃ + 2NaOH + 3H₂O → 2Na[Al(OH)₄].
It reacts with both an acid and a base, so it is amphoteric.

Answer: Amphoteric oxide.

Memory hook: Amphoteric means it behaves as a base towards acids and an acid towards bases.

Tricks to remember Electron Gain Enthalpy, Electronegativity and Nature of Oxides

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

Left basic, right acidic
Oxides go from basic through amphoteric to acidic across a period.
Use it when: Classifying oxides.
Cl beats F for gain, F beats Cl for pull
Cl has the most negative electron gain enthalpy, but F is the most electronegative.
Use it when: MCQs on halogens.

Electron Gain Enthalpy, Electronegativity and Nature of Oxides: solved examples

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

Example 1: Most electronegative element · 1 mark · MCQBoard level

The most electronegative element in the periodic table is:
(a) oxygen
(b) chlorine
(c) fluorine
(d) nitrogen

Try it first, then show the solutionSolution

On the Pauling scale F = 4.0, the highest.

Answer: (c) fluorine

Example 2: Bond polarity from electronegativity · 3 marks · Short answerBoard level

Using Pauling values H = 2.1, F = 4.0, Cl = 3.0, Na = 0.9, find the electronegativity differences in HF, HCl and NaCl and classify the bonds.

Try it first, then show the solutionSolution

HF: 4.0 − 2.1 = 1.9; HCl: 3.0 − 2.1 = 0.9; NaCl: 3.0 − 0.9 = 2.1. (1.5 marks)
NaCl (2.1) is ionic; HCl (0.9) is polar covalent. HF (1.9) is by the 1.7 rule borderline, but 1.7 is only an approximate guide (about 50% ionic character); HF is in fact a polar covalent molecule with a high degree of ionic character. (1.5 marks)

Answer: NaCl is ionic; HF is polar covalent with high ionic character (borderline by the 1.7 rule); HCl is polar covalent.

Example 3: Classifying the oxides · 4 marks · Short answerHarder board level

Classify Na₂O, Al₂O₃, SiO₂ and SO₃ as acidic, basic or amphoteric, giving one equation each for Na₂O, SO₃ and Al₂O₃ to support your answer.

Try it first, then show the solutionSolution

Na₂O is basic: Na₂O + H₂O → 2NaOH. (1 mark)
SO₃ is acidic: SO₃ + H₂O → H₂SO₄. (1 mark)
Al₂O₃ is amphoteric: with acid, Al₂O₃ + 6HCl → 2AlCl₃ + 3H₂O; with alkali, Al₂O₃ + 2NaOH + 3H₂O → 2Na[Al(OH)₄]. (1.5 marks)
SiO₂ is acidic (it reacts with hot alkali). (0.5 mark)

Answer: Na₂O basic; Al₂O₃ amphoteric; SiO₂ and SO₃ acidic.

Example 4: Maximum oxidation states · 4 marks · Short answerHarder board level

State the maximum oxidation state shown by sulphur and by chlorine and show the calculation in SO₃ and Cl₂O₇. Explain why the second-period element nitrogen cannot show +7 or +6.

Try it first, then show the solutionSolution

SO₃: x + 3(−2) = 0, so S = +6. (1 mark)
Cl₂O₇: 2x + 7(−2) = 0, so Cl = +7. (1 mark)
The maximum oxidation state equals the number of valence electrons (6 for S, 7 for Cl). (1 mark)
N has no d orbitals and only four orbitals in its valence shell (octet), so its maximum is +5 (as in HNO₃). (1 mark)

Answer: S: +6; Cl: +7; N: at most +5.

Example 5: Electron gain enthalpy of halogens · 5 marks · Long answerHOTS / competency

The electron gain enthalpies (kJ mol⁻¹) are listed as F −328, Cl −349, Br −325, I −295 and O −141, S −200 (as in standard tables). Explain why Cl has a more negative value than F, and why the gain of a second electron by oxygen is not exothermic.

Try it first, then show the solutionSolution

The trend down the halogen group is generally less negative, but F is an exception. (1 mark)
F is very small, so the incoming electron enters a compact 2p subshell and experiences strong repulsion from the existing electrons. (1.5 marks)
In the larger 3p subshell of Cl there is less repulsion, so more energy is released. (1 mark)
O⁻ + e⁻ → O²⁻: the electron must be added to an already negative ion, so electrostatic repulsion makes the process endothermic. (1.5 marks)

Answer: Cl > F in magnitude because of electron-electron repulsion in the small 2p subshell of F; the second electron gain of O is endothermic because of repulsion by the negative ion.

Where students lose marks in Electron Gain Enthalpy, Electronegativity and Nature of Oxides

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

Mistake: Confusing electron gain enthalpy with electronegativity
Treating them as the same quantity.
Do this: Electron gain enthalpy is for an isolated atom; electronegativity is for an atom in a bond.
Usually costs: 1 to 2 marks.
Mistake: No equation for oxide nature
Stating "Al₂O₃ is amphoteric" without evidence.
Do this: Write a balanced equation with both an acid and a base.
Usually costs: 1 to 2 marks.
Mistake: Calling O²⁻ formation exothermic
Saying the second electron gain releases energy.
Do this: It is endothermic because of repulsion.
Usually costs: 1 mark.

Exam tips

Give the reason in terms of size, nuclear charge and electron repulsion.
Use balanced equations to show the nature of oxides.
State the electronegativity values you use.
For oxidation states, show the sum equals the charge.

Practice questions on Electron Gain Enthalpy, Electronegativity and Nature of Oxides

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
Which of the following is an amphoteric oxide?
(a) Na₂O
(b) Al₂O₃
(c) SO₃
(d) Cl₂O₇
Hint
It reacts with both acids and bases.
Answer
(b) Al₂O₃
Q2 · 2 marks · Short answerBoard level
Which has the higher electronegativity, nitrogen or carbon? Give a reason.
Hint
Compare nuclear charge in the same period.
Answer
Nitrogen (3.0) is higher than carbon (2.5): across a period the nuclear charge rises while the size falls, so the attraction for shared electrons increases.
Q3 · 3 marks · Short answerHarder board level
State the nature of the oxides MgO, P₄O₁₀ and CO, with one reason or equation for each.
Hint
Think of left basic, right acidic, and neutral oxides.
Answer
MgO is basic (MgO + 2HCl → MgCl₂ + H₂O). P₄O₁₀ is acidic (P₄O₁₀ + 6H₂O → 4H₃PO₄). CO is neutral and reacts with neither acids nor bases.
Q4 · 3 marks · Short answerHarder board level
Explain the diagonal relationship between lithium and magnesium, giving two similar properties.
Hint
Think about size and charge/radius ratio.
Answer
Li⁺ and Mg²⁺ have similar polarising power (similar charge/radius ratio) and electronegativity. Both form nitrides with nitrogen (Li₃N and Mg₃N₂) and both carbonates decompose on heating to give the oxide and CO₂.
Q5 · 3 marks · Short answerHarder board level
Calculate the oxidation state of Cl in ClO₄⁻ and of S in S₂O₃²⁻ (average), and say which shows the maximum for its element.
Hint
Use the sum of the oxidation states equal to the net charge.
Answer
ClO₄⁻: x + 4(−2) = −1, so Cl = +7 (the maximum). S₂O₃²⁻: 2x + 3(−2) = −2, so S = +2 on average; this is not the maximum (+6).
Q6 · 4 marks · Case-basedHOTS / competency
Case study: Suppose a student compares the third-period oxides Na₂O, MgO, Al₂O₃, SiO₂, P₄O₁₀, SO₃ and Cl₂O₇.
(i) How does the acidic character change from Na₂O to Cl₂O₇? (1 mark)
(ii) Which oxide is amphoteric? (1 mark)
(iii) Write the equation for the reaction of Cl₂O₇ with water. (1 mark)
(iv) Explain the trend in terms of electronegativity. (1 mark)
Hint
Cl₂O₇ + H₂O gives HClO₄.
Answer
(i) Acidic character increases steadily across the period. (ii) Al₂O₃. (iii) Cl₂O₇ + H₂O → 2HClO₄. (iv) The more electronegative the element, the more covalent the E–O bond and the more the oxide behaves as an acid-former; electropositive elements give ionic, basic oxides.

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.Name the element whose atom releases the most energy on gaining an electron.Show answer
Chlorine.
2.Pauling electronegativity of F?Show answer
4.0.
3.Name an amphoteric oxide.Show answer
Al₂O₃.
4.Which pair shows a diagonal relationship?Show answer
Li and Mg (also Be–Al, B–Si).

Electron Gain Enthalpy, Electronegativity and Nature of Oxides: frequently asked questions

Why is the electron gain enthalpy of noble gases positive?

They have complete shells, so an added electron must go into the next shell, which is higher in energy; energy must be supplied.

Does electronegativity have units?

No. It is a relative number on a scale, not a measured energy.

Why are oxides of metals basic?

The M–O bond is largely ionic, so the oxide supplies O²⁻ ions that react with water to give OH⁻.