Classification of Elements and Periodicity in Properties ISC Class 11

What Classification of Elements and Periodicity in Properties is about

Weightage: Classification of Elements and Periodicity in Properties belongs to unit 3 of the ISC Class XI Chemistry syllabus. CISCE weights units 1-6 together (Physical Chemistry) at 41 of the 70 marks in Paper I (Theory). Class XI is examined by the school, so your school sets the paper within this weightage.Source: CISCE ISC Chemistry syllabus, examination year 2028, Class XI unit weightage

Key concepts

Need for classification and Mendeleev's periodic law

Classification groups elements with similar properties so they can be studied systematically. Early attempts include Döbereiner's triads (the middle element's atomic mass is about the average of the other two) and Newlands' law of octaves. Mendeleev's periodic law: the properties of elements are a periodic function of their atomic masses. Merits: he grouped similar elements, corrected some atomic masses, and left gaps with predicted properties (eka-aluminium = gallium, eka-silicon = germanium). Limitations: the position of hydrogen was uncertain; isotopes (different masses) could not be placed; some pairs were in reverse order of mass (Ar 39.9 before K 39.1; Co before Ni; Te before I); and there was no place for the lanthanoids and actinoids in the main table.

Worked example

Show that Li (7), Na (23) and K (39) form a Döbereiner triad, and give one anomaly in Mendeleev's table.

Try it first, then show the solutionSolution

1. Average of Li and K = (7 + 39)/2 = 23 = atomic mass of Na
2. Anomaly: argon (39.9) is placed before potassium (39.1) although heavier, to keep each with similar elements

Answer: Triad confirmed; Ar–K is an anomalous pair

Modern periodic law and the long form

Modern periodic law: physical and chemical properties of elements are periodic functions of their atomic numbers. In the long form, a period corresponds to the filling of a new principal shell (period number = n of the outermost shell) and a group collects elements with the same outer configuration. Blocks: s-block (groups 1–2, last electron in ns), p-block (groups 13–18, np), d-block (groups 3–12, (n − 1)d, transition elements), f-block (lanthanoids and actinoids, (n − 2)f, placed below). Group number (IUPAC): s-block = number of valence electrons; p-block = 10 + ns + np electrons; d-block = ns + (n − 1)d electrons.

Worked example

Find the period, group and block of elements with Z = 35 and Z = 26.

Try it first, then show the solutionSolution

1. Z = 35: [Ar]3d¹⁰4s²4p⁵: n = 4, last electron in 4p ⇒ period 4, p-block, group 10 + 2 + 5 = 17 (bromine)
2. Z = 26: [Ar]3d⁶4s²: period 4, last electron in 3d ⇒ d-block, group 6 + 2 = 8 (iron)

Answer: Period 4, group 17, p-block; period 4, group 8, d-block

IUPAC names for elements with Z > 100

Until a permanent name is approved, an element is named from its atomic number using the roots 0 nil, 1 un, 2 bi, 3 tri, 4 quad, 5 pent, 6 hex, 7 sept, 8 oct, 9 enn, followed by -ium. The symbol is the first letters of the roots (three letters). When 'enn' is followed by 'nil', or 'bi'/'tri' by '-ium', one letter is dropped to ease pronunciation (enn + nil = ennil; bi + ium = bium; tri + ium = trium).

Worked example

Write the IUPAC temporary names and symbols for elements 109 and 120.

Try it first, then show the solutionSolution

1. 109: un + nil + enn + ium = unnilennium, Une
2. 120: un + bi + nil + ium = unbinilium, Ubn

Answer: Unnilennium (Une); unbinilium (Ubn)

Factors affecting periodic properties: shielding and effective nuclear charge

Outer electrons are attracted by the nucleus (charge +Ze) but repelled by inner electrons, which shield or screen them. The net attraction is described by the effective nuclear charge Z_eff = Z − σ, where σ is the screening constant. Inner shells shield strongly; electrons in the same shell shield poorly; s electrons shield better than p, p better than d, d better than f. Across a period, Z increases by 1 each step but shielding by same-shell electrons is small, so Z_eff rises and outer electrons are held more tightly. Down a group, a new shell is added each time, so distance and shielding increase and outer electrons are held less tightly.

Worked example

Use effective nuclear charge to explain why the atomic radius of chlorine is smaller than that of sodium, although chlorine has more electrons.

Try it first, then show the solutionSolution

1. Both have outer electrons in n = 3
2. Cl has Z = 17, Na has Z = 11; the extra protons are poorly shielded by the extra 3s/3p electrons, so Z_eff is much larger for Cl
3. The stronger pull draws the n = 3 shell closer to the nucleus

Answer: Higher Z_eff in Cl contracts its outer shell

Atomic and ionic radii

Covalent radius: half the distance between two bonded identical atoms; metallic radius: half the internuclear distance in a metal crystal; van der Waals radius: half the distance between non-bonded neighbouring atoms (largest). Trends: decrease across a period (rising Z_eff), increase down a group (new shells). A cation is smaller than its atom (fewer electrons, often a shell lost); an anion is larger (more repulsion, same Z). In an isoelectronic series (same number of electrons), the radius decreases as nuclear charge increases.

Worked example

Arrange N³⁻, O²⁻, F⁻, Na⁺, Mg²⁺, Al³⁺ in decreasing order of radius.

Try it first, then show the solutionSolution

1. All have 10 electrons (isoelectronic)
2. Nuclear charges: 7, 8, 9, 11, 12, 13; more protons pull the same electron cloud tighter

Answer: N³⁻ > O²⁻ > F⁻ > Na⁺ > Mg²⁺ > Al³⁺

Ionisation enthalpy

First ionisation enthalpy Δ_iH₁ is the energy needed to remove the most loosely held electron from an isolated gaseous atom in its ground state: M(g) → M⁺(g) + e⁻. Successive values increase (Δ_iH₂ > Δ_iH₁), with a very large jump when an electron is removed from a new, inner shell; this reveals the number of valence electrons. Trend: increases across a period, decreases down a group. Exceptions: Be > B and Mg > Al (removing a p electron, which is higher in energy and shielded by the s electrons, is easier); N > O and P > S (N has a stable half-filled 2p³; in O the paired 2p electron is repelled by its partner).

Worked example

An element has successive ionisation enthalpies 738, 1451 and 7733 kJ mol⁻¹. To which group does it belong?

Try it first, then show the solutionSolution

1. The jump from Δ_iH₂ to Δ_iH₃ (about five-fold) shows the third electron comes from an inner shell
2. So there are two valence electrons

Answer: Group 2 (the data are those of magnesium)

Electron gain enthalpy and electronegativity

Electron gain enthalpy Δ_egH is the enthalpy change when an electron is added to an isolated gaseous atom: X(g) + e⁻ → X⁻(g). It is usually negative (energy released), becoming more negative across a period and less negative down a group. Exceptions: noble gases and Be, Mg, N have positive or near-zero values (stable configurations); Cl is more negative than F, because the compact 2p shell of F repels the incoming electron. Electronegativity is the tendency of an atom in a bond to attract the shared pair towards itself; it is not measurable directly, and the Pauling scale gives F = 4.0 (highest). It increases across a period and decreases down a group. Metallic character runs the opposite way.

Worked example

Arrange F, Cl, Br, I in order of (i) electron gain enthalpy (most negative first) (ii) electronegativity.

Try it first, then show the solutionSolution

1. (i) Cl is most negative; F is less negative than Cl because of electron repulsion in its small 2p shell; then Br, then I
2. (ii) Electronegativity simply falls down the group

Answer: (i) Cl > F > Br > I (ii) F > Cl > Br > I

Valency, oxidation states, anomalous second period and diagonal relationship

Valency of s- and p-block elements depends on outer electrons: equal to the number of valence electrons (up to 4), or 8 minus that number. Typical oxide and hydride formulae follow (group 13: E₂O₃, EH₃; group 16: H₂E). Transition and heavier p-block elements show variable oxidation states. Second-period elements (Li to F) differ from the rest of their groups because of small size, high electronegativity, high charge/radius ratio and no available d orbitals (maximum covalency 4; e.g. N forms only NCl₃ while P forms PCl₅). The diagonal relationship: Li resembles Mg, Be resembles Al, B resembles Si, because moving right increases and moving down decreases charge density, giving similar polarising power.

Worked example

Give one similarity between Li and Mg, with an equation.

Try it first, then show the solutionSolution

1. Both burn in air to form nitrides as well as oxides
2. $6Li+N_2\to 2L{i}_{3}N$ and $3Mg+N_2\to M{g}_3{N}_2$

Answer: Both form nitrides directly with nitrogen

Acidic and basic nature of oxides

Across a period, metallic character falls and the oxides change from basic to amphoteric to acidic: Na₂O and MgO are basic, Al₂O₃ is amphoteric, and SiO₂, P₄O₁₀, SO₃ and Cl₂O₇ are acidic (increasingly so). Down a group, oxides become more basic. Basic oxides react with water to give alkalis and with acids to give salts; acidic oxides give acids with water and react with bases; amphoteric oxides react with both acids and bases.

Worked example

Write equations showing the nature of Na₂O, SO₃ and Al₂O₃.

Try it first, then show the solutionSolution

1. Basic: $N{a}_{2}O+H_{2}O\to 2NaOH$
2. Acidic: $S{O}_3+H_{2}O\to H_{2}S{O}_4$
3. Amphoteric: $A{l}_2{O}_3+6HCl\to 2AlC{l}_3+3H_{2}O$ and $A{l}_2{O}_3+2NaOH\to 2NaAl{O}_2+H_{2}O$

Answer: Na₂O basic; SO₃ acidic; Al₂O₃ amphoteric

Chemistry in real life: Classification of Elements and Periodicity in Properties

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.

Topics in this chapter

• Atomic Radii and Ionisation Enthalpy Trends
• Electron Gain Enthalpy, Electronegativity and Nature of Oxides

Classification of Elements and Periodicity in Properties formula sheet

Effective nuclear charge

$Z_{\text{eff}}=Z-\sigma$

σ = screening constant.

Group number (IUPAC)

$\text{s: }{e}_{ns};\text{p: }10+e_{ns}+e_{np};\text{d: }{e}_{ns}+e_{(n-1)d}$

Period = outermost n.

First ionisation

$M(g)\to M^{+}(g)+e^{-}$

Always endothermic.

Electron gain

$X(g)+e^{-}\to X^{-}(g)$

Usually exothermic (negative).

Valency rule

$\text{valency}=n_{\text{valence}}\text{or}8-n_{\text{valence}}$

For s- and p-block elements.

IUPAC roots

$0\text{nil},1\text{un},2\text{bi},3\text{tri},4\text{quad},5\text{pent},6\text{hex},7\text{sept},8\text{oct},9\text{enn}$

Add -ium.

Classification of Elements and Periodicity in Properties: diagrams to practise

Practise each one on paper: draw it from the checklist, then tick off every label.

How the school exam asks Classification of Elements and Periodicity in Properties

The question patterns to expect, and the method that works for each.

Tricks to remember Classification of Elements and Periodicity in Properties

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

• Across shrinks, down grows

  Radius decreases across a period and increases down a group; ionisation enthalpy and electronegativity do the opposite.

  Use it when: Any trend question.

• Half-full and full resist change

  N (2p³) and Be (2s²) hold their electrons better than neighbours.

  Use it when: Exceptions in ionisation enthalpy.

• Fluorine is too crowded

  F's small 2p shell repels an extra electron, so Cl has the more negative Δ_egH.

  Use it when: Electron gain enthalpy exception.

• Diagonal pairs

  Li–Mg, Be–Al, B–Si: one step right, one step down.

  Use it when: Diagonal relationship.

• Base, both, acid

  Oxides across period 3: Na₂O, MgO (base), Al₂O₃ (both), SiO₂ … Cl₂O₇ (acid).

  Use it when: Nature of oxides.

Classification of Elements and Periodicity in Properties: exam-level solved questions

Ordered from exam level to HOTS. Cover the solution, write your own, then compare step by step.

Case-study practice (Section D style)

Read the situation, then answer every part. In a school exam set on the board pattern this is a 5-mark question split into 1 + 1 + 1 + 2 marks.

Where the marks are

How a typical answer is split into step marks. Show every step: a right final answer alone rarely earns full marks.

Where students lose marks in Classification of Elements and Periodicity in Properties

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

• ✗Mistake: Confusing electronegativity with electron gain enthalpy

  Both describe attraction for electrons.

  Wrong working

  Cl is more electronegative than F because its Δ_egH is more negative.

  Right working

  Electronegativity (atom in a bond): F > Cl; Δ_egH (isolated atom): Cl more negative than F.

  ✓Do this: Electronegativity: bonded atom; electron gain enthalpy: gaseous isolated atom.

  Usually costs: 1-2 marks.

• ✗Mistake: Wrong group number for p-block elements

  Students use the old A/B numbering or forget to add 10.

  Wrong working

  Selenium (4s²4p⁴) is in group 6.

  Right working

  IUPAC group = 10 + 2 + 4 = 16.

  ✓Do this: The syllabus requires IUPAC numbering 1–18.

  Usually costs: 1 mark.

• ✗Mistake: Ignoring the exceptions in ionisation enthalpy

  Students apply the general trend blindly.

  Wrong working

  O has higher Δ_iH₁ than N.

  Right working

  N > O because of the half-filled 2p³.

  ✓Do this: Check Be/B, N/O, Mg/Al, P/S pairs.

  Usually costs: 1 mark.

• ✗Mistake: Comparing isoelectronic ions by mass

  Students think heavier means bigger.

  Wrong working

  Al³⁺ is larger than F⁻ because Al is heavier.

  Right working

  Same electrons: higher nuclear charge ⇒ smaller: Al³⁺ < F⁻.

  ✓Do this: Count electrons; then compare protons.

  Usually costs: 1 mark.

• ✗Mistake: Calling Al₂O₃ basic

  Students classify every metal oxide as basic.

  Wrong working

  Al₂O₃ reacts only with acids.

  Right working

  It reacts with acids and with NaOH: amphoteric.

  ✓Do this: Write one equation with an acid and one with a base.

  Usually costs: 1 mark.

Practice questions on Classification of Elements and Periodicity in Properties

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

1. Q1 · 1 mark · MCQExam level

   The most electronegative element is:
   (a) O
   (b) Cl
   (c) F
   (d) N

   Hint

   Pauling 4.0.

   Answer

   (c)

2. Q2 · 1 mark · MCQExam level

   Which oxide is amphoteric?
   (a) Na₂O
   (b) Al₂O₃
   (c) SO₃
   (d) P₄O₁₀

   Hint

   Middle of period 3.

   Answer

   (b)

3. Q3 · 1 mark · Very short answerExam level

   To which block does the element with Z = 29 belong?

   Hint

   [Ar]3d¹⁰4s¹.

   Answer

   d-block

4. Q4 · 1 mark · Very short answerExam level

   Write the IUPAC temporary symbol for Z = 107.

   Hint

   un-nil-sept.

   Answer

   Uns (unnilseptium)

5. Q5 · 2 marks · Short answerExam level

   Why do noble gases have very high ionisation enthalpies?

   Hint

   Outer configuration.

   Answer

   They have completely filled, very stable ns²np⁶ (or 1s² for He) configurations with high effective nuclear charge, so removing an electron needs a lot of energy.

6. Q6 · 2 marks · Short answerHarder exam level

   Arrange Cl, Cl⁻ and Cl⁺ in decreasing order of size and explain.

   Hint

   Same nuclear charge.

   Answer

   Cl⁻ > Cl > Cl⁺: with the same 17 protons, more electrons mean more repulsion and a larger cloud.

7. Q7 · 2 marks · Short answerHarder exam level

   The first ionisation enthalpy of lithium is 520 kJ mol⁻¹. Find the longest wavelength of light whose photons could ionise a single gaseous Li atom. (h = 6.626 × 10⁻³⁴ J s, c = 3.0 × 10⁸ m s⁻¹)

   Hint

   Divide by N_A for one atom, then λ = hc/E.

   Answer

   E per atom = 520 000/6.022 × 10²³ = 8.64 × 10⁻¹⁹ J; λ = hc/E = 1.988 × 10⁻²⁵/8.64 × 10⁻¹⁹ = 2.30 × 10⁻⁷ m = 230 nm (ultraviolet)

8. Q8 · 3 marks · Short answerHarder exam level

   Predict the formulae of (i) the oxide of the period 3, group 2 element (ii) the hydride of the period 2, group 16 element (iii) the chloride of the period 3, group 13 element.

   Hint

   Use valencies.

   Answer

   (i) MgO (ii) H₂O (iii) AlCl₃

9. Q9 · 3 marks · Short answerHarder exam level

   Give three limitations of Mendeleev's periodic table.

   Hint

   Hydrogen, isotopes, order.

   Answer

   Hydrogen's position is uncertain (it resembles both alkali metals and halogens); isotopes of one element would need different places; some pairs (Ar–K, Co–Ni, Te–I) are placed against increasing atomic mass.

10. Q10 · 2 marks · Short answerHarder exam level

    The first five ionisation enthalpies of a period 3 element M are 786, 1577, 3232, 4356 and 16 091 kJ mol⁻¹. Identify its group and write the formulae of its oxide and hydride.

    Hint

    Where is the jump?

    Answer

    The jump comes after the fourth value, so M has four valence electrons: group 14 (silicon). Oxide MO₂, hydride MH₄.

11. Q11 · 3 marks · Short answerHarder exam level

    Give three reasons for the anomalous behaviour of second-period elements, with one example.

    Hint

    Size, electronegativity, d orbitals.

    Answer

    Small size, high electronegativity (high charge density) and no available d orbitals (maximum covalency 4). Example: nitrogen forms NCl₃ but not NCl₅, whereas phosphorus forms PCl₅.

12. Q12 · 3 marks · Short answerHarder exam level

    Arrange the oxides Na₂O, MgO, Al₂O₃, SiO₂, P₄O₁₀, SO₃ and Cl₂O₇ in increasing acidic character and explain the trend.

    Hint

    Across period 3.

    Answer

    Na₂O < MgO < Al₂O₃ < SiO₂ < P₄O₁₀ < SO₃ < Cl₂O₇: metallic character falls and electronegativity rises across the period.

13. Q13 · 5 marks · Long answerHarder exam level

    (a) Find the period, group and block of Z = 56. [1]
    (b) Find the block and period of Z = 64, whose configuration is [Xe]4f⁷5d¹6s². [2]
    (c) Write the IUPAC temporary name of Z = 116. [1]
    (d) To which group and block does Z = 118 belong? [1]

    Hint

    Last electron decides the block.

    Answer

    (a) [Xe]6s²: period 6, group 2, s-block. (b) f-block (a lanthanoid), period 6. (c) Ununhexium (Uuh). (d) Group 18, p-block.

14. Q14 · 3 marks · Short answerHOTS / competency

    Fluorine has the highest electronegativity but chlorine has the most negative electron gain enthalpy. Explain why these two facts do not contradict each other.

    Hint

    Bonded vs isolated atom.

    Answer

    Electronegativity refers to an atom pulling a shared pair in a bond, where F's small size gives it the strongest pull. Electron gain enthalpy refers to an isolated gaseous atom accepting a whole extra electron; in tiny F the extra electron is strongly repelled by the crowded 2p electrons, so less energy is released than for Cl.

15. Q15 · 3 marks · Short answerHOTS / competency

    In each pair, which has the higher first ionisation enthalpy? Give reasons: (i) Mg, Al (ii) P, S (iii) Na, K.

    Hint

    Two exceptions and one group trend.

    Answer

    (i) Mg: Al loses a 3p electron that is higher in energy and shielded by 3s². (ii) P: half-filled 3p³ is extra stable. (iii) Na: K's outer electron is in a farther, more shielded shell.

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. 1.Modern periodic law?Show answer

   Properties are a periodic function of atomic number.

2. 2.Group of [Ar]3d¹⁰4s²4p⁵?Show answer

   17 (period 4, p-block).

3. 3.IUPAC root for 9?Show answer

   enn.

4. 4.Which is larger: Na or Na⁺?Show answer

   Na.

5. 5.Why is Δ_iH₁ of N greater than O?Show answer

   N has a half-filled 2p subshell.

6. 6.Element with most negative electron gain enthalpy?Show answer

   Chlorine.

7. 7.Diagonal partner of Be?Show answer

   Al.

8. 8.Nature of Cl₂O₇?Show answer

   Strongly acidic.

Quick revision checklist

The night before the exam: can you do each of these without looking?

• ☐Can you state three limitations of Mendeleev's table?
• ☐Can you find the period, group and block of any element from Z?
• ☐Can you write IUPAC names for Z = 101 to 120?
• ☐Can you explain effective nuclear charge?
• ☐Can you order an isoelectronic series by size?
• ☐Can you explain Be > B and N > O in ionisation enthalpy?
• ☐Can you explain why Cl has a more negative Δ_egH than F?
• ☐Can you show that Al₂O₃ is amphoteric with equations?

Classification of Elements and Periodicity in Properties: frequently asked questions