Periodic Properties and Variations of Properties ICSE Class 10: Trends Explained

What Periodic Properties and Variations of Properties is about

Weightage: CISCE publishes no chapter or unit weightage for ICSE Chemistry. Periodic Properties and Variations of Properties is in the 1. Periodic Properties and variations of Properties – Physical and Chemical part of the 2027 syllabus and can be asked in both Section A and Section B of the 80-mark paper.Source: CISCE ICSE Chemistry (Science Paper 2) syllabus, examination year 2027

Key concepts

Periods, groups and electronic configuration

Write the electronic configuration by shells (K, L, M, N) with at most 2, 8, 8 electrons in the first three shells for the first 20 elements; potassium (19) is 2, 8, 8, 1 and calcium (20) is 2, 8, 8, 2.

• Period number = number of occupied shells.
• Group number: for 1 or 2 valence electrons, group 1 or 2 (IA, IIA). For 3 to 7 valence electrons, group 13 to 17 (IIIA to VIIA), that is, add 10. A full outer shell (2 or 8) means group 18 (zero group).
• Valency for metals = valence electrons; for non-metals = 8 minus valence electrons.

Hydrogen (1 electron) is unusual: like group 1 it has one electron to lose, like group 17 it needs one electron to complete its shell, so it can be placed with either.

Worked example

An element X has atomic number 16. Write its electronic configuration and state its period, group and valency.

Try it first, then show the solutionSolution

1. Fill shells: 2, 8, 6.
2. Three shells are occupied, so period 3.
3. Six valence electrons, so group 16 (VIA).
4. It needs 2 more electrons for an octet, so valency 2.

Answer: 2, 8, 6; period 3; group 16 (VIA); valency 2 (it is sulphur).

Atomic size (atomic radius)

Atomic size is the distance between the centre of the nucleus and the outermost shell of an atom.

• Across a period (left to right): decreases. The number of shells is the same, but the nuclear charge increases by one unit at each step, so the outer shell is pulled closer. Example: Na > Mg > Al > Si > P > S > Cl.
• Down a group: increases. A new shell is added at each step; this outweighs the rise in nuclear charge. Example: Li < Na < K.

Noble gases are usually left out of size comparisons across a period, because their size is measured in a different way.

Worked example

Arrange Mg, Na, Cl and Si in decreasing order of atomic size and give a reason.

Try it first, then show the solutionSolution

1. All four are in period 3 (three shells).
2. Nuclear charge: Na 11, Mg 12, Si 14, Cl 17.
3. Greater nuclear charge with the same shells pulls the outer shell inwards.

Answer: Na > Mg > Si > Cl; nuclear charge rises across the period while the number of shells stays three.

Ionisation potential (ionisation energy)

Ionisation potential is the energy needed to remove an electron from the outermost shell of an isolated gaseous atom to form a positive ion: $M(g)\to M^{+}(g)+e^{-}$.

• Across a period: increases, because the outer electron is held more tightly by a larger nuclear charge and a smaller atom.
• Down a group: decreases, because the outer electron is further from the nucleus and is shielded by more inner shells.
• Noble gases have the highest ionisation potential in their period, because a full outer shell is very stable.

Low ionisation potential means an atom loses electrons easily, so it is a strong metal and a good reducing agent.

Worked example

Which has the higher ionisation potential, sodium or potassium? Explain.

Try it first, then show the solutionSolution

1. Both are in group 1.
2. Potassium (2, 8, 8, 1) has four shells; sodium (2, 8, 1) has three.
3. Potassium's outer electron is further from the nucleus and better shielded, so it is easier to remove.

Answer: Sodium has the higher ionisation potential.

Electron affinity

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

• Across a period: increases, because a smaller atom with a larger nuclear charge attracts the added electron more strongly. The halogens have the highest electron affinity in each period.
• Down a group: generally decreases, because the incoming electron enters a shell further from the nucleus.
• Noble gases: about zero, because a full outer shell has no tendency to accept another electron.

One point beyond the basic rule: chlorine releases slightly more energy than fluorine on gaining an electron, because fluorine's very small outer shell is already crowded. If a question simply asks for the general trend, give the general trend.

Worked example

Among Mg, Ar, Li and Cl, which has the highest electron affinity, and why does argon not?

Try it first, then show the solutionSolution

1. Halogens need only one electron to complete the octet, so they release the most energy on gaining one.
2. Argon already has a stable octet; it has almost no tendency to gain an electron.

Answer: Chlorine; argon's full outer shell gives it an electron affinity of about zero.

Electronegativity

Electronegativity is the tendency of an atom in a molecule to attract the shared pair of electrons towards itself. Do not confuse it with electron affinity, which is about an isolated atom gaining an electron.

• Across a period: increases (larger nuclear charge, smaller atom).
• Down a group: decreases (outer shell further out).
• Fluorine is the most electronegative element of all; in period 3, chlorine is the most electronegative (argon is not given a value, since it does not normally form bonds).

Electronegativity difference decides bond type: a large difference (Na and Cl) gives an ionic bond; a small difference (H and Cl) gives a polar covalent bond; no difference (Cl and Cl) gives a non-polar bond.

Worked example

Name the most electronegative element of period 2 and of period 3.

Try it first, then show the solutionSolution

1. Electronegativity rises to the right of a period, stopping before the noble gas.
2. Period 2 ends with F before Ne; period 3 ends with Cl before Ar.

Answer: Fluorine (period 2) and chlorine (period 3).

Metallic and non-metallic character

Metallic character is the tendency of an atom to lose electrons and form a positive ion; non-metallic character is the tendency to gain electrons and form a negative ion.

• Across a period: metallic character decreases and non-metallic character increases (atoms hold electrons harder). Period 3 runs from the very metallic Na to the non-metal Cl, with silicon a metalloid in between.
• Down a group: metallic character increases (outer electron easier to lose), non-metallic character decreases.

The same idea explains oxides: metals form basic oxides ($N{a}_{2}O$, $MgO$), non-metals form acidic oxides ($S{O}_2$, $P_2{O}_5$), and some metals near the dividing line form amphoteric oxides ($A{l}_2{O}_3$, $ZnO$).

Worked example

X, Y and Z are three metals in the same group in that order from top to bottom. Which is the strongest reducing agent?

Try it first, then show the solutionSolution

1. A reducing agent gives away electrons.
2. Down a group, the outer electron is further from the nucleus, so it is lost more easily.
3. Z is at the bottom, so it has the largest atom and the lowest ionisation potential.

Answer: Z, the largest atom with the lowest ionisation potential.

Alkali metals and halogens: two families compared

Alkali metals (group 1: Li, Na, K) have one valence electron. Down the group, atomic size increases and ionisation potential decreases, so reactivity increases: lithium reacts steadily with water, sodium vigorously, potassium so violently that the hydrogen catches fire.

$2Na+2H_{2}O\to 2NaOH+H_2$

Halogens (group 17: F, Cl, then Br and I lower down) have seven valence electrons and react by gaining one. Down the group, atomic size increases, so the pull on an incoming electron weakens and reactivity decreases. A more reactive halogen therefore displaces a less reactive one from its salt solution:

$C{l}_2+2NaBr\to 2NaCl+B{r}_2$

So the two families show opposite reactivity trends down the group, for the same underlying reason (bigger atom, outer shell further out).

Worked example

Why does reactivity increase down group 1 but decrease down group 17?

Try it first, then show the solutionSolution

1. Group 1 metals react by losing an electron; a larger atom loses it more easily.
2. Group 17 non-metals react by gaining an electron; a larger atom attracts it less strongly.
3. Both families get larger down the group.

Answer: Increasing size helps electron loss (group 1) but hinders electron gain (group 17).

Chemistry in real life: Periodic Properties and Variations of 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

• Periods, Groups and Electronic ConfigurationComing soon
• Ionisation Potential, Electron Affinity and ElectronegativityComing soon
• Atomic Size and Metallic CharacterComing soon
• Ionisation Potential, Electron Affinity and Electronegativity
• Periodicity, Atomic Size and Metallic Character

Periodic Properties and Variations of Properties formula sheet

Trend across a period (left to right)

$\text{Size}\downarrow ,\text{IP}\uparrow ,\text{EA}\uparrow ,\text{EN}\uparrow ,\text{metallic}\downarrow ,\text{non-metallic}\uparrow$

Same number of shells; nuclear charge increases.

Trend down a group

$\text{Size}\uparrow ,\text{IP}\downarrow ,\text{EA}\downarrow ,\text{EN}\downarrow ,\text{metallic}\uparrow ,\text{non-metallic}\downarrow$

A new shell is added at each step.

Ionisation potential

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

Energy is absorbed.

Electron affinity

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

Energy is released.

Group from valence electrons

$v=1,2\Rightarrow \text{group }v;v=3\text{ to }7\Rightarrow \text{group }v+10$

Full outer shell: group 18 (0).

Valency

$\text{metal: }v;\text{non-metal: }8-v$

v = number of valence electrons.

Periodic Properties and Variations of Properties: diagrams to practise

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

How the board asks Periodic Properties and Variations of Properties

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

Tricks to remember Periodic Properties and Variations of Properties

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

• Across: the nucleus wins

  Across a period, the shells stay the same and the nucleus gets stronger, so everything that depends on 'holding on' (IP, EA, EN, non-metallic character) goes up, and size and metallic character go down.

  Use it when: Any across-the-period trend in one sentence.

• Down: the shell wins

  Down a group, a new shell is added. Bigger atom, looser grip: size and metallic character up, IP, EA and EN down.

  Use it when: Any down-the-group trend.

• Bottom-left, top-right

  The most metallic element is at the bottom left, the most non-metallic at the top right (fluorine), noble gases aside.

  Use it when: Quickly spotting the extreme element in a list.

• Affinity is alone, negativity is in a bond

  Electron Affinity: an Atom Alone gains an electron. ElectroNegativity: a Neighbour is sharing.

  Use it when: Not mixing up the two definitions.

Periodic Properties and Variations of Properties: board-level solved questions

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

Case-study practice (Section B style)

Read the situation, then answer every part. In the board paper this is a 4-mark question split into 2 + 1 + 1 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 Periodic Properties and Variations of Properties

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

• ✗Mistake: Calling the noble gas configuration unstable

  CISCE examiners' comments, 2024: when asked why neon has the highest ionisation potential in period 2, candidates chose 'unstable electronic configuration'.

  Wrong working

  Neon has the maximum ionisation potential because its configuration is unstable.

  Right working

  Neon's outer shell is completely filled (2, 8), a very stable arrangement, so the most energy is needed to remove an electron.

  ✓Do this: A full shell is the most stable arrangement: high ionisation potential, almost zero electron affinity.

  Usually costs: 1 mark (MCQ).

• ✗Mistake: Giving a noble gas the highest electron affinity

  CISCE examiners' comments, 2024: candidates chose argon or lithium for the highest electron affinity instead of bromine, forgetting that inert gases have almost no electron affinity.

  Wrong working

  Electron affinity is highest for Ar because it is at the end of the period.

  Right working

  Electron affinity is highest for the halogen; the noble gas has none.

  ✓Do this: Electron affinity and electronegativity peak at group 17, not group 18; ionisation potential peaks at group 18.

  Usually costs: 1 mark.

• ✗Mistake: Linking a small atom with the best reducing agent

  CISCE examiners' comments, 2025: for three metals in a group, candidates picked the smallest atom as the best reducing agent.

  Wrong working

  X has the smallest radius, so it is the best reducing agent.

  Right working

  Z, the largest atom, loses its outer electron most easily, so it is the best reducing agent.

  ✓Do this: Reducing agent = electron giver = large metal atom with low ionisation potential.

  Usually costs: 1 mark.

• ✗Mistake: Using the wrong symbol or direction in an order

  CISCE examiners' comments, 2024: candidates reversed the order or used the wrong < or > signs when arranging elements.

  Wrong working

  Increasing ionisation potential: Li > Na > K

  Right working

  Increasing ionisation potential: K < Na < Li

  ✓Do this: Write 'increasing' with < from left to right; check the first element is the smallest value.

  Usually costs: 1 mark per order.

• ✗Mistake: Roman numerals for periods and confusing valency with valence electrons

  CISCE examiners' comments, 2025: candidates wrote period III instead of 3, gave valency 5 for a group 15 element, and wrote the name instead of the formula.

  Wrong working

  Element with 2, 8, 5: period III, valency 5.

  Right working

  Period 3, valency 3 (8 - 5).

  ✓Do this: Periods in Arabic numerals; non-metal valency = 8 minus valence electrons.

  Usually costs: 1 mark each.

• ✗Mistake: Mixing up ionisation potential and electronegativity

  CISCE examiners' comments, 2025: asked for the highest ionisation potential in Ne, O and F, candidates wrote fluorine.

  Wrong working

  Highest ionisation potential among Ne, O, F: F (most electronegative).

  Right working

  Ne: a noble gas has the highest ionisation potential in its period.

  ✓Do this: Ionisation potential includes noble gases; electronegativity and electron affinity do not.

  Usually costs: 1 mark.

Practice questions on Periodic Properties and Variations of 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 · MCQBoard level

   Which of these has the smallest atomic size?
   (a) Na
   (b) Mg
   (c) S
   (d) Cl

   Hint

   All are in period 3.

   Answer

   (d) Cl

2. Q2 · 1 mark · MCQBoard level

   The element with configuration 2, 8, 8, 2 is in:
   (a) period 2, group 8
   (b) period 4, group 2
   (c) period 3, group 2
   (d) period 2, group 2

   Hint

   Count shells and outer electrons.

   Answer

   (b) period 4, group 2 (calcium).

3. Q3 · 1 mark · MCQHarder board level

   Statement I: Electronegativity of chlorine is greater than that of sulphur.
   Statement II: Chlorine has a greater nuclear charge than sulphur but the same number of shells.
   (a) Both statements are true, and II explains I.
   (b) Both statements are true, but II does not explain I.
   (c) I is true, II is false.
   (d) I is false, II is true.

   Hint

   Same period.

   Answer

   (a)

4. Q4 · 1 mark · Assertion-reasonHarder board level

   Assertion (A): Fluorine is the most reactive halogen.
   Reason (R): Fluorine has the largest atomic size in group 17.
   (a) Both (A) and (R) are true, and (R) is the correct explanation for (A).
   (b) Both (A) and (R) are true, but (R) is not the correct explanation for (A).
   (c) (A) is true and (R) is false.
   (d) (A) is false and (R) is true.

   Hint

   Where is fluorine in the group?

   Answer

   (c): fluorine is the smallest halogen, which is why it gains an electron most readily.

5. Q5 · 3 marks · Very short answerBoard level

   Name: (a) the alkali metal in period 4; (b) the most electronegative element of period 3; (c) the element with configuration 2, 8, 6.

   Hint

   Elements 1-20.

   Answer

   (a) Potassium (b) Chlorine (c) Sulphur

6. Q6 · 3 marks · Short answerBoard level

   An element has atomic number 12. (a) Is it a metal or a non-metal? (b) Write the formula of its oxide and its chloride. (c) Is its oxide acidic or basic?

   Hint

   Configuration 2, 8, 2.

   Answer

   (a) Metal (b) MgO-type: XO; chloride XCl₂ (c) Basic

7. Q7 · 3 marks · Short answerHarder board level

   Arrange: (a) Be, Mg, Ca (increasing metallic character); (b) Li, C, N, F (decreasing atomic size); (c) Cl, F (increasing ionisation potential).

   Hint

   Check the direction of each.

   Answer

   (a) Be < Mg < Ca (b) Li > C > N > F (c) Cl < F

8. Q8 · 4 marks · Short answerHarder board level

   Elements A, B and C have atomic numbers 3, 11 and 19. (a) Which property is the same for all three? (b) Which has the highest ionisation potential? (c) Which reacts most vigorously with water? Write the equation for that reaction using its symbol.

   Hint

   Group 1.

   Answer

   (a) One valence electron (valency 1, group 1). (b) A (lithium, smallest). (c) C (potassium): $2K+2H_{2}O\to 2KOH+H_2$.

9. Q9 · 2 marks · Short answerHarder board level

   Explain why the ionisation potential of magnesium is greater than that of sodium, but less than that of beryllium.

   Hint

   One across, one down.

   Answer

   Mg has a greater nuclear charge than Na with the same three shells, so it holds its outer electrons more tightly. Be has only two shells, so its outer electrons are closer to the nucleus than Mg's.

10. Q10 · 3 marks · Short answerHarder board level

    Bromine water is added to (i) potassium chloride solution and (ii) potassium iodide solution. State what you observe in each and write the equation where a reaction occurs.

    Hint

    A halogen displaces only those below it.

    Answer

    (i) No change: bromine is less reactive than chlorine. (ii) The solution turns brown as iodine is set free: $B{r}_2+2KI\to 2KBr+I_2$.

11. Q11 · 3 marks · Short answerHOTS / competency

    Element Y forms the ion Y²⁻ with the configuration 2, 8, 8. (a) Find the atomic number of Y. (b) State its period and group. (c) Compare its atomic size with that of the element after it in the same period.

    Hint

    The ion has 2 extra electrons.

    Answer

    (a) 18 - 2 = 16 (b) Period 3, group 16 (c) Y is larger than the next element (chlorine), which has one more unit of nuclear charge and the same number of shells.

12. Q12 · 2 marks · Short answerHOTS / competency

    A student says 'Since chlorine has a higher electron affinity than fluorine, chlorine must be more electronegative.' Is the student right? Explain.

    Hint

    Two different properties.

    Answer

    No. Electronegativity concerns an atom pulling a shared pair in a molecule; fluorine, being smaller, does this most strongly and is the most electronegative element. Electron affinity concerns an isolated atom, where fluorine's crowded small shell slightly lowers the energy released.

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.Define ionisation potential.Show answer

   The energy needed to remove an electron from the outermost shell of an isolated gaseous atom.

2. 2.Define electron affinity.Show answer

   The energy released when an isolated gaseous atom gains an electron to form a negative ion.

3. 3.Define electronegativity.Show answer

   The tendency of an atom in a molecule to attract the shared electron pair towards itself.

4. 4.How does atomic size change across a period?Show answer

   It decreases, because nuclear charge increases while the number of shells stays the same.

5. 5.Most electronegative element?Show answer

   Fluorine.

6. 6.Most metallic element up to calcium?Show answer

   Potassium.

7. 7.Electronic configuration of calcium?Show answer

   2, 8, 8, 2.

8. 8.How does reactivity change down the halogen group?Show answer

   It decreases.

9. 9.Which element can be placed in both group 1 and group 17?Show answer

   Hydrogen.

Quick revision checklist

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

• ☐Can you state the period, group and valency of any element up to calcium from its atomic number?
• ☐Can you give all six trends across a period and down a group, with a reason for each?
• ☐Do you know why noble gases have the highest ionisation potential but almost zero electron affinity?
• ☐Can you explain why reactivity increases down group 1 but decreases down group 17?
• ☐Can you write a halogen displacement equation, balanced?
• ☐Can you answer a letter-table question without naming the elements?

Periodic Properties and Variations of Properties: frequently asked questions