Atomic Size and Atomic Radius | ICSE Class 10 Chemistry

011Concept

What atomic size means

Atoms are extremely small, but they are not all the same size. Some have more electron shells, some feel a stronger pull from the nucleus, some have gained or lost electrons. Every one of those changes shows up as a change in size.

Think of an atom as a building. The nucleus sits at the centre, the shells are the floors around it, and the outermost shell is the outside wall:

Outermost shell ┌─────────────────┐ │ │ │ Inner shell │ │ ┌─────────┐ │ │ │ Nucleus │ │ │ └─────────┘ │ │ │ └─────────────────┘

The farther the outermost shell is from the nucleus, the larger the atom. That single sentence is the whole of this lesson in disguise — everything else just explains what pushes the outer wall out or pulls it in.

The textbook definition

Atomic size (atomic radius)

The distance between the centre of the nucleus of an atom and its outermost shell.

Centre of nucleus ───────────→ Outermost shell Atomic radius

In your own words

Atomic radius tells us how far the outer edge of an atom is from its nucleus.

Why is it called a radius?

A radius is the distance from the centre of a circle to its outer edge. An atom is treated the same way:

Centre of atom → Outer boundary of atom

So the size of an atom is expressed as its radius, not as its diameter. Half the width, measured outwards from the middle.

Check your understanding · 1 of 14

Atomic radius is the distance between:

Correct answer: B. Atomic size, or atomic radius, is the distance between the centre of the nucleus of an atom and its outermost shell. Option D describes the diameter, which is twice the radius.

022Concept

Half the internuclear distance

There is a practical problem with the first definition: an atom has no sharp, clearly visible boundary. You cannot lay a ruler against a fuzzy edge.

So chemists measure something they can pin down — the distance between the two nuclei of two atoms that are joined together — and then halve it.

Atomic radius — alternative definition

Atomic radius can also be defined as half the internuclear distance between two combined atoms in a molecule.

Internuclear distance

The distance between the nuclei of two combined atoms.

Atom A Atom B Nucleus A ----------- Nucleus B Distance

Because the two atoms are identical, each one owns exactly half of that distance:

Atomic radius = ½ × internuclear distance

Worked out

Suppose the distance between the nuclei of two identical combined atoms is 200 pm.

Atomic radius = 200 ÷ 2 = 100 pm

Try it — the halving calculator

Type any internuclear distance in picometres and halve it. Do the division in your head first, then check.

Atomic radius = ½ × internuclear distance
The two joined atoms are identical, so the halfway point between the nuclei is the outer edge of each one.

Check your understanding · 2 of 14

The internuclear distance between two identical combined atoms is 250 pm. What is the atomic radius of the element?

Correct answer: C — 125 pm. Atomic radius is half the internuclear distance, so 250 ÷ 2 = 125 pm. Option A doubles instead of halving — the internuclear distance is already the whole width across two atoms.

033Concept

The units: ångström and picometre

Atomic sizes are far too small for millimetres. The chapter uses two units.

Ångström

1 Å = 10⁻¹⁰ metre

Picometre

1 pm = 10⁻¹² metre

The symbol for picometre is:

pm

The atomic-radius values in this chapter are mainly given in picometres, so that is the unit to write after every number in an answer.

Worth noticing

Because 10⁻¹⁰ is a hundred times bigger than 10⁻¹², one ångström is the same as 100 picometres. So sodium's radius of 186 pm could also be written as 1.86 Å. Both units describe the same tiny distance; only the number in front changes.

Check your understanding · 3 of 14

One picometre is equal to:

Correct answer: B — 10⁻¹² metre. Option A is the ångström (1 Å = 10⁻¹⁰ m). Keep the pair straight: the ångström is the larger of the two, and 1 Å = 100 pm.

044Concept

The two factors that control size

Every size question in this chapter is settled by exactly two things pulling in opposite directions.

According to the chapter, atomic size mainly depends upon:

Number of electron shells
Nuclear charge

ATOMIC SIZE │ ┌───────────┴───────────┐ │ │ Number of shells Nuclear charge │ │ More shells usually Stronger nuclear pull make atom larger usually makes atom smaller

These two factors often act against each other. Deciding which one wins is the whole skill.

Factor 1 · The number of shells

More occupied shells generally make an atom larger. Compare three members of Group 1:

Lithium: 2, 1 Sodium: 2, 8, 1 Potassium: 2, 8, 8, 1

Counting the occupied shells
Element	Number of occupied shells
Lithium	2
Sodium	3
Potassium	4

Each new shell lies farther from the nucleus than the one before it:

2 shells → smaller atom 3 shells → larger atom 4 shells → still larger atom

Building analogy

Imagine three buildings:

Building 1: 2 floors Building 2: 3 floors Building 3: 4 floors

The top floor of the four-storey building is farther from the ground than the top floor of the two-storey building. In the same way, an atom with more shells generally has its outermost electrons farther from the nucleus.

Factor 2 · Nuclear charge

The nucleus contains positively charged protons. As the number of protons increases, the positive charge of the nucleus increases. This stronger positive nucleus pulls the negatively charged electrons more strongly.

Greater nuclear charge ↓ Stronger attraction for electrons ↓ Electrons pulled closer ↓ Smaller atomic size

Magnet analogy

Imagine the nucleus as a magnet and the electrons as objects attracted towards it.

Weak magnet → weaker pull → object stays farther away Strong magnet → stronger pull → object moves closer

A stronger nuclear charge pulls the electron shells closer, so the atom shrinks.

Check your understanding · 4 of 14

Atomic size mainly depends upon which two factors?

Correct answer: B. The two factors are the number of electron shells and the nuclear charge. More shells push the outer edge outwards; a greater nuclear charge pulls it inwards. Everything in this lesson comes from weighing those two against each other.

055Concept

Down a group: atoms get bigger

Moving down a group means moving from top to bottom of a vertical column.

Top ↓ Element Element Element Element ↓ Bottom

Atomic size increases down a group.

Down a group ↓ Atomic size increases

Why does it increase?

When we move down a group:

one new electron shell is added at each step;
the outermost shell becomes farther from the nucleus;
nuclear charge also increases;
however, the effect of the additional shells is greater than the increased nuclear pull.

Therefore, the atom becomes larger.

New shell added ↓ Outermost electrons farther away ↓ Effect of extra shells outweighs increased nuclear charge ↓ Atomic size increases

The sentence examiners want

Notice that point 3 is not ignored — the nuclear charge does increase down a group. A full-mark answer says so, and then says that the effect of the extra shells is the greater of the two.

Example: Group 1

Group 1 — the chapter's values
Element	Atomic radius
Hydrogen	37 pm
Lithium	152 pm
Sodium	186 pm
Potassium	231 pm
Rubidium	244 pm
Caesium	262 pm

H < Li < Na < K < Rb < Cs 37 → 152 → 186 → 231 → 244 → 262 pm

A rough picture of the same thing — the circles are only a simple comparison, not accurate scale drawings:

Hydrogen ○ Lithium ◯ Sodium ◯ Potassium ◎ Rubidium ◎ Caesium ◉ Down the group → size increases

Example: Group 17

Group 17 — the halogens
Element	Atomic radius
Fluorine	64 pm
Chlorine	99 pm
Bromine	114 pm
Iodine	133 pm
Astatine	140 pm

F < Cl < Br < I < At

Atomic size increases down Group 17 as well — the same rule, a different column.

The radius chart — tap any bar

Switch between the two groups and the two periods. The height of each bar is that element's atomic radius, all four sets drawn to the same scale.

Top of the groupBar height = atomic radiusBottom of the group

Atomic radius in pm Currently selected

Thinking check

Fluorine and chlorine both have seven valence electrons. Why is chlorine larger?

Answer

Chlorine has one more occupied electron shell than fluorine. Therefore, its outermost shell is farther from the nucleus. (Fluorine is 2, 7 with two shells; chlorine is 2, 8, 7 with three.)

Check your understanding · 5 of 14

Why does atomic size increase down a group?

Correct answer: C. One new electron shell is added at each successive step, placing the outermost shell farther from the nucleus. Nuclear charge does increase too, but the effect of the additional shells is greater — so the atom ends up larger. Option A is simply false: nuclear charge rises down a group.

Check your understanding · 6 of 14

Fluorine (64 pm) and chlorine (99 pm) both have seven valence electrons. Chlorine is larger because:

Correct answer: B. Chlorine has one more occupied electron shell than fluorine, so its outermost shell is farther from the nucleus. Option C is a trap — both have seven valence electrons, which is exactly why they are in the same group.

066Concept

Across a period: atoms get smaller

Moving across a period means moving from left to right along a horizontal row.

Left → → → → → → Right

Atomic size decreases from left to right across a period.

Across a period Left → Right Atomic size decreases

Why does it decrease?

Across a period:

the number of occupied shells remains the same;
the atomic number increases by one from element to element;
therefore, nuclear charge increases;
the stronger nucleus pulls the electrons closer;
the atom becomes smaller.

Same number of shells + Increasing nuclear charge ↓ Stronger attraction ↓ Electrons pulled closer ↓ Atomic size decreases

Tug-of-war analogy

Imagine the nucleus and the electrons playing tug-of-war. Across a period, the electrons are being added to the same main shell, but the nucleus gains more positive charge with every step.

Stronger nucleus ← pulls electrons inward

The nucleus wins a little more each time, so the atom gradually becomes smaller.

Example: Period 2

Period 2 — lithium to fluorine
Element	Li	Be	B	C	N	O	F
Atomic radius	152	112	88	77	70	66	64

Li > Be > B > C > N > O > F 152 → 112 → 88 → 77 → 70 → 66 → 64 pm

Why is lithium the largest in Period 2?

Lithium has the lowest nuclear charge among the listed Period 2 elements. Its nucleus pulls the electrons less strongly than the nuclei of the elements to its right.

Why is fluorine the smallest in the given Period 2 sequence?

Fluorine has a higher nuclear charge while still having the same number of occupied shells as the other Period 2 elements. Therefore, it pulls its electrons closer.

Example: Period 3

Na > Mg > Al > Si > P > S > Cl

Period 3 — approximate values
Element	Atomic radius
Sodium	186 pm
Magnesium	160 pm
Aluminium	143 pm
Silicon	117 pm
Phosphorus	110 pm
Sulphur	104 pm
Chlorine	99 pm

Atomic size decreases from sodium to chlorine. Switch the chart in Concept 5 to Period 2 → or Period 3 → and watch the bars fall instead of climb.

The two trends side by side

The comparison examiners ask for
Movement	Number of shells	Nuclear charge	Atomic size
Down a group	Increases	Increases	Increases
Across a period	Remains the same	Increases	Decreases

Look carefully at the middle column: nuclear charge increases both ways. That is why it can never be the whole explanation on its own. The deciding column is the number of shells.

Down a group Extra shells have the stronger effect ↓ Atom becomes larger Across a period No new shell is added Nuclear charge increases ↓ Atom becomes smaller

Memory trick

Down = Diameter grows Across = Atom contracts

Down, more floors. Across, stronger pull.

The noble-gas exception

The chapter notes that inert-gas atoms are shown as larger than expected. It explains this by stating that their outermost shells are complete and that electronic repulsions are maximum.

Remember for this chapter

Noble gases are treated as an exception to the usual decrease in atomic size across a period. Do not apply the ordinary left-to-right size pattern blindly to the noble gas at the end of a period when answering questions based on this chapter.

Sort these — down a group, or across a period?

One tap each. Answers lock in.

A new electron shell is added at each step

The number of occupied shells remains the same

Nuclear charge increases, yet atomic size increases too

Nuclear charge increases and atomic size decreases

Li → Na → K → Rb → Cs

Na → Mg → Al → Si → P → S → Cl

Check your understanding · 7 of 14

Why does atomic size decrease from left to right across a period?

Correct answer: B. The atomic number rises by one at every step, so nuclear charge increases; but no new shell is added, so the stronger nucleus simply pulls the same outer shell closer. Electrons are being added, not removed — they just go into the shell that is already outermost.

Check your understanding · 8 of 14

In the sequence Na, Mg, Al, Si, P, S, Cl, which atom is the smallest?

Correct answer: C — chlorine, at 99 pm. All seven are Period 3 elements with three occupied shells, so the shells cannot separate them. Chlorine has the greatest nuclear charge of the seven, so it pulls its electrons in the tightest: Na > Mg > Al > Si > P > S > Cl.

077Concept

Ions are never the size of their parent atom

An atom may lose or gain electrons and form an ion. The moment it does, its size changes — and it changes in a completely predictable direction.

Atom loses electron(s) → Cation Atom gains electron(s) → Anion

A cation is smaller than its parent atom

A cation is formed when an atom loses one or more electrons.

Na → Na⁺ + e⁻

A cation is always smaller than its parent atom.

Why?

When electrons are removed:

the number of electrons decreases;
electron-electron repulsion decreases;
the nucleus pulls the remaining electrons more strongly;
sometimes the outermost shell may be lost completely.

Atom loses electrons ↓ Fewer electrons and less repulsion ↓ Remaining electrons pulled closer ↓ Cation is smaller

The point most students miss

The number of protons does not change when an ion forms. So in Na⁺ the same 11 protons are now holding on to only 10 electrons — more pull spread over fewer electrons. That is the real reason a cation shrinks.

Sodium

Na: 2, 8, 1 Na⁺: 2, 8

Sodium loses the electron in its third shell. The Na⁺ ion now has only two occupied shells, so an entire shell has disappeared:

Na⁺ is smaller than Na

Magnesium

Mg: 2, 8, 2 Mg²⁺: 2, 8

Magnesium loses two outermost electrons. Therefore:

Mg²⁺ is smaller than Mg

An anion is larger than its parent atom

An anion is formed when an atom gains one or more electrons.

Cl + e⁻ → Cl⁻

An anion is larger than its parent atom.

Why?

When electrons are added:

the number of electrons increases;
electron-electron repulsion increases;
the electron cloud spreads outward;
the ion becomes larger.

Atom gains electrons ↓ More electron-electron repulsion ↓ Electron cloud expands ↓ Anion is larger

Chlorine

Cl: 2, 8, 7 Cl⁻: 2, 8, 8

Chlorine gains one electron. The extra electron increases repulsion in the outermost shell, so the shell puffs out:

Cl⁻ is larger than Cl

The picture to keep in your head

Na⁺Cation — one whole shell gone

NaParent atom · 2, 8, 1

ClParent atom · 2, 8, 7

Cl⁻Anion — electron cloud expands

A comparison only — the circles are not accurate scale drawings.

Cation < Parent atom < Anion

In words: losing electrons makes the particle smaller; gaining electrons makes the particle larger.

Balloon analogy

Imagine electrons as people inside a balloon.

Losing electrons Fewer people push against the balloon ↓ Less outward pressure → smaller size Gaining electrons More people crowd inside ↓ More outward pressure → larger size

Check your understanding · 9 of 14

Which is smaller, Na or Na⁺, and why?

Correct answer: B — Na⁺ is smaller. Sodium (2, 8, 1) loses its single third-shell electron to become Na⁺ (2, 8), leaving only two occupied shells. Fewer electrons also means less electron-electron repulsion, so the same 11 protons hold the remaining 10 electrons more tightly.

Check your understanding · 10 of 14

Why is Cl⁻ larger than Cl?

Correct answer: C. Cl is 2, 8, 7 and Cl⁻ is 2, 8, 8 — the number of shells is unchanged, so option D is wrong. What changes is the crowding: the added electron increases repulsion inside the outermost shell and the electron cloud spreads outward. The number of protons never changes when an ion forms, which rules out A.

088Concept

Isoelectronic ions

Now a harder comparison: what if two ions have exactly the same number of electrons? Something else has to decide the size.

Iso = same Electronic = electrons

Isoelectronic ions

Isoelectronic ions are ions having the same number of electrons.

The chapter's example

O²⁻, F⁻, Na⁺, Mg²⁺

Count the electrons
Ion	Number of protons	Number of electrons
O²⁻	8	10
F⁻	9	10
Na⁺	11	10
Mg²⁺	12	10

All four have ten electrons. Therefore, they are isoelectronic.

What decides their size?

Since all have the same number of electrons, we compare their nuclear charges.

More protons ↓ Greater nuclear charge ↓ Stronger pull on the same number of electrons ↓ Smaller ion

Greater nuclear charge → smaller ionic size

Working out the order

O²⁻ = 8 protons F⁻ = 9 protons Na⁺ = 11 protons Mg²⁺ = 12 protons

The ion with the lowest nuclear charge is largest; the ion with the highest nuclear charge is smallest. Therefore:

O²⁻ > F⁻ > Na⁺ > Mg²⁺

O²⁻8 protons · largest

F⁻9 protons

Na⁺11 protons

Mg²⁺12 protons · smallest

Ten electrons in every one of them — only the number of protons differs. A comparison only, not to scale.

Simple analogy

Imagine four equally sized groups of ten children, each being held by a different number of adults.

More adults pulling inward → group held more tightly

Similarly, when the same number of electrons is attracted by more protons, the ion becomes smaller.

Check your understanding · 11 of 14

Among O²⁻, F⁻, Na⁺ and Mg²⁺, which ion is the smallest?

Correct answer: D — Mg²⁺. All four ions have ten electrons, so the electrons cannot separate them. Magnesium's nucleus has 12 protons — more than any of the others — so it holds those ten electrons the most tightly. The full order is O²⁻ > F⁻ > Na⁺ > Mg²⁺.

Sort these — which particle of each pair is larger?

One tap each. Answers lock in.

Na or Na⁺

Cl or Cl⁻

Mg or Mg²⁺

O or O²⁻

Na or K

Mg or Cl

F⁻ or Na⁺

09WWorked

The four-step comparison method

Every "which is larger?" question in this chapter is answered by the same four steps, in the same order, every single time.

STEP 1: What is the relationship? ↓ Same group · same period · atom and its own ion · isoelectronic STEP 2: What changes between them? ↓ Shells · nuclear charge · number of electrons STEP 3: Which factor wins? ↓ STEP 4: State the answer with the reason

Step through five comparisons

Pick a pair, decide the answer in your head, then reveal the reasoning one line at a time.

Worked example 1

Which is larger: sodium or potassium?

Full solution

Step 1: Both belong to Group 1.

Step 2: Potassium is below sodium and has one extra electron shell.

Answer: Potassium is larger than sodium because atomic size increases down a group.

Worked example 2

Which is smaller: magnesium or chlorine?

Full solution

Both are in Period 3, so both have three occupied shells. Across a period, atomic size decreases from left to right, and chlorine lies to the right of magnesium.

Answer: Chlorine is smaller than magnesium. (Mg 160 pm, Cl 99 pm.)

Worked example 3

Arrange in increasing order of size: Li, Na, K

Full solution

All three are Group 1 elements, and down Group 1 atomic size increases.

Answer: Li < Na < K

Worked example 4

Arrange in decreasing order of atomic size: Li, Be, B, C

Full solution

All four are Period 2 elements, and across Period 2 atomic size decreases from left to right.

Answer: Li > Be > B > C (152, 112, 88, 77 pm)

Worked example 5

Which is smaller: Na or Na⁺?

Full solution

Na⁺ is formed by loss of an electron. Fewer electrons means less electron-electron repulsion, and sodium loses its entire third shell in the process.

Answer: Na⁺ < Na

Worked example 6

Which is larger: Cl or Cl⁻?

Full solution

Cl⁻ is formed by gaining an electron. The added electron increases repulsion and the electron cloud expands.

Answer: Cl⁻ > Cl

Worked example 7

Arrange the following in decreasing size: O²⁻, F⁻, Na⁺, Mg²⁺

Full solution

They are isoelectronic — all four have ten electrons. Greater nuclear charge produces a smaller ion, and the proton counts are 8, 9, 11 and 12.

Answer: O²⁻ > F⁻ > Na⁺ > Mg²⁺

Use the stepper

Go back to the panel above and run examples 1, 2, 5, 6 and 7 through it yourself. Deciding the answer before you tap is worth far more than watching the steps appear.

10RRecall

Concept check — answer without looking back

Say each answer aloud first, then open the strip to check it.

1 · What is atomic radius?

It is the distance between the centre of the nucleus and the outermost shell.

2 · Name two units used for atomic size.

Ångström and picometre.

3 · State the two main factors affecting atomic size.

Number of electron shells and nuclear charge.

4 · What happens to atomic size down a group?

It increases.

5 · What happens to atomic size across a period?

It decreases.

6 · Why does atomic size increase down a group?

New shells are added, placing the outermost electrons farther from the nucleus.

7 · Why does atomic size decrease across a period?

Nuclear charge increases while the number of shells remains the same.

8 · Which is smaller: Na or Na⁺?

Na⁺.

9 · Which is larger: Cl or Cl⁻?

Cl⁻.

10 · Which is smallest among O²⁻, F⁻, Na⁺ and Mg²⁺?

Mg²⁺.

Type it out — three answers in full sentences

Recall · 12 of 14

Define atomic size.

Model answer: Atomic size, or atomic radius, is the distance between the centre of the nucleus of an atom and its outermost shell. It can also be defined as half the internuclear distance between two combined atoms in a molecule.

Recall · 13 of 14

Why does atomic size increase down a group?

Model answer: Atomic size increases down a group because a new electron shell is added at each successive step. The effect of the increased number of shells outweighs the effect of increased nuclear charge.

Recall · 14 of 14

Why is a cation smaller than its parent atom?

Model answer: A cation is formed by the loss of electrons. This reduces electron-electron repulsion and allows the nucleus to attract the remaining electrons more strongly. Therefore, the cation is smaller.

11✗Traps

Mistakes that cost marks

Read the wrong statement, decide the fix in your head, then tap to confirm.

Although electrons are added, they enter the same main shell. Nuclear charge also increases and pulls the electrons closer. Therefore, atomic size decreases.

Nuclear charge does increase, but new shells are also added. The effect of the additional shells is greater, so atomic size increases.

A positive ion forms by losing electrons. Fewer electrons and stronger attraction make it smaller than its parent atom. The charge is a consequence of losing electrons, not a cause of swelling.

An anion has extra electrons. The additional electron-electron repulsion causes the electron cloud to expand. Therefore, an anion is larger.

Isoelectronic ions have the same number of electrons but different numbers of protons. The ion with more protons is smaller — which is why O²⁻ > F⁻ > Na⁺ > Mg²⁺.

12EExam

Exam notes

Definitions to learn word-for-word

Atomic size / atomic radius

Atomic size or atomic radius is the distance between the centre of the nucleus of an atom and its outermost shell.

Atomic radius — alternative definition

Atomic radius is half the internuclear distance between two combined atoms in a molecule.

Internuclear distance

The distance between the nuclei of two combined atoms.

Isoelectronic ions

Isoelectronic ions are ions having the same number of electrons.

The two trend answers, written out

Down a group

Atomic size increases down a group because a new electron shell is added at each successive step. The effect of the increased number of shells outweighs the effect of increased nuclear charge.

Across a period

Atomic size decreases from left to right across a period because nuclear charge increases while the number of occupied shells remains the same. The increased nuclear charge pulls the outermost electrons closer to the nucleus.

Important “give reason” answers

Why is potassium larger than sodium?

Potassium has one more occupied electron shell than sodium. Therefore, its outermost shell is farther from the nucleus and its atomic size is greater.

Why is fluorine smaller than lithium?

Lithium and fluorine are in the same period and have the same number of occupied shells. Fluorine has a greater nuclear charge, which pulls its electrons closer to the nucleus. Therefore, fluorine is smaller.

Why is a cation smaller than its parent atom?

A cation is formed by the loss of electrons. This reduces electron-electron repulsion and allows the nucleus to attract the remaining electrons more strongly. Therefore, the cation is smaller.

Why is an anion larger than its parent atom?

An anion is formed by the gain of electrons. The added electrons increase electron-electron repulsion, causing the electron cloud to expand. Therefore, the anion is larger.

Why is Mg²⁺ smaller than Na⁺?

Mg²⁺ and Na⁺ have the same number of electrons, but Mg²⁺ has a greater number of protons. Therefore, its nucleus pulls the electrons more strongly, making Mg²⁺ smaller.

Memory tricks worth keeping

Periodic trends

Down the group → Size goes UP Across the period → Size goes DOWN Atomic size increases: ←────── ↓

Atomic size is generally greater towards the lower-left side of the table discussed in the chapter.

Ions

Lose electrons → Less size Gain electrons → Greater size

Positive pulls in; negative spreads out.

Isoelectronic ions

Same electrons: more protons means smaller size.

13QPractice

Practice questions

Level 1 — easy

Define atomic radius.
Answer
Atomic radius is the distance between the centre of the nucleus of an atom and its outermost shell. It may also be defined as half the internuclear distance between two combined atoms in a molecule.
State the unit picometre in metres.
Answer
1 pm = 10⁻¹² metre.
What happens to atomic size down a group?
Answer
It increases.
What happens to atomic size across a period?
Answer
It decreases from left to right.
Which is larger: Li or Na?
Answer
Sodium. Both are Group 1 elements, and sodium lies below lithium with one extra shell. (Li 152 pm, Na 186 pm.)
Which is smaller: B or F?
Answer
Fluorine. Both are Period 2 elements; fluorine is farther to the right and has the greater nuclear charge. (B 88 pm, F 64 pm.)
Which is larger: Cl or Cl⁻?
Answer
Cl⁻, because an anion is larger than its parent atom.
Which is smaller: Mg or Mg²⁺?
Answer
Mg²⁺, because a cation is always smaller than its parent atom.

Level 2 — understanding

Explain why atomic size increases down Group 1.
Answer
Going down Group 1, one new electron shell is added at each step, so the outermost shell lies farther from the nucleus. Nuclear charge also increases, but the effect of the additional shells is greater. Therefore atomic size increases: H < Li < Na < K < Rb < Cs.
Explain why atomic size decreases from lithium to fluorine.
Answer
Lithium and fluorine are both Period 2 elements, so the number of occupied shells stays at two throughout. The atomic number increases from 3 to 9, so nuclear charge increases and pulls the outermost electrons closer. Atomic size therefore falls from 152 pm to 64 pm.
Why is sodium larger than chlorine?
Answer
Both are Period 3 elements with three occupied shells. Chlorine has a greater nuclear charge (17 protons against sodium's 11), so it pulls its electrons closer. Sodium is therefore larger: 186 pm against 99 pm.
Why is potassium larger than lithium?
Answer
Both belong to Group 1, but potassium has four occupied shells while lithium has only two. Its outermost shell is much farther from the nucleus, so potassium is larger (231 pm against 152 pm).
Why is Na⁺ smaller than Na?
Answer
Na⁺ is formed by the loss of one electron, and that electron was the only one in sodium's third shell — so a whole shell is lost. Fewer electrons also means less electron-electron repulsion, so the nucleus holds the remaining electrons more strongly.
Why is O²⁻ larger than O?
Answer
O²⁻ is formed by the gain of two electrons. The number of protons is unchanged, so the extra electrons increase electron-electron repulsion and the electron cloud expands. An anion is larger than its parent atom.
What are isoelectronic ions?
Answer
Isoelectronic ions are ions having the same number of electrons. For example O²⁻, F⁻, Na⁺ and Mg²⁺ each have ten electrons.
Why do isoelectronic ions not necessarily have the same size?
Answer
Although they have the same number of electrons, they have different numbers of protons. The ion with the greater nuclear charge attracts the same ten electrons more strongly and is therefore smaller.

Level 3 — arrange in increasing atomic size

Li, Na, K
Answer
Li < Na < K — down Group 1, size increases.
F, O, N
Answer
F < O < N — all Period 2. Moving right to left, nuclear charge falls, so size rises: F 64 pm, O 66 pm, N 70 pm.
Cl, Br, I
Answer
Cl < Br < I — down Group 17, size increases: 99, 114, 133 pm.
Na, Mg, Al
Answer
Al < Mg < Na — all Period 3. Aluminium is farthest right and has the greatest nuclear charge of the three: 143, 160, 186 pm.

Level 3 — arrange in decreasing atomic size

Li, Be, B, C
Answer
Li > Be > B > C — across Period 2, size decreases: 152, 112, 88, 77 pm.
F, Cl, Br
Answer
Br > Cl > F — down Group 17 size increases, so read the group upwards for decreasing size.
Na, Mg, Si, Cl
Answer
Na > Mg > Si > Cl — across Period 3, size decreases: 186, 160, 117, 99 pm.
O²⁻, F⁻, Na⁺, Mg²⁺
Answer
O²⁻ > F⁻ > Na⁺ > Mg²⁺ — isoelectronic ions; the greater the nuclear charge, the smaller the ion (8, 9, 11 and 12 protons).

ICSE-style questions

State how atomic size varies (i) down a group and (ii) across a period. Give a reason for each variation.
Answer

(i) Down a group, atomic size increases. A new electron shell is added at each successive step, so the outermost shell lies farther from the nucleus. Nuclear charge also increases, but the effect of the increased number of shells outweighs it.

(ii) Across a period, atomic size decreases. Nuclear charge increases while the number of occupied shells remains the same, so the outermost electrons are pulled closer to the nucleus.
The atomic radii of some Period 2 elements are 152 pm, 112 pm, 88 pm, 77 pm, 70 pm, 66 pm, 64 pm. Explain why the values gradually decrease from left to right.
Answer
These values belong to Li, Be, B, C, N, O and F. All are Period 2 elements, so every one of them has two occupied shells and no new shell is added. However, the atomic number increases by one from element to element, so nuclear charge increases steadily. The stronger nucleus attracts the outermost electrons more strongly and pulls them closer, so the atomic radius falls gradually from 152 pm to 64 pm.
Choose the larger particle in each pair and give a reason: (i) Na or Na⁺, (ii) Cl or Cl⁻, (iii) Li or K, (iv) Mg or Cl, (v) F⁻ or Na⁺.
Answer

(i) Na — Na⁺ is a cation formed by the loss of an electron, and a cation is always smaller than its parent atom.

(ii) Cl⁻ — an anion is larger than its parent atom, because the extra electron increases electron-electron repulsion and the electron cloud expands.

(iii) K — both are Group 1 elements, and potassium has two more occupied shells than lithium; atomic size increases down a group.

(iv) Mg — both are Period 3 elements, and chlorine has the greater nuclear charge, so it pulls its electrons closer; size decreases across a period.

(v) F⁻ — the two ions are isoelectronic (ten electrons each), but F⁻ has only 9 protons against Na⁺'s 11, so its weaker nuclear charge holds the electrons less tightly.
The ions O²⁻, F⁻, Na⁺ and Mg²⁺ each possess ten electrons. (i) What name is given to such ions? (ii) Arrange them in decreasing order of size. (iii) Explain the order.
Answer
See the model answer written out immediately below.

Model answer to question 28

Write it like this

The ions are called isoelectronic ions because they contain the same number of electrons.

Their decreasing order of size is:

O²⁻ > F⁻ > Na⁺ > Mg²⁺

They contain the same number of electrons, but their nuclear charges increase from O²⁻ to Mg²⁺. A greater nuclear charge attracts the electrons more strongly and therefore produces a smaller ionic size.

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One-minute revision

ATOMIC SIZE │ ├── Definition: │ Distance from nucleus to outermost shell │ ├── Also: │ Half the internuclear distance │ ├── Units: │ Å and pm │ ├── Depends on: │ Number of shells │ Nuclear charge │ ├── Down a group: │ Shells increase │ Size increases │ ├── Across a period: │ Shells remain the same │ Nuclear charge increases │ Size decreases │ ├── Cation: │ Smaller than parent atom │ ├── Anion: │ Larger than parent atom │ └── Isoelectronic ions: More protons → smaller size

The whole lesson as one map

ATOMIC SIZE │ ┌──────────────────┴──────────────────┐ │ │ Atom trends Ion trends │ │ ┌───────┴────────┐ ┌────────┴────────┐ │ │ │ │ Down a group Across a period Cation Anion │ │ │ │ Shells increase Nuclear charge Smaller Larger Size increases increases Size decreases │ Isoelectronic ions │ Same number of electrons │ More protons → smaller ion

Preview — Lesson 5

You have just found that the largest atoms sit towards the lower-left of the table. In Lesson 5 — Metallic Character and Non-Metallic Character you will meet a second trend that runs in exactly the same direction, because an atom that holds its outer electrons loosely is also the one that gives them away most easily. Do not use that idea in an answer yet — this lesson is marked on size and its two causes, shells and nuclear charge.

Mastery check

I can say this aloud, without looking: “Atomic size is the distance from the nucleus to the outermost shell. It increases down a group because new electron shells are added. It decreases across a period because nuclear charge increases while the number of shells remains the same. A cation is smaller than its parent atom, while an anion is larger. Among isoelectronic ions, the ion with the greatest nuclear charge is the smallest.”

Next → Lesson 5: Metallic Character and Non-Metallic Character