ChemistryICSE Class 10 · Periodic Properties and Variations of Properties

Periodicity, Atomic Size and Metallic Character (ICSE Class 10)

Q: Why does atomic size decrease across a period?

The nuclear charge increases but the number of shells stays the same, so the electrons are pulled in.

Q: Why does atomic size increase down a group?

Each element has one more shell than the one above it.

Q: Which element is the most metallic in the first three alkali metals?

Potassium, since its outer electron is farthest from the nucleus.

Q: How do I find the group from the electron arrangement?

Use the outer electrons: 1 or 2 give Group 1 or 2, and 3 to 8 outer electrons give Groups 13 to 18 by adding 10.

Updated 17 October 2026 · Fact-checked

The modern periodic law states that the properties of elements are a periodic function of their atomic number. Across a period, atomic size decreases and metallic character decreases (non-metallic character increases) because the nuclear charge rises while the number of shells stays the same. Down a group, atomic size and metallic character increase because new shells are added. A period number equals the number of shells; the group is found from the outer electrons.

Across a period atomic size falls as nuclear charge rises with the same shells; down a group it rises as shells are added, and metallic character follows size.

Illustration drawn for QuestionBuddy.

Understand Periodicity, Atomic Size and Metallic Character

1. Modern periodic law and layout. Elements are arranged in order of increasing atomic number. The table has 18 groups (vertical columns) and 7 periods (horizontal rows). In the ICSE course you are exposed to the whole table, but questions are asked only up to the fourth period (up to calcium).

2. Period and group from the electron arrangement.

Period number = number of electron shells in the atom.
Group number: for elements with 1 or 2 outer electrons it is that number (Group 1 or 2). For elements with 3 to 8 outer electrons, group number = 10 + number of outer electrons (Group 13 to 18). Helium, with 2 electrons, belongs to Group 18.
Examples: Na (2,8,1) is Period 3, Group 1. Cl (2,8,7) is Period 3, Group 17. K (2,8,8,1) is Period 4, Group 1. Ca (2,8,8,2) is Period 4, Group 2.
Valency depends on the outer electrons, so elements of one group have similar chemistry.

3. Atomic size is the distance from the centre of the nucleus to the outermost shell of the atom.

Along a period (left to right): decreases. The number of shells is constant, but the nuclear charge (protons) increases by one for each element. The outer electrons are pulled closer. Example: size order Na > Mg > Al > Si > P > S > Cl.
Down a group: increases. Each element has one more shell than the one above, and this outweighs the rise in nuclear charge. Example: Li < Na < K, and F < Cl.
The noble gas at the end of a period is a special case in measurement, but the syllabus asks for the general trend.

4. Metallic character is the tendency to lose electrons and form positive ions.

Down a group: increases. Outer electrons are farther from the nucleus and more shielded, so they are lost more easily. Li < Na < K; also Be < Mg < Ca.
Along a period: decreases. With higher nuclear charge and the same shells, atoms hold electrons more tightly and lose them with more difficulty.
The most metallic elements are at the left and bottom of the table (alkali metals).

5. Non-metallic character is the tendency to gain electrons. It follows the opposite pattern: it increases across a period and decreases down a group. Fluorine is the most non-metallic element; the elements at the right (halogens) are strongly non-metallic.

6. Examples with Period 3. From Na to Cl: atomic number rises from 11 to 17; shells stay at 3; size falls, metallic character falls, and the oxides change from basic (Na, Mg) through amphoteric (Al) to acidic (Si to Cl).

Periodicity, Atomic Size and Metallic Character: formulas and key facts

Period and group: $P er i o d = n u mb er o f s h e l l s ∣ G r o u p (1, 2) = o u t er e l ec t r o n s ∣ G r o u p (13 t o 18) = 10 + o u t er e l ec t r o n s$
Trend in atomic size: $A cr oss a p er i o d : d ecr e a ses ∣ D o w na g r o u p : in cr e a ses$
Trend in metallic character: $A cr oss a p er i o d : d ecr e a ses ∣ D o w na g r o u p : in cr e a ses$
Trend in non-metallic character: $A cr oss a p er i o d : in cr e a ses ∣ D o w na g r o u p : d ecr e a ses$
Reason (ICSE expects nuclear charge and shells): Across a period: nuclear charge rises, shells same | Down a group: shells increase

How to solve Periodicity, Atomic Size and Metallic Character questions

Explain any periodic trend in three short statements.

1Write the electron arrangement and decide the period (number of shells) and group (outer electrons).
2State the direction of the trend (increases or decreases) for the movement asked: across a period or down a group.
3Give the reason in terms of nuclear charge and number of shells only (not orbitals).
4Across a period, say: shells constant, nuclear charge increases, pull on outer electrons increases.
5Down a group, say: a new shell is added, so the outer electrons are farther away and less strongly held.
6Link atomic size to metallic character if asked: larger size with weaker pull means easier loss of electrons.

Periodicity, Atomic Size and Metallic Character 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 sodium is stored under oil but magnesium ribbon can sit in a drawer

Suppose a lab technician notices that a piece of sodium (Period 3, Group 1) must be kept under oil, while a ribbon of magnesium (Period 3, Group 2) stays shiny in a dry drawer for days.

Explain, using the periodic trend, why sodium is more reactive than magnesium.

Try it first, then show the solutionSolution

Both have three shells, but magnesium has one more proton (nuclear charge +12 against +11), so its outer electrons are held more tightly and it is smaller.
Sodium therefore loses its single outer electron more easily, so it is more metallic and more reactive.

Answer: Sodium is more metallic than magnesium because of the lower nuclear charge in the same shell.

Memory hook: Left side of a period: bigger, more metallic, more reactive.

Tricks to remember Periodicity, Atomic Size and Metallic Character

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

Across: pull in, down: shells out
Across a period the pull on the outer shell grows (size shrinks). Down a group a new shell is added (size grows).
Use it when: Atomic size questions.
Metal trend follows size
The bigger the atom, the easier it is to lose electrons, so metallic character grows with size: down a group and to the left of a period.
Use it when: Choosing the more metallic element.
Group = 10 + outer electrons
Outer electrons 3 to 8 give Groups 13 to 18: Cl with 7 outer electrons is Group 17.
Use it when: Placing an element from its electron arrangement.

Periodicity, Atomic Size and Metallic Character: solved examples

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

Example 1: Place an element · 2 marks · Short answerBoard level

An element has atomic number 14. Write its electron arrangement and state its period and group.

Try it first, then show the solutionSolution

Electron arrangement: 2,8,4.
Three shells, so Period 3; four outer electrons, so Group 10 + 4 = 14.

Answer: 2,8,4; Period 3, Group 14.

Example 2: Order the atomic size · 2 marks · Short answerBoard level

Arrange Na, Cl, Mg, Al in decreasing order of atomic size. Give the reason.

Try it first, then show the solutionSolution

All are in Period 3, with three shells each; atomic size decreases from left to right as nuclear charge rises.
Order of atomic number: Na (11), Mg (12), Al (13), Cl (17).

Answer: Na > Mg > Al > Cl; nuclear charge increases with the same number of shells.

Example 3: Group trend with reason · 3 marks · Short answerHarder board level

Why is potassium (Z = 19) larger than sodium (Z = 11), although potassium has more protons?

Try it first, then show the solutionSolution

K is 2,8,8,1 (four shells) and Na is 2,8,1 (three shells).
The extra shell puts the outer electron farther from the nucleus, and this effect is larger than the increase in nuclear charge from +11 to +19.
So potassium is larger.

Answer: Potassium has an extra shell; its outer electron is farther from the nucleus.

Example 4: Compare two elements · 4 marks · Long answerHarder board level

Elements A (2,8,1) and B (2,8,3). (a) Which has the larger atomic size? (b) Which is more metallic? (c) Give a reason for both.

Try it first, then show the solutionSolution

A is sodium (Period 3, Group 1) and B is aluminium (Period 3, Group 13).
(a) A is larger.
(b) A is more metallic.
(c) Both have three shells, but B has 13 protons against 11 in A, so B has the greater pull on its outer electrons and a smaller radius; A loses electrons more easily.

Answer: (a) A (b) A (c) The nuclear charge of B is higher while the shells are the same.

Example 5: Unknown element reasoning · 4 marks · Long answerHOTS / competency

An element X is in Period 3 and has a lower atomic size than the element in Group 1 of the same period but a higher atomic size than the element in Group 17 of the same period. (a) What can you say about its group? (b) Name the type (metal or non-metal) it is likely to be if it forms an acidic oxide. (c) State why elements in a period change from metal to non-metal.

Try it first, then show the solutionSolution

(a) It lies between Group 1 and Group 17 in Period 3, so it is in Group 2, 13, 14, 15 or 16 (Mg, Al, Si, P or S). It is not an alkali metal or a halogen.
(b) Elements that form acidic oxides are non-metals (towards the right, e.g. Si, P, S).
(c) Along the period the nuclear charge increases, outer electrons are held tighter, so tendency to lose electrons falls and tendency to gain electrons rises.

Answer: (a) Group 2, 13, 14, 15 or 16 of Period 3 (Mg, Al, Si, P or S). (b) A non-metal. (c) Rising nuclear charge with the same shells.

Where students lose marks in Periodicity, Atomic Size and Metallic Character

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

Mistake: Giving the trend without a reason
The question says "explain" or "give a reason", but only "decreases" is written.
Do this: Add: nuclear charge increases and the number of shells is the same.
Usually costs: 1 mark.
Mistake: Using orbitals instead of shells
The syllabus asks for explanations on the basis of nuclear charge and shells, not orbitals.
Do this: Write shells and nuclear charge only.
Usually costs: 1 mark.
Mistake: Wrong group for elements with 3 to 8 outer electrons
Group is written as the outer electron count instead of 10 plus that number.
Wrong working
Chlorine, 7 outer electrons, Group 7.
Right working
Chlorine, Group 17.
Do this: Remember 10 + outer electrons for Groups 13 to 18.
Usually costs: 1 mark.
Mistake: Reversing the group trend
Atomic size increases down a group, but candidates confuse it with ionisation potential.
Do this: Size and metallic character rise down a group; ionisation energy falls.
Usually costs: 1 mark.

Exam tips

Always state the electron arrangement before you decide the period and group.
Give the reason in a full sentence: say which factor stays constant and which changes.
Use only elements up to calcium (Z = 20) in your examples.
Keep metallic and non-metallic character trends as exact opposites.

Practice questions on Periodicity, Atomic Size and Metallic Character

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 element has the largest atomic size among Li, Be, B and C?
(a) Li
(b) Be
(c) B
(d) C
Hint
They are in the same period.
Answer
(a) Li.
Q2 · 1 mark · MCQBoard level
The most metallic element in Group 1 among Li, Na and K is:
(a) Li
(b) Na
(c) K
(d) all are equal
Hint
Metallic character increases down a group.
Answer
(c) K.
Q3 · 2 marks · Short answerBoard level
State the modern periodic law. How many groups and periods are there in the modern periodic table?
Hint
Atomic number.
Answer
Properties of elements are a periodic function of their atomic number. 18 groups and 7 periods.
Q4 · 4 marks · Long answerHarder board level
Element Y has atomic number 20. Give its electron arrangement, period and group, and say whether its atomic size is larger or smaller than that of magnesium (Z = 12). Explain.
Hint
Count shells.
Answer
2,8,8,2; Period 4, Group 2. Larger than Mg because it has four shells against three.
Q5 · 2 marks · Short answerHarder board level
Explain why non-metallic character increases from left to right across a period.
Hint
Use nuclear charge and shells.
Answer
Nuclear charge increases with the same number of shells, so the atom holds its electrons more strongly and gains electrons more readily.
Q6 · 1 mark · Assertion-reasonHarder board level
Assertion (A): Atomic size decreases from sodium to chlorine in Period 3.
Reason (R): Nuclear charge increases while the number of shells remains the same.
(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
Check the reason for the trend.
Answer
(a).
Q7 · 3 marks · Short answerHarder board level
Arrange F, Cl and Li in order of increasing non-metallic character, with a reason. (F: 2,7; Cl: 2,8,7; Li: 2,1)
Hint
F and Cl are in the same group.
Answer
Li < Cl < F. Li is a metal; F and Cl are in Group 17, and non-metallic character decreases down the group, so F is more non-metallic than Cl.
Q8 · 5 marks · Long answerHOTS / competency
Elements P (Z = 3), Q (Z = 11) and R (Z = 19) are in the same group. (a) Write the group and the number of shells in each. (b) Arrange them in increasing order of atomic size and of metallic character. (c) Which one loses its outer electron most easily and why?
Hint
These are Li, Na and K.
Answer
(a) Group 1; 2, 3 and 4 shells. (b) P < Q < R for both. (c) R: outermost electron is farthest from the nucleus and most shielded.

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.What gives the period number?Show answer
The number of electron shells.
2.Group of an element with 5 outer electrons?Show answer
Group 15.
3.Atomic size trend across a period?Show answer
Decreases.
4.Atomic size trend down a group?Show answer
Increases.
5.Which side of a period is metallic?Show answer
The left side.

Periodicity, Atomic Size and Metallic Character: frequently asked questions

Why does atomic size decrease across a period?

The nuclear charge increases but the number of shells stays the same, so the electrons are pulled in.

Why does atomic size increase down a group?

Each element has one more shell than the one above it.

Which element is the most metallic in the first three alkali metals?

Potassium, since its outer electron is farthest from the nucleus.

How do I find the group from the electron arrangement?

Use the outer electrons: 1 or 2 give Group 1 or 2, and 3 to 8 outer electrons give Groups 13 to 18 by adding 10.