ChemistryCBSE Class 11 · Classification of Elements and Periodicity in Properties

Atomic Radius, Electronegativity and Chemical Periodicity Class 11

Updated 17 October 2026 · Fact-checked

Atomic radius decreases across a period (rising effective nuclear charge) and increases down a group (extra shells); a cation is smaller and an anion larger than its atom, and among isoelectronic species the radius falls as the nuclear charge rises. Electronegativity increases across a period and decreases down a group, with F the highest. Across period 3 the oxides change from strongly basic (Na₂O) through amphoteric (Al₂O₃) to strongly acidic (Cl₂O₇).

Across a period circles shrink as nuclear charge grows; down a group they swell. With 10 electrons each, more protons pull the ion tighter.

Illustration drawn for QuestionBuddy.

Understand Atomic Radius, Electronegativity and Chemical Periodicity

Atomic radius. Covalent radius is half the distance between two bonded identical atoms (e.g. half the Cl–Cl bond length); metallic radius is half the distance between neighbouring atoms in a metallic crystal; van der Waals radius applies to non-bonded atoms and is larger. Across a period the radius decreases (nuclear charge rises while shielding changes little); down a group it increases (new shells). Bond lengths are approximately additive: $d (A - B) \approx r_{A} + r_{B}$ .

Ionic radius. A cation is smaller than its atom (loss of electrons or a whole shell, greater $Z_{e f f}$ per electron); an anion is larger than its atom (more electron repulsion). For a given element, the higher the positive charge, the smaller the ion (Fe³⁺ < Fe²⁺). Isoelectronic species have the same number of electrons; the radius decreases as nuclear charge increases: $O^{2 -} > F^{-} > Na^{+} > Mg^{2 +} > Al^{3 +}$ .

Electronegativity. The tendency of an atom in a bond to attract the shared electron pair. On the Pauling scale (H 2.1, C 2.5, N 3.0, O 3.5, F 4.0, Cl 3.0, Na 0.9), it increases across a period and decreases down a group. A large difference ( $≳ 1.7$ ) gives a largely ionic bond; a smaller difference gives a polar covalent bond. The greater the difference, the more polar the bond.

Chemical periodicity. Valence (combining capacity) and oxidation states follow the group. Across period 3 the oxides change in nature:

Na₂O, MgO: basic (react with acids/water to give alkalis);
Al₂O₃: amphoteric (reacts with both acids and bases);
SiO₂: weakly acidic; P₄O₁₀, SO₃, Cl₂O₇: acidic, increasing in strength.

Metallic character falls across a period and rises down a group.

Anomalous behaviour and diagonal relationship. The second-period element differs from its group (small size, high charge density, no d orbitals, covalency limited to 4). Diagonal pairs (Li–Mg, Be–Al, B–Si) show similar properties because the charge-to-radius ratios of the ions are comparable.

Atomic Radius, Electronegativity and Chemical Periodicity: formulas and key facts

Bond length (additive): $d_{A - B} \approx r_{A} + r_{B}$; Covalent radii.
Charge-to-radius ratio: $\frac{z}{r}$; Compares ions (diagonal pairs).
Isoelectronic radius: $r \propto \frac{1}{Z}$; Same number of electrons.
Bond polarity: $Δ χ = ∣ χ_{A} - χ_{B} ∣$; Larger means more polar.
Basic oxide in water: $Na_{2} O + H_{2} O \to 2 NaOH$; Alkali.
Acidic oxide in water: $SO_{3} + H_{2} O \to H_{2} SO_{4}$; Acid.

How to solve Atomic Radius, Electronegativity and Chemical Periodicity questions

For every trend, name the controlling factor (nuclear charge, shells, electron-electron repulsion) before giving the order.

1Locate the elements in the periodic table (period and group).
2Choose the trend: across a period or down a group.
3For ions, compare the number of electrons first, then the nuclear charge.
4For polarity, take the difference of electronegativities.
5For oxides, classify by the position: left basic, middle amphoteric, right acidic, and write the balanced equations.
6Support every order with a one-line reason.

Atomic Radius, Electronegativity and Chemical Periodicity 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.

Predicting a bond length

Suppose the covalent radii of Cl and C are 99 pm and 77 pm.

Estimate the C–Cl bond length.

Try it first, then show the solutionSolution

d(C–Cl) ≈ r_C + r_Cl = 77 + 99 = 176 pm.

Answer: About 176 pm.

Memory hook: Add the covalent radii.

Antacids

Suppose an antacid contains Mg(OH)₂, the hydroxide of a basic oxide.

Why does it neutralise stomach acid?

Try it first, then show the solutionSolution

MgO is basic, so Mg(OH)₂ is a base.
Mg(OH)₂ + 2HCl → MgCl₂ + 2H₂O.

Answer: It is a base that reacts with HCl.

Memory hook: Metal oxides on the left are basic.

Tricks to remember Atomic Radius, Electronegativity and Chemical Periodicity

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

Isoelectronic: more Z, smaller
O²⁻ > F⁻ > Na⁺ > Mg²⁺ > Al³⁺.
Use it when: Radius order.
Cation small, anion big
Compare an ion with its atom.
Use it when: Radius questions.
Left basic, right acidic
Period 3 oxide nature changes with position.
Use it when: Oxide questions.
Diagonal likeness
Similar charge density gives Li–Mg, Be–Al, B–Si likeness.
Use it when: Anomalous behaviour.

Atomic Radius, Electronegativity and Chemical Periodicity: solved examples

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

Example 1: Bond length from covalent radii · 2 marks · Very short answerBoard level

The Cl–Cl bond length is 198 pm. If the covalent radius of carbon is 77 pm, predict the C–Cl bond length.

Try it first, then show the solutionSolution

r(Cl) = 198/2 = 99 pm. (1)
d(C–Cl) ≈ 77 + 99 = 176 pm. (1)

Answer: 176 pm.

Example 2: Isoelectronic species · 3 marks · Short answerBoard level

Arrange O²⁻, F⁻, Na⁺ and Mg²⁺ in increasing order of ionic radius, giving the number of electrons and a reason.

Try it first, then show the solutionSolution

Each species has 10 electrons (isoelectronic with Ne). (1)
Nuclear charges are 8, 9, 11 and 12. (1)
More protons pull the same 10 electrons closer: Mg²⁺ < Na⁺ < F⁻ < O²⁻. (1)

Answer: Mg²⁺ < Na⁺ < F⁻ < O²⁻.

Example 3: Polarity of the hydrogen halides · 3 marks · Short answerHarder board level

Using Pauling electronegativities H 2.1, F 4.0, Cl 3.0, Br 2.8 and I 2.5, arrange HF, HCl, HBr and HI in decreasing order of bond polarity, and state which has the most ionic character.

Try it first, then show the solutionSolution

Differences: HF 1.9, HCl 0.9, HBr 0.7, HI 0.4. (1)
Polarity: HF > HCl > HBr > HI. (1)
HF has the most ionic character because F is the most electronegative element. (1)

Answer: HF > HCl > HBr > HI.

Example 4: Oxides of the third period · 4 marks · Long answerHarder board level

Write balanced equations to show that (a) Na₂O is basic, (b) P₄O₁₀ is acidic, (c) Cl₂O₇ is acidic, and (d) SiO₂ reacts with sodium hydroxide. Arrange Na₂O, MgO, SiO₂, P₄O₁₀ and Cl₂O₇ in order of increasing acidic character.

Try it first, then show the solutionSolution

(a) Na₂O + H₂O → 2NaOH (a strong base). (1)
(b) P₄O₁₀ + 6H₂O → 4H₃PO₄. (c) Cl₂O₇ + H₂O → 2HClO₄. (1)
(d) SiO₂ + 2NaOH → Na₂SiO₃ + H₂O. (1)
Order: Na₂O < MgO < SiO₂ < P₄O₁₀ < Cl₂O₇ (acidic character grows across the period). (1)

Answer: Equations as above; Na₂O < MgO < SiO₂ < P₄O₁₀ < Cl₂O₇.

Example 5: Diagonal relationship of Be and Al · 4 marks · Long answerHOTS / competency

Take ionic radii Li⁺ 76 pm, Mg²⁺ 72 pm, Be²⁺ 45 pm and Al³⁺ 54 pm. (a) Compute the charge-to-radius ratio z/r (in units of e per pm) for each. (b) Which diagonal pair has the closer ratios and what does this suggest? (c) Show with equations that Be(OH)₂, like Al(OH)₃, is amphoteric.

Try it first, then show the solutionSolution

(a) Li⁺ 1/76 = 0.0132; Mg²⁺ 2/72 = 0.0278; Be²⁺ 2/45 = 0.0444; Al³⁺ 3/54 = 0.0556. (1)
(b) Be/Al ratios (0.0444 vs 0.0556) are closer than Li/Mg, suggesting very similar polarising power and chemistry. (1)
(c) Be(OH)₂ + 2HCl → BeCl₂ + 2H₂O (acts as a base). (1)
Be(OH)₂ + 2NaOH → Na₂[Be(OH)₄] (acts as an acid). (1)

Answer: (a) 0.0132, 0.0278, 0.0444, 0.0556; (b) Be–Al; (c) amphoteric.

Where students lose marks in Atomic Radius, Electronegativity and Chemical Periodicity

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

Mistake: Saying cations are larger than their atoms.
Reversing the trend.
Do this: Cations are smaller, anions larger.
Usually costs: Concept mark.
Mistake: Ordering isoelectronic ions by group number.
Ignoring nuclear charge.
Do this: Compare the number of protons.
Usually costs: The order.
Mistake: Mixing covalent and van der Waals radii.
Comparing different kinds of radius.
Do this: Use the same kind of radius for comparison.
Usually costs: 1 mark.
Mistake: Writing an unbalanced oxide equation.
Missing coefficients.
Do this: Check atoms on both sides.
Usually costs: The equation mark.

Exam tips

State the factor responsible for each trend.
Write the electron count when comparing ions.
Give balanced equations for the acidic/basic nature of oxides.
Use the Pauling difference to label polar or ionic.
Show additive radii for bond-length questions.

Practice questions on Atomic Radius, Electronegativity and Chemical Periodicity

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 · MCQHarder board level
The correct order of ionic radii is
(a) K⁺ > Ca²⁺ > Cl⁻ > S²⁻ (b) S²⁻ > Cl⁻ > K⁺ > Ca²⁺ (c) Ca²⁺ > K⁺ > Cl⁻ > S²⁻ (d) Cl⁻ > S²⁻ > Ca²⁺ > K⁺
Hint
All four have 18 electrons.
Answer
(b).
Q2 · 1 mark · Assertion-reasonHarder board level
Assertion (A): The radius of a neon atom is larger than that of a fluorine atom in the same period.
Reason (R): The radius of a noble gas is a van der Waals radius, while that of fluorine is a covalent radius.
(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
Different kinds of radius.
Answer
(a).
Q3 · 2 marks · Very short answerBoard level
Using the Pauling scale (Na 0.9, Cl 3.0, H 2.1), find the electronegativity differences in NaCl and HCl and say which bond is ionic.
Hint
Compare with about 1.7.
Answer
NaCl: 3.0 − 0.9 = 2.1 (ionic); HCl: 3.0 − 2.1 = 0.9 (polar covalent).
Q4 · 3 marks · Short answerHarder board level
The atomic radii (pm) of Li, Be, B, C, N, O and F are 152, 111, 88, 77, 74, 66 and 64. Find the percentage decrease from Li to F and the average decrease per element, and explain the trend.
Hint
Six steps from Li to F.
Answer
(152 − 64)/152 × 100 = 57.9%; average decrease = 88/6 = 14.7 pm per element. The nuclear charge rises by one unit each time while the electrons enter the same shell, so the pull increases and the radius falls.
Q5 · 4 marks · Long answerHOTS / competency
An element has the outer configuration 4s²4p³. (a) Give its atomic number, period, group and block. (b) Compare its radius, electronegativity and metallic character with nitrogen and phosphorus. (c) Predict the formulas of its hydride and its two oxides and the nature of the higher oxide.
Hint
Same group as N and P.
Answer
(a) Z = 18 + 15 = 33, period 4, group 15, p-block. (b) Larger radius, lower electronegativity and more metallic than N and P (they increase/decrease down the group). (c) Hydride EH₃; oxides E₂O₃ and E₂O₅; the higher oxide is acidic.

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.Radius across a period?Show answer
Decreases.
2.Isoelectronic radius order?Show answer
Decreases with increasing nuclear charge.
3.Most electronegative element?Show answer
Fluorine.
4.Nature of Al₂O₃?Show answer
Amphoteric.
5.Diagonal pairs?Show answer
Li–Mg, Be–Al, B–Si.
6.Strongest acidic oxide in period 3?Show answer
Cl₂O₇.

Atomic Radius, Electronegativity and Chemical Periodicity: frequently asked questions

Why is an anion bigger than its atom?

Added electrons increase repulsion while the nuclear charge is unchanged.

Why does electronegativity increase across a period?

Effective nuclear charge rises and the atoms get smaller, so they attract bonding electrons more.

What makes an oxide amphoteric?

It reacts with both acids and bases, as Al₂O₃ and ZnO do.

Why do Li and Mg resemble each other?

Their ions have similar charge-to-size ratios, so similar polarising power.

Why can second-period elements not exceed a covalency of 4?

They have no d orbitals, only one s and three p in the valence shell.