Chapter 1 · Periodic Table, Periodic Properties and Variations of Properties
Complete Chapter Revision and ICSE Practice
Lesson 12 — every definition, every trend and every question type in the chapter, in one place.
Work through it on screen · tap every question · print it as your final revision file
By the end of this lesson you can
state every definition in the chapter in the wording ICSE expects;
name the four scientists and say what each one contributed;
find period, group and valency from an atomic number or an electronic configuration;
calculate protons, electrons and neutrons from atomic number and mass number;
state and explain every trend across a period and down a group;
compare ionisation energy, electron affinity and electronegativity;
arrange elements and ions in order of size, reactivity and electronegativity;
answer give-reason and correct-the-statement questions in full marks form.
01SStory
The whole chapter in one story
Eleven lessons, one idea. Line the elements up by atomic number and their behaviour starts to repeat — and everything else in this chapter is a consequence of that.
The Periodic Table arranges elements in increasing order of atomic number.
Increasing atomic number
↓
Electronic configurations change regularly
↓
Similar outer configurations reappear
↓
Similar properties reappear
↓
Periodicity
An element's behaviour depends mainly on:
the number of occupied shells;
the number of valence electrons;
atomic size;
nuclear charge.
These four factors control whether an atom:
loses electrons;
gains electrons;
attracts shared electrons;
reacts easily or with difficulty.
How to use this lesson
Do not read it straight through like a chapter. Answer every check before you look, use the two calculators to test yourself on configurations you make up, and only then print it. The printed copy comes out with every answer opened.
Check your understanding · 1 of 23
The modern Periodic Table arranges the elements in increasing order of:
Correct answer: B — atomic number. Atomic mass was Mendeleev's basis. Moseley established atomic number as the modern basis, and the Modern Periodic Law is stated in terms of atomic numbers.
02DDefine
Every definition, word for word
Sixteen definitions carry most of the definition marks in this chapter. Learn the wording, not the gist — a paraphrase that drops "isolated gaseous atom" loses the mark.
1 · Modern Periodic Law
The physical and chemical properties of elements are periodic functions of their atomic numbers.
2 · Periodic Table
A tabular arrangement of elements in groups and periods, showing regular trends in their properties.
3 · Group
A vertical column in the Periodic Table. There are 18 groups.
4 · Period
A horizontal row in the Periodic Table. There are 7 periods.
5 · Periodicity
The recurrence of similar properties at regular intervals when elements are arranged in increasing order of atomic number.
6 · Periodic properties
Properties that reappear at regular intervals or show regular variation in the Periodic Table.
The periodic properties named in this chapter are atomic size, metallic character, non-metallic character, ionisation energy, electron affinity and electronegativity.
7 · Valence electrons
The electrons present in the outermost shell of an atom.
8 · Valency
The combining capacity of an atom.
9 · Atomic radius
The distance between the centre of the nucleus and the outermost shell of an atom. It may also be defined as half the internuclear distance between two combined atoms.
10 · Metallic character
The tendency of an atom to lose valence electrons and form a positive ion.
11 · Non-metallic character
The tendency of an atom to gain electrons and form a negative ion.
12 · Ionisation energy
The energy required to remove an electron from a neutral isolated gaseous atom and convert it into a positively charged gaseous ion.
M(g) + I.E. → M⁺(g) + e⁻
13 · Electron affinity
The energy released when a neutral isolated gaseous atom gains an electron and forms a negatively charged gaseous ion.
X(g) + e⁻ → X⁻(g) + E.A.
14 · Electronegativity
The tendency of an atom in a molecule to attract the shared pair of electrons towards itself. It has no unit.
15 · Atomic number
The number of protons present in the nucleus of an atom.
Z = Number of protons
For a neutral atom:
Number of protons = Number of electrons
16 · Mass number
The sum of the numbers of protons and neutrons present in the nucleus.
A = p + n
Therefore:
n = A − Z
The words examiners look for
Ionisation energy:required, remove, neutral isolated gaseous atom. Electron affinity:released, gains. Electronegativity:in a molecule, shared pair. Miss the gaseous-atom phrase and the definition is only half-right.
Type them out
Write from memory, then compare with the model answer.
Definition check · 2 of 23
State the Modern Periodic Law.
Model answer: The physical and chemical properties of elements are periodic functions of their atomic numbers.
Definition check · 3 of 23
Define periodicity.
Model answer: The recurrence of similar properties at regular intervals when elements are arranged in increasing order of atomic number.
Definition check · 4 of 23
Define ionisation energy.
Model answer: The energy required to remove an electron from a neutral isolated gaseous atom and convert it into a positively charged gaseous ion.
Definition check · 5 of 23
Define electronegativity.
Model answer: The tendency of an atom in a molecule to attract the shared pair of electrons towards itself.
03HHistory
Four scientists, four steps
Four names, and one thing to remember about each. Tap a card to open it.
Grouped similar elements into triads — sets of three elements with related properties.
Proposed the Law of Octaves: arranged in order, every eighth element resembled the first, like the notes of a musical scale.
Arranged the elements mainly according to atomic mass. This is the single most tested distinction in the chapter — Mendeleev used mass, not number.
Established atomic number as the modern basis of classification. Everything in the modern table follows from his work.
Döbereiner → Newlands → Mendeleev → Moseley
Memory sentence
Do Not Miss Meals. Döbereiner, Newlands, Mendeleev, Moseley — in that order.
Check your understanding · 6 of 23
Who established atomic number as the modern basis for classifying the elements?
Correct answer: D — Moseley. Mendeleev is the tempting wrong answer: he built the famous table, but he arranged it mainly by atomic mass. Moseley replaced mass with atomic number.
04FFamily
Groups and their family names
Eighteen columns, and nine names worth memorising. Questions often ask for the family rather than the group number.
The group families
Group
Family name
1
Alkali metals
2
Alkaline earth metals
3–12
Transition elements
13
Boron family
14
Carbon family
15
Nitrogen family
16
Oxygen family or chalcogens
17
Halogens
18
Noble gases or inert gases
The representative or normal elements are found in:
Groups 1, 2 and 13–18
Those are exactly the groups where the simple valence-electron rule for finding the group works. The transition elements in the middle are not assigned by that rule.
Check your understanding · 7 of 23
An element has seven electrons in its outermost shell. Which family does it belong to?
Correct answer: C — the halogens. Seven valence electrons gives group 10 + 7 = 17, and Group 17 is the halogen family. Chalcogens are Group 16, with six valence electrons.
05AAddress
Period, group and valency in four steps
Given any electronic configuration, run the same four steps in the same order every time. Here it is on 2, 8, 6.
Step 1 · Find the period
Count the occupied shells:
3 occupied shells → Period 3
Step 2 · Find the valence electrons
The last shell contains:
6 valence electrons
Step 3 · Find the group
For representative elements with 3–8 valence electrons:
Group number = 10 + valence electrons
Group = 10 + 6 = 16
Step 4 · Find the valency
Valency = 8 − 6 = 2
Final answer
Period 3, Group 16, Valency 2.
Quick group rule
Valence electrons to group, for representative elements
Valence electrons
Group
1
1
2
2
3
13
4
14
5
15
6
16
7
17
8
18
Quick valency rule
1, 2, 3 or 4 valence electrons:
Valency = same number
5, 6 or 7 valence electrons:
Valency = 8 − valence electrons
Complete outer shell:
Valency = 0
Across a period this produces the rise and fall you have already met:
Valency: 1 → 2 → 3 → 4 → 3 → 2 → 1 → 0
The element profile calculator
Pick an element or type any atomic number and mass number, then step through the whole profile one line at a time.
What the calculator will and will not do
It fills shells by the 2, 8, 8, 2 rule this chapter uses, so it is reliable for atomic numbers 1 to 20. Beyond calcium the transition elements begin and the simple filling order no longer holds.
Check your understanding · 8 of 23
An element has the electronic configuration 2, 8, 6. Its period, group and valency are:
Correct answer: B. Three occupied shells → Period 3. Six valence electrons → Group 10 + 6 = 16. Six is more than four, so valency = 8 − 6 = 2. Option D is the classic slip of reporting the valence electrons as the valency.
06TTrends
The master trend tables
Two tables carry most of the marks in this chapter. Everything else is an explanation of a line in one of them.
Across a period: left to right
Property
Trend
Number of shells
Remains the same
Valence electrons
Increase from 1 to 8
Valency
1 → 4, then 4 → 0
Atomic size
Decreases
Metallic character
Decreases
Non-metallic character
Increases
Ionisation energy
Generally increases
Electron affinity
Generally increases
Electronegativity
Increases
Chemical reactivity
First decreases, then increases
Nature of oxides
Basic → amphoteric → acidic
Melting and boiling points
Usually increase to Group 14, then decrease
Density
Increases towards a maximum, then may decrease
Down a group: top to bottom
Property
Trend
Number of shells
Increases
Valence electrons
Remain the same
Valency
Remains the same
Atomic size
Increases
Metallic character
Increases
Non-metallic character
Decreases
Ionisation energy
Decreases
Electron affinity
Generally decreases
Electronegativity
Decreases
Reactivity of metals
Increases
Reactivity of non-metals
Decreases
Density
Generally increases
The trend lab
Tap a property to see both directions at once, with the reason underneath.
IncreasesDecreasesStays the same, or a mixed pattern
Sort these — across a period, or down a group?
One tap each. Answers lock in.
Atomic size decreases
The number of occupied shells increases
Metallic character increases
Ionisation energy decreases
Electronegativity increases
The number of valence electrons remains the same
Non-metallic character increases
Chemical reactivity first decreases and then increases
Check your understanding · 9 of 23
Moving from left to right across a period, atomic size:
Correct answer: B — it decreases. Option A states the trap directly: electrons are indeed being added, but they go into the same shell while the nuclear charge keeps rising, so the whole electron cloud is pulled in tighter.
07?Why
Why the trends occur
Two chains of reasoning. Learn these and you can rebuild any row of the master tables from scratch.
Across a period
Number of shells remains the same
+
Nuclear charge increases
↓
Electrons pulled closer
↓
Atomic size decreases
That single result then drives everything else:
Metallic character decreases
Non-metallic character increases
Ionisation energy increases
Electron affinity generally increases
Electronegativity increases
Down a group
New electron shells are added
↓
Atomic size increases
↓
Outermost electrons become farther from nucleus
And again, the consequences follow:
Metallic character increases
Non-metallic character decreases
Ionisation energy decreases
Electronegativity decreases
The one-line version
Down, more floors; across, stronger pull. Going down you add shells, so the atom grows. Going across you add protons without adding shells, so the atom shrinks.
Check your understanding · 10 of 23
Why does atomic size decrease across a period?
Correct answer: B. Both halves matter and a full-mark answer states both: rising nuclear charge and an unchanged number of shells. Shells are never removed across a period — that is what makes it one period.
08RRadius
Atomic size and ionic size
Atoms shrink across and grow down. Ions are a separate question — and the answer depends entirely on whether electrons were lost or gained.
Atomic size
Across a period:
Atomic size decreases →
Li > Be > B > C > N > O > F
Down a group:
Atomic size increases ↓
Li < Na < K < Rb < Cs
More electrons
↓
Greater electron-electron repulsion
↓
Electron cloud expands
↓
Larger size
Isoelectronic ions
Ions with the same number of electrons are called isoelectronic ions.
O²⁻, F⁻, Na⁺, Mg²⁺
All four have ten electrons. When the electron count is tied, the nucleus decides:
Among isoelectronic ions, greater nuclear charge means smaller size.
O²⁻ > F⁻ > Na⁺ > Mg²⁺
Check your understanding · 11 of 23
Which is larger, a sodium atom or a sodium ion?
Correct answer: A — the atom. Na⁺ is a cation: it has lost an electron, so repulsion between the remaining electrons drops and the nucleus draws them in. A cation is always smaller than its parent atom.
Check your understanding · 12 of 23
Among the isoelectronic ions O²⁻, F⁻, Na⁺ and Mg²⁺, which is the largest?
Correct answer: A — O²⁻. All four have ten electrons, so size is decided by nuclear charge: oxygen has the fewest protons (8) and therefore the weakest pull on those ten electrons. The full order is O²⁻ > F⁻ > Na⁺ > Mg²⁺.
09MMetal
Metallic and non-metallic character
One is the tendency to give electrons away; the other is the tendency to take them. They move in opposite directions everywhere in the table.
Metallic character
Atom loses electrons
↓
Positive ion forms
↓
Metallic character
Across a period: decreases
Down a group: increases
Non-metallic character
Atom gains electrons
↓
Negative ion forms
↓
Non-metallic character
Across a period: increases
Down a group: decreases
Period 3, seen in one line
Na → Mg → Al → Si → P → S → Cl
Metallic character:
High ─────────────────────→ Low
Non-metallic character:
Low ──────────────────────→ High
Memory line
Across, metal fades; down, metal grows. The reason is size: a big atom holds its outer electrons loosely and gives them up easily, which is exactly what being metallic means.
Check your understanding · 13 of 23
Moving down a group, metallic character:
Correct answer: A. Option C is tempting because valence electrons genuinely do stay the same down a group — but metallic character depends on how easily those electrons are lost, and a larger atom loses them more readily.
10RxReact
Reactivity
The one trend that is not a straight line. Across a period reactivity dips in the middle and climbs again — write "it increases" and you lose the mark.
Across a period
Chemical reactivity:
First decreases
Then increases
Na → Mg → Al → Si → P → S → Cl
Reactive metal Least reactive Reactive non-metal
Silicon is shown as the least reactive element in this sequence. The metals on the left react by losing electrons and the non-metals on the right react by gaining them; the elements in the middle do neither easily.
Down Group 1
Li < Na < K < Rb < Cs
Reactivity increases because electron loss becomes easier as the atom grows.
Down Group 17
F > Cl > Br > I
Reactivity decreases because gaining an electron becomes more difficult as the atom grows.
The pair that trips everyone
Down a group, metal reactivity increases but non-metal reactivity decreases. Both follow from the same fact — the atom is getting bigger — because metals need to lose and non-metals need to gain.
Check your understanding · 14 of 23
Going from sodium to chlorine across Period 3, chemical reactivity:
Correct answer: C. Sodium is a highly reactive metal, silicon in the middle is the least reactive of the sequence, and chlorine at the far end is a highly reactive non-metal. Any answer describing a single direction is wrong.
11EEnergy
Ionisation energy, electron affinity, electronegativity
Three properties that sound alike and are constantly swapped in exams. One removes an electron, one adds an electron, one only pulls on a shared pair.
1 · Ionisation energy
Energy needed to remove an electron.
Low I.E. → Electron removed easily
High I.E. → Electron removed with difficulty
Across a period:
Generally increases, with exceptions
Down a group:
Decreases
Values stated in the chapter
Helium: highest ionisation energy
Caesium: lowest determined ionisation energy
Metals → Usually low I.E.
Non-metals → Usually high I.E.
2 · Electron affinity
Energy released when an electron is added to a neutral isolated gaseous atom.
Across a period:
Generally increases
Down a group:
Generally decreases
Group 17 → High electron affinity
The two exceptions you must know
Electron affinity of Cl > F
Electron affinity of S > O
The chapter explains these through the small size and greater electron-electron repulsion in fluorine and oxygen: their outer shells are so tightly packed that an incoming electron is partly pushed away.
The chapter's table assigns the noble gases zero electron affinity, because their outer shells are already complete.
3 · Electronegativity
Electron-pulling power of an atom in a molecule.
Dimensionless
No unit
Pauling scale
Fluorine = 4.0, highest
Caesium = 0.7, lowest value stated
Across a period:
Increases
Down a group:
Decreases
Diagonal relationship
The chapter lists three diagonal pairs from Periods 2 and 3 that resemble each other in certain properties:
Li–Mg
Be–Al
B–Si
The three side by side
The comparison examiners ask for
Property
Ionisation energy
Electron affinity
Electronegativity
What happens?
Electron removed
Electron added
Shared electrons attracted
Atom considered
Neutral isolated gaseous atom
Neutral isolated gaseous atom
Atom in a molecule
Ion formed
Positive ion
Negative ion
No complete ion formation required
Energy
Supplied
Released according to the source definition
Only a tendency
Unit
eV/atom, kJ mol⁻¹
eV/atom, kJ mol⁻¹
No unit
Across period
Generally increases
Generally increases
Increases
Down group
Decreases
Generally decreases
Decreases
Sort these — which property is being described?
One tap each. Answers lock in.
An electron is removed from a neutral isolated gaseous atom
An electron is added to a neutral isolated gaseous atom
A shared pair of electrons is attracted inside a molecule
It is dimensionless and has no unit at all
Helium has the highest value; caesium the lowest determined value
Chlorine's value is greater than fluorine's, against the general trend
Check your understanding · 15 of 23
Moving down a group, ionisation energy:
Correct answer: B — it decreases. Each step down adds a shell, so the outermost electron sits farther from the nucleus and is held less tightly. Less energy is needed to remove it.
Check your understanding · 16 of 23
Which has the greater electron affinity, chlorine or fluorine?
Correct answer: A — chlorine. This is a stated exception to the general trend. Fluorine's outer shell is very small and crowded, so an incoming electron meets greater electron-electron repulsion. Sulphur beats oxygen for the same reason. (Careful: fluorine still wins on electronegativity.)
Check your understanding · 17 of 23
The unit of electronegativity is:
Correct answer: C — no unit. Electronegativity is dimensionless; it is a tendency, measured on a comparative scale where fluorine is 4.0. Options A and B are the units of ionisation energy and electron affinity.
12ZZ and A
Atomic number and mass number
Numerical questions almost always start here: two numbers on the symbol, and everything else follows.
Protons = Z
Electrons = Z
for a neutral atom
Neutrons = A − Z
Mass number:
A = p + n
Worked example — chlorine-35
For an atom written as mass number 35, atomic number 17:
Full solution
Protons = 17 (that is the atomic number).
Electrons = 17, because the atom is neutral.
Neutrons = 35 − 17 = 18.
Electronic configuration = 2, 8, 7.
Period = 3 (three occupied shells). Group = 10 + 7 = 17. Valency = 8 − 7 = 1.
The element is chlorine, a halogen.
The light-element pattern
For the first few elements the chapter notes a rough pattern between A and Z, with named exceptions.
Even atomic number
A is often 2Z
Exceptions listed: Be and Ar
Odd atomic number
A is often 2Z + 1
Exceptions listed: N and H
Use it as a check, not a rule
This pattern is a sanity check on an answer you have already worked out — it is not a way of finding the mass number. If a question gives you A, use the given value.
Check your understanding · 18 of 23
An atom has mass number 35 and atomic number 17. How many neutrons does it contain?
Correct answer: B — 18. Neutrons = A − Z = 35 − 17 = 18. Option A gives the protons and the electrons; option D adds the two numbers instead of subtracting.
13171 vs 17
Alkali metals versus halogens
The two ends of the table, and a favourite comparison question. One group gives an electron away; the other takes one.
Group 1 against Group 17
Property
Alkali metals
Halogens
Group
1
17
Valence electrons
1
7
Electron behaviour
Lose one
Gain one
Ion formed
M⁺
X⁻
Nature
Metals
Non-metals
Conduction
Good conductors
Non-conductors
Ionisation energy
Low
High
Electron affinity
Low
High
Electronegativity
Low
High
Reactivity down group
Increases
Decreases
Chemical role
Reducing agents
Oxidising agents
Atomic size down group
Increases
Increases
Melting and boiling points down group
Decrease
Increase
Memory line
Alkali gives; halogen takes. An element that gives electrons away is a reducing agent; one that takes them is an oxidising agent.
Sort these — alkali metals or halogens?
One tap each. Answers lock in.
Seven valence electrons
Forms M⁺ ions
Acts as a reducing agent
Reactivity decreases down the group
Good conductors of electricity
High ionisation energy, electron affinity and electronegativity
Check your understanding · 19 of 23
The alkali metals act as:
Correct answer: A — reducing agents. They lose their single valence electron readily and hand it to something else. The halogens, which take an electron, are the oxidising agents.
14ΣMap
One-minute trend map and memory tricks
If you have one minute before the paper starts, read this page and nothing else.
ACROSS A PERIOD →
Atomic size ↓
Metallic character ↓
Non-metallic character ↑
Ionisation energy ↑ generally
Electron affinity ↑ generally
Electronegativity ↑
Valence electrons ↑
Reactivity ↓ then ↑
DOWN A GROUP ↓
Atomic size ↑
Metallic character ↑
Non-metallic character ↓
Ionisation energy ↓
Electron affinity ↓ generally
Electronegativity ↓
Metal reactivity ↑
Non-metal reactivity ↓
Eight memory tricks
1 · Group and period
Group goes down.
Period passes across.
2 · Atomic size
Down, more floors; across, stronger pull.
3 · Metallic character
Across, metal fades; down, metal grows.
4 · Ionisation energy
Across it climbs; down it declines.
5 · Electron affinity
Across it generally attracts more; down the attraction becomes weaker.
6 · Electronegativity
Across it grows; down it slows.
7 · Ions
Lose electrons → Smaller cation
Gain electrons → Larger anion
8 · Alkali metals and halogens
Alkali gives; halogen takes.
15✗Traps
Mistakes that cost marks
Read the wrong statement, decide the fix in your head, then tap to confirm.
Fifteen common traps
Mendeleev used atomic mass. Moseley established atomic number as the modern basis.
A group is vertical; a period is horizontal. Group goes down, period passes across.
For oxygen, valence electrons = 6 but valency = 2. They are not the same.
Do not use the total number of electrons. Period number = number of occupied shells, so sodium is in Period 3.
It means Group 17, not Group 7 — add ten once you are past two valence electrons.
Atomic size decreases, because nuclear charge increases while the number of shells remains the same.
A cation is smaller than its parent atom; an anion is larger.
It first decreases and then increases. Do not write that it moves in a single direction.
Energy is supplied to remove an electron. That is what ionisation energy means.
According to the source definition, energy is released when an electron is added.
It has no unit, and it applies to an atom in a molecule.
Not at electron affinity: Cl > F, and S > O. Fluorine does win on electronegativity.
Down Group 1 metal reactivity increases; down Group 17 non-metal reactivity decreases.
Hydrogen is a non-metal, even though it is placed above Group 1.
Mass number = protons + neutrons. Neutrons alone are A − Z.
Correct the statement
Eight statements as a careless student would write them. Decide the correction, then tap.
Groups are vertical columns.
Mendeleev arranged elements mainly according to atomic mass.
Atomic size decreases across a period.
A cation is smaller than its parent atom.
Electronegativity is dimensionless and has no unit.
Halogen reactivity decreases down Group 17.
Alkali metals act as reducing agents.
Mass number equals the number of protons plus neutrons.
16QRapid
Rapid oral quiz
Twenty questions. Answer each one aloud without looking back, then open the strip to check.
1 · What is the basis of the modern Periodic Table?
Atomic number.
2 · How many groups and periods are present?
18 groups and 7 periods.
3 · What determines the period number?
The number of occupied shells.
4 · What determines the group of a representative element?
The number of valence electrons.
5 · Why do elements in the same group have similar chemical properties?
They have similar outer electronic configurations.
6 · What happens to atomic size across a period?
It decreases.
7 · What happens to atomic size down a group?
It increases.
8 · Which is larger: Na or Na⁺?
Na.
9 · Which is larger: Cl or Cl⁻?
Cl⁻.
10 · What happens to metallic character across a period?
It decreases.
11 · What happens to non-metallic character down a group?
It decreases.
12 · What happens to ionisation energy down a group?
It decreases.
13 · Which element has the highest electronegativity?
Fluorine.
14 · Does electronegativity have a unit?
No.
15 · Which has greater electron affinity: chlorine or fluorine?
Chlorine.
16 · What is the valency of an element with seven valence electrons?
One.
17 · What is the formula for neutrons?
n = A − Z
18 · Which group contains alkali metals?
Group 1.
19 · Which group contains halogens?
Group 17.
20 · Which family acts as oxidising agents?
Halogens.
17PPractice
Practice questions
Level 1 — easy
An element has electronic configuration 2, 8, 3. Find (i) its period, (ii) its group, (iii) its valence electrons, (iv) its valency, (v) whether it is more metallic or non-metallic than chlorine in the same period.
Answer
Period = 3 · Group = 13 · Valence electrons = 3 · Valency = 3. It is more metallic than chlorine, because metallic character decreases from left to right and this element lies to the left of chlorine in Period 3. (It is aluminium.)
An element has configuration 2, 7. State (i) its group, (ii) its period, (iii) its valency, (iv) its family, (v) the charge on its ion.
Answer
Group = 17 · Period = 2 · Valency = 1 · Family = halogens · Ion charge = −1. (It is fluorine.)
Arrange them — tap the chips into order
Tap chips into the slot, first to last. Tap a placed chip to take it back. It checks itself once the last one is placed.
Arrange · easy 3
Arrange in increasing atomic size: Li, Na, K.
Smallest first
Correct order: Li < Na < K. All three are Group 1. Down a group each element has one more occupied shell, so the atom grows.
Arrange · easy 4
Arrange in decreasing electronegativity: F, Cl, Br, I.
Most electronegative first
Correct order: F > Cl > Br > I. Electronegativity decreases down a group as the atom grows and its pull on a shared pair weakens. Fluorine is 4.0, the highest on the Pauling scale.
Level 2 — medium
Arrange · medium 1
Arrange in increasing metallic character: Na, Mg, Al, Si.
Least metallic first
Correct order: Si < Al < Mg < Na. These four are consecutive in Period 3, and metallic character decreases left to right — so reading right to left gives increasing metallic character.
Arrange · medium 2
Arrange in increasing ionisation energy: K, Na, Li.
Lowest ionisation energy first
Correct order: K < Na < Li. Ionisation energy decreases down a group, so the lowest belongs to the largest atom, potassium. Lithium holds its outer electron most tightly.
Arrange · medium 3
Arrange in decreasing non-metallic character: C, Si, Ge, Sn.
Most non-metallic first
Correct order: C > Si > Ge > Sn. All four are Group 14. Non-metallic character decreases down a group as the atom grows and gaining electrons becomes harder.
Arrange · medium 4
Arrange these isoelectronic ions in decreasing size: O²⁻, F⁻, Na⁺, Mg²⁺.
Largest first
Correct order: O²⁻ > F⁻ > Na⁺ > Mg²⁺. All four have ten electrons, so nuclear charge decides: 8, 9, 11 and 12 protons respectively. More protons pull the same ten electrons in tighter.
An element has atomic number 16 and mass number 32. Find (i) protons, (ii) electrons, (iii) neutrons, (iv) electronic configuration, (v) group, (vi) period, (vii) valency.
Answer
Elements in the same group have similar chemical properties.
Answer
They possess the same number of valence electrons and similar outer electronic configurations. Since chemical properties depend mainly on valence electrons, their chemical properties are similar.
Atomic size decreases across a period.
Answer
Nuclear charge increases while the number of occupied shells remains the same. The electrons are pulled closer to the nucleus, so atomic size decreases.
Atomic size increases down a group.
Answer
A new electron shell is added at each successive step. The outermost electrons become farther from the nucleus, so atomic size increases.
Metallic character increases down a group.
Answer
Atomic size increases and valence electrons are lost more easily. Therefore, metallic character increases.
Non-metallic character increases across a period.
Answer
Atomic size decreases and nuclear charge increases. The tendency to gain electrons therefore increases.
Ionisation energy decreases down a group.
Answer
Additional shells increase atomic size. The outermost electron is farther from the nucleus and is removed more easily.
Fluorine is more electronegative than chlorine.
Answer
Fluorine has a smaller atomic size and attracts the shared electron pair more strongly.
Chlorine has greater electron affinity than fluorine.
Answer
Fluorine's very small outer shell is crowded, so an incoming electron experiences greater electron-electron repulsion. Chlorine's larger shell produces less repulsion.
A cation is smaller than its parent atom.
Answer
Electron loss reduces electron-electron repulsion and allows the nucleus to attract the remaining electrons more strongly.
An anion is larger than its parent atom.
Answer
Electron gain increases electron-electron repulsion, causing the electron cloud to expand.
ICSE-standard mixed questions
Element X has atomic number 11, and element Y has atomic number 17. (i) Write their electronic configurations. (ii) State their groups and periods. (iii) State their valencies. (iv) Which is more metallic? (v) Which has greater ionisation energy? (vi) Which has greater electronegativity? (vii) What ions will they form?
Answer
Property
X
Y
Atomic number
11
17
Configuration
2, 8, 1
2, 8, 7
Group
1
17
Period
3
3
Valency
1
1
Ion
X⁺
Y⁻
X is more metallic. Y has greater ionisation energy. Y has greater electronegativity.
The elements A, B, C and D have the configurations A: 2, 1 · B: 2, 8, 1 · C: 2, 7 · D: 2, 8, 7. (i) Which belong to the same group? (ii) Which are alkali metals? (iii) Which are halogens? (iv) Which is larger, A or B? (v) Which is more electronegative, C or D? (vi) Which metal is more reactive? (vii) Which halogen is more reactive?
Answer
(i) A and B belong to Group 1; C and D belong to Group 17.
(ii) A and B. (iii) C and D.
(iv) B is larger — it has three shells against A's two.
(v) C is more electronegative, being the smaller atom higher up Group 17.
(vi) B is the more reactive metal — metal reactivity increases down a group.
(vii) C is the more reactive halogen — non-metal reactivity decreases down a group.
For the elements Na, Mg, Al, Si, P, S, Cl, state the direction of change from sodium to chlorine in (i) atomic size, (ii) metallic character, (iii) non-metallic character, (iv) ionisation energy, (v) electronegativity, (vi) reactivity.
Answer
(i) Atomic size decreases. (ii) Metallic character decreases. (iii) Non-metallic character increases. (iv) Ionisation energy generally increases. (v) Electronegativity increases.
(vi) Reactivity first decreases towards silicon and then increases towards chlorine.
ICSE-standard multiple choice
ICSE MCQ · 20 of 23
The element with the greatest atomic size is:
Correct answer: C — K. Li, Na and K are all Group 1 and size increases down a group, so potassium is the largest of those three. Fluorine is in Period 2 on the far right, where atoms are smallest of all.
ICSE MCQ · 21 of 23
The most electronegative element is:
Correct answer: C — fluorine. Fluorine is 4.0, the highest value on the Pauling scale. Remember that it loses only on electron affinity, where chlorine is greater.
ICSE MCQ · 22 of 23
The particle larger than its parent atom is:
Correct answer: C — Cl⁻. It is the only anion in the list. Gaining an electron increases repulsion and the electron cloud expands; the three cations have all lost electrons and are smaller than their atoms.
ICSE MCQ · 23 of 23
Ionisation energy generally:
Correct answer: C — increases across a period. Options A and B are both the trends reversed. Across a period the atom shrinks and holds its electrons harder; down a group it grows and holds them more loosely.
18✓Close
The final one-page revision sheet
MODERN PERIODIC TABLE
│
├── Basis: Increasing atomic number
├── 18 groups
├── 7 periods
│
├── Period number = Number of occupied shells
├── Group relates to valence electrons
└── Same group → Similar chemical properties
ACROSS A PERIOD →
│
├── Shells: Same
├── Valence electrons: Increase
├── Atomic size: Decreases
├── Metallic character: Decreases
├── Non-metallic character: Increases
├── Ionisation energy: Generally increases
├── Electron affinity: Generally increases
├── Electronegativity: Increases
└── Reactivity: First decreases, then increases
DOWN A GROUP ↓
│
├── Shells: Increase
├── Valence electrons: Same
├── Atomic size: Increases
├── Metallic character: Increases
├── Non-metallic character: Decreases
├── Ionisation energy: Decreases
├── Electron affinity: Generally decreases
├── Electronegativity: Decreases
├── Metal reactivity: Increases
└── Non-metal reactivity: Decreases
ION SIZE
│
├── Cation < Parent atom
├── Anion > Parent atom
└── Isoelectronic: More protons → Smaller ion
ATOMIC CALCULATIONS
│
├── Protons = Z
├── Electrons = Z
├── Neutrons = A − Z
└── Mass number = Protons + Neutrons
GROUP 1
│
├── One valence electron
├── Forms +1 ions
├── Low I.E., E.A. and E.N.
├── Reactivity increases down group
└── Reducing agents
GROUP 17
│
├── Seven valence electrons
├── Forms −1 ions
├── High I.E., E.A. and E.N.
├── Reactivity decreases down group
└── Oxidising agents
Final mastery check
End of Chapter 1 · Periodic Table, Periodic Properties and Variations of Properties
