Alkali Metals and Halogens | ICSE Class 10 Chemistry

011Concept

Two families at opposite ends

A plug and a socket are nothing alike, and that is exactly why they fit. Alkali metals and halogens are the chemistry version of that pair: one has a spare electron to hand over, the other has a gap that fits it perfectly.

Alkali metals and halogens sit near opposite ends of the Periodic Table.

Left side Right side Group 1 Group 17 Alkali metals Halogens Lose 1 electron Gain 1 electron Form +1 ions Form −1 ions

They behave almost like opposites:

alkali metals have one valence electron;
halogens have seven valence electrons;
alkali metals easily give away one electron;
halogens readily accept one electron.

That is why these two families are so useful for revision: learn one trend properly and you have very nearly learned the other, because it usually runs the opposite way. This lesson is one long comparison, so read every point in pairs.

How to use this lesson

Every concept from here on has an alkali-metal half and a halogen half. When you revise, cover one half with your hand and try to state it from the other.

022Concept

Meet the two families

Before comparing anything, you must be able to name the members. These are the elements the chapter lists.

2.1 The alkali metals — Group 1

Li, Na, K, Rb, Cs, Fr

Group 1 — the alkali metals
Symbol	Name
Li	Lithium
Na	Sodium
K	Potassium
Rb	Rubidium
Cs	Caesium
Fr	Francium

The trap in the first column

Hydrogen is printed above Group 1, but the chapter clearly treats it as a non-metal, not an alkali metal. Never list hydrogen as a member of this family.

2.2 The halogens — Group 17

F, Cl, Br, I, At

Group 17 — the halogens
Symbol	Name
F	Fluorine
Cl	Chlorine
Br	Bromine
I	Iodine
At	Astatine

Check your understanding · 1 of 16

Which of these is not an alkali metal?

Correct answer: B — hydrogen. It is printed above Group 1, but the chapter treats hydrogen as a non-metal. The alkali metals listed are Li, Na, K, Rb, Cs and Fr.

033Concept

Physical state and appearance

You could tell these two families apart from across the room. One is a soft shiny solid you can cut with a knife; the other changes from gas to liquid to solid as you read down the column.

3.1 The alkali metals

According to the comparison table, alkali metals are:

soft;
capable of being cut with a knife;
shiny when freshly cut;
soon dull, because they react with air.

The chapter states that lithium is the hardest among them.

Freshly cut alkali metal ↓ Shiny surface ↓ Reacts with air ↓ Surface becomes dull

3.2 The halogens

Appearance and physical state given in the chapter
Halogen	Appearance or physical state
Fluorine	Pale yellow gas
Chlorine	Poisonous yellowish-green gas
Bromine	Poisonous reddish-brown volatile liquid
Iodine	Dark-grey crystalline solid

The visible change down Group 17

Fluorine → Gas Chlorine → Gas Bromine → Liquid Iodine → Solid

This agrees with the earlier trend that the melting and boiling points of halogens increase down the group. A substance that melts and boils at a higher temperature is more likely to be found as a liquid or a solid at room temperature.

Check your understanding · 2 of 16

Which description matches bromine?

Correct answer: C. Bromine is the halogen that is a liquid at room temperature. Option A is fluorine, B is chlorine and D is iodine — worth learning all four together, because examiners ask for the colour as well as the state.

044Concept

Valence electrons and the ions they form

This is the most important comparison in the lesson. Everything else — the ions, the reactivity, the reducing and oxidising behaviour — is a consequence of these two numbers.

4.1 Alkali metals: one electron to give away

Alkali metals possess:

1 valence electron

Li: 2, 1 Na: 2, 8, 1 K: 2, 8, 8, 1

They lose this one electron and form positive ions carrying a single positive charge.

M → M⁺ + e⁻

Na → Na⁺ + e⁻ K → K⁺ + e⁻

Valency of alkali metals = 1 Ion formed = +1 cation

4.2 Halogens: one electron short

Halogens possess:

7 valence electrons

F: 2, 7 Cl: 2, 8, 7

They gain one electron and form negative ions carrying a single negative charge.

X + e⁻ → X⁻

F + e⁻ → F⁻ Cl + e⁻ → Cl⁻

Valency of halogens = 1 Ion formed = −1 anion

4.3 Opposite electron behaviour

ALKALI METAL HALOGEN 1 valence electron 7 valence electrons ↓ ↓ Loses 1 electron Gains 1 electron ↓ ↓ Forms M⁺ Forms X⁻

Same number, opposite meaning

Both families have a valency of 1. That does not make them similar. Valency counts how many electrons are involved; it says nothing about the direction. One electron out is not one electron in.

Check your understanding · 3 of 16

A halogen atom forms which kind of ion?

Correct answer: B. A halogen has seven valence electrons and gains exactly one to complete its outermost shell: X + e⁻ → X⁻. Option A describes an alkali metal.

Check your understanding · 4 of 16

A classmate says: “Alkali metals and halogens both have valency 1, so they must behave the same way.” What is wrong?

Correct answer: C. Valency is the combining capacity — the number of electrons involved. Both involve one electron, but in opposite directions: M → M⁺ + e⁻ against X + e⁻ → X⁻. Option B is the common slip of confusing valence electrons (7 for a halogen) with valency (1).

055Concept

Metals, non-metals and the two “-tive” words

Electropositive and electronegative are easy to mix up. Read them as directions: positive is what an alkali metal becomes; negative is what a halogen attracts.

5.1 Alkali metals

The chapter describes alkali metals as:

good conductors of electricity;
highly reactive;
electropositive;
metallic in nature.

Electropositive

An electropositive element readily loses electrons and forms positive ions.

5.2 Halogens

The chapter describes halogens as:

non-conductors of electricity;
highly reactive;
electronegative;
non-metallic in nature.

Electronegative

An electronegative element has a strong tendency to attract or gain electrons.

Both are “highly reactive”

Notice that the chapter calls both families highly reactive. Being reactive is not the difference between them — what they do when they react is.

5.3 Quick comparison

Four lines worth memorising as a block
Alkali metals	Halogens
Metals	Non-metals
Good conductors	Non-conductors
Electropositive	Electronegative
Lose electrons	Gain electrons

Check your understanding · 5 of 16

Which pair of words correctly completes this sentence? “Alkali metals are ______ and good conductors; halogens are ______ and non-conductors.”

Correct answer: B. An electropositive element readily loses electrons and forms positive ions — that is the alkali metal. An electronegative element has a strong tendency to attract or gain electrons — that is the halogen.

066Concept

Melting and boiling points

Here is the first trend that runs in opposite directions, and it is the one you can actually see: the halogens change state as you read down the column, the alkali metals do not.

6.1 Alkali metals — they fall

The melting and boiling points of alkali metals:

Decrease down the group

Li ↓ Na ↓ K Melting point decreases Boiling point decreases

6.2 Halogens — they rise

The melting and boiling points of halogens:

Increase down the group

F ↓ Cl ↓ Br ↓ I Melting point increases Boiling point increases

This is why fluorine and chlorine are gases, bromine is a liquid and iodine is a solid.

The opposite trend, side by side
Down the group	Alkali metals	Halogens
Melting point	Decreases	Increases
Boiling point	Decreases	Increases

Check your understanding · 6 of 16

Going down each group, the melting points:

Correct answer: B. Alkali metals melt and boil at lower temperatures as you go down; halogens at higher ones. The halogen half is easy to check against the physical states — gas, gas, liquid, solid.

077Concept

Atomic size

In any period, the alkali metal is the giant on the left and the halogen is one of the smallest atoms on the right — yet both families get bigger as you go down.

7.1 Alkali metals

The chapter states that alkali metals have the largest atomic size in their respective periods. Their atomic size increases down Group 1:

Li < Na < K < Rb < Cs

Why?

Each element down the group has one additional electron shell.

More shells ↓ Outermost electron farther away ↓ Atomic size increases

7.2 Halogens

The chapter states that halogens have the smallest atomic size in their respective periods, among the elements being compared before the noble gas. Their atomic size also increases down Group 17:

F < Cl < Br < I

7.3 The comparison within one period

In the same period:

Group 1 atom Group 17 atom Larger Smaller

For example, in Period 3:

Na is much larger than Cl.

This is because atomic size decreases from left to right across a period.

Two directions, do not mix them

Down a group both families grow. Across a period atoms shrink, which is why the Group 1 atom is always bigger than the Group 17 atom in the same row.

Check your understanding · 7 of 16

In Period 3, how does the size of a sodium atom compare with a chlorine atom?

Correct answer: A. Both have three occupied shells, but across the period the nuclear charge rises and pulls the shells in, so size falls from left to right. The alkali metal is the largest atom of its period; the halogen is among the smallest.

088Concept

Ionisation energy, electron affinity, electronegativity

Three properties, one pattern: alkali metals are low in all three, halogens are high in all three. Learn the pattern first, the reasons second.

8.1 Ionisation energy

Ionisation energy is the energy needed to remove an electron from a neutral isolated gaseous atom.

Alkali metals

Low ionisation energy

They lose their one valence electron easily. Their ionisation energy decreases down the group:

Li > Na > K > Rb > Cs

Halogens

High ionisation energy

Their electrons are held strongly and are not removed easily. The chapter notes that their ionisation energies are lower than those of noble gases. Ionisation energy also decreases down Group 17:

F > Cl > Br > I

Ionisation energy compared
Alkali metals	Halogens
Low ionisation energy	High ionisation energy
Electron removed easily	Electron removed with difficulty
I.E. decreases down the group	I.E. decreases down the group

The line students lose marks on

The values are opposite (low against high), but the trend is the same: ionisation energy decreases down both groups. Do not turn this one into an opposite as well.

8.2 Electron affinity

Electron affinity is connected with the energy released when an isolated gaseous atom gains an electron.

Alkali metals

Low electron affinity

They do not strongly tend to gain an electron. Their electron affinity generally decreases down the group.

Halogens

High electron affinity

They readily gain one electron to complete their outermost shell. Their electron-affinity values generally decrease down the group, with the fluorine–chlorine exception already studied.

Why are the halogens' electron affinities high?

7 valence electrons ↓ Need only 1 more electron ↓ Strong tendency to accept an electron ↓ High electron affinity

8.3 Electronegativity

Alkali metals

Low electronegativity

They do not strongly attract shared electrons. Electronegativity decreases down Group 1.

Halogens

High electronegativity

They strongly attract shared electrons. Electronegativity decreases down Group 17:

F > Cl > Br > I

Fluorine has the highest electronegativity.

8.4 All three together

Electron-related properties compared
Property	Alkali metals	Halogens
Ionisation energy	Low	High
Electron affinity	Low	High
Electronegativity	Low	High
Main tendency	Lose electrons	Gain or attract electrons

Check your understanding · 8 of 16

Which family has low ionisation energy, low electron affinity and low electronegativity?

Correct answer: A. All three properties are about holding on to or attracting electrons, and an alkali metal wants to do the opposite — give one away. The halogens are high in all three.

Check your understanding · 9 of 16

Why do halogens have high electron affinities?

Correct answer: B. Seven valence electrons means the shell is one short of eight, so an incoming electron is strongly welcomed. Option C describes a noble gas, and option D describes an alkali metal.

099Concept

Reactivity down the group

Both families are highly reactive, but their reactivity changes in opposite directions — and this is the single most examined line in the whole comparison.

9.1 Alkali metals — reactivity increases

Reactivity increases down Group 1.

Li < Na < K < Rb < Cs

Why?

Down the group:

atomic size increases;
the valence electron moves farther from the nucleus;
ionisation energy decreases;
the electron is lost more easily;
reactivity increases.

Larger atomic size ↓ Lower ionisation energy ↓ Electron lost more easily ↓ Greater reactivity

9.2 Halogens — reactivity decreases

Reactivity decreases down Group 17.

F > Cl > Br > I

Why?

Down the group:

atomic size increases;
attraction for an incoming electron becomes weaker;
electronegativity and electron affinity generally decrease;
the tendency to gain an electron decreases;
reactivity decreases.

Larger atomic size ↓ Weaker attraction for incoming electron ↓ Electron gained less readily ↓ Lower reactivity

DOWN GROUP 1 DOWN GROUP 17 Reactivity increases Reactivity decreases

Memory sentence

Down the group, metals give more easily; halogens receive less easily. The same cause — a bigger atom — helps one family and hinders the other.

9.3 The reactivity ladder — tap any element

Bar length is reactivity. Read each column downwards and watch the bars grow on the left but shrink on the right.

Group 1 · alkali metals

Group 17 · halogens

↓ reactivity increasesBars show reactivityreactivity decreases ↓

Two columns, two directions.
Down Group 1 the bars get longer — reactivity increases. Down Group 17 they get shorter — reactivity decreases. Tap any element to see why.

Alkali metal reactivity Halogen reactivity Currently selected

9.4 Down the group — increases or decreases?

One tap each. Answers lock in.

Atomic size of the alkali metals

Melting point of the alkali metals

Melting point of the halogens

Ionisation energy of the halogens

Electronegativity of the alkali metals

Reactivity of the alkali metals

Reactivity of the halogens

Oxidising power of the halogens

Check your understanding · 10 of 16

Moving down Group 17, reactivity:

Correct answer: B. A halogen reacts by taking an electron. As the atom grows, the nucleus pulls an incoming electron less strongly, so the electron is gained less readily and reactivity falls. Option C is the correct reasoning for Group 1, where reactivity rises instead.

1010Concept

Reaction with water and acids

A short section, but a favourite one-mark question.

10.1 Alkali metals

The chapter states that alkali metals react vigorously with water and acids, liberating hydrogen.

Alkali metal + water or acid ↓ Reaction occurs ↓ Hydrogen is liberated

It also states that their reactivity with water increases down the group — the same direction as their general reactivity.

10.2 Halogens

The comparison table states that halogens generally do not react with acids and water in the same manner.

Keep to the textbook comparison here
Alkali metals	Halogens
React vigorously with water and acids	Generally do not react with acids and water in the same way
Hydrogen is liberated	No corresponding alkali-metal-type reaction stated

1111Concept

Reducing and oxidising nature

Two labels that follow straight from “loses electrons” and “gains electrons”. If you can say which way the electron moves, you can name the agent.

11.1 Alkali metals are reducing agents

Alkali metals lose electrons easily.

Metal atom → positive ion + electron

Therefore, they act as reducing agents. The chapter states:

Reducing power increases down Group 1.

Why? Down the group, electrons are lost more easily.

11.2 Halogens are oxidising agents

Halogens gain electrons readily.

Halogen atom + electron → negative ion

Therefore, they act as oxidising agents. The chapter states:

Oxidising power decreases down Group 17.

Why? Down the group, the tendency to gain electrons decreases.

ALKALI METALS HALOGENS Lose electrons Gain electrons ↓ ↓ Reducing agents Oxidising agents ↓ ↓ Reducing power increases Oxidising power decreases down the group down the group

Preview — from a later chapter

The full electron-transfer definitions of oxidation and reduction belong to the later chapter on redox and electrolysis. For this chapter you only need the rule used above: an element that loses electrons acts as a reducing agent, and an element that gains electrons acts as an oxidising agent.

Check your understanding · 11 of 16

Alkali metals act as reducing agents because they:

Correct answer: B. Elements that lose electrons act as reducing agents; elements that gain electrons act as oxidising agents. So alkali metals reduce, halogens oxidise — and the two powers move in opposite directions down their groups.

1212Concept

The compounds they form

12.1 Alkali metals

The chapter states that alkali metals form electrovalent compounds with non-metals. Examples include:

NaCl KBr

The metal loses an electron, while the non-metal gains it.

Na → Na⁺ + e⁻ Cl + e⁻ → Cl⁻ Na⁺ + Cl⁻ → NaCl

12.2 Halogens

1 · Electrovalent compounds with metals

KCl CaCl₂

2 · Covalent compounds with hydrogen and other non-metals

HCl HBr

Preview — from a later chapter

Electrovalent (ionic) and covalent bonding are studied in full in the Chemical Bonding chapter. All you need here is the pattern: a metal and a non-metal transfer electrons and give an electrovalent compound; two non-metals share electrons and give a covalent compound.

Compounds compared
Alkali metals	Halogens
Electrovalent compounds with non-metals (NaCl, KBr)	Electrovalent compounds with metals (KCl, CaCl₂) and covalent compounds with hydrogen and other non-metals (HCl, HBr)

1313Concept

Why they combine so readily

An alkali-metal atom has one electron it can lose. A halogen atom needs exactly one. They fit together naturally.

Alkali metal Halogen Has 1 extra outer electron Needs 1 electron \ / \ / Electron transfer ↓ M⁺ and X⁻ formed ↓ Electrovalent compound

13.1 Sodium and chlorine, worked out

Na: 2, 8, 1 Cl: 2, 8, 7

Sodium transfers one electron to chlorine:

Na⁺: 2, 8 Cl⁻: 2, 8, 8

The oppositely charged ions form sodium chloride:

NaCl

Look at what each ion gained from the deal: sodium is left with a complete outer shell of eight, and chlorine has filled its shell to eight as well. Both are more stable than they were as atoms.

13.2 Step through the transfer yourself

Pick a pair, then take it one step at a time. Predict each line before you tap.

Metal

Halogen

Check your understanding · 12 of 16

Element X has configuration 2, 8, 1 and element Y has configuration 2, 8, 7. Which statement is correct?

Correct answer: B. X has one valence electron, so it is an alkali metal: it loses that electron, forms X⁺ and therefore reduces. Y has seven, so it is a halogen: it gains one, forms Y⁻ and therefore oxidises. Together they would form the electrovalent compound XY.

14CCompare

The complete comparison

Every line of the chapter's comparison table is in the panel below. Tap a property to see the two families side by side, or open every line at once for revision.

Alkali metals · Group 1

—

Halogens · Group 17

—

Sort these — alkali metal or halogen?

One tap each. Answers lock in.

Has one valence electron

Has seven valence electrons

Forms an ion carrying a single negative charge

Good conductor of electricity

Electropositive in character

Acts as an oxidising agent

Largest atomic size in its period

Includes a poisonous reddish-brown volatile liquid

Reacts vigorously with water, liberating hydrogen

Forms covalent compounds with hydrogen, such as HCl

15RRecall

Pause and think

Answer these without scrolling back, then open each strip to check.

1 · How many valence electrons do alkali metals have?

One.

2 · How many valence electrons do halogens have?

Seven.

3 · What type of ion is formed by an alkali metal?

A positive ion with charge +1.

4 · What type of ion is formed by a halogen?

A negative ion with charge −1.

5 · Which family has low ionisation energy?

Alkali metals.

6 · Which family has high electronegativity?

Halogens.

7 · How does alkali-metal reactivity change down the group?

It increases.

8 · How does halogen reactivity change down the group?

It decreases.

9 · Which family acts as reducing agents?

Alkali metals.

10 · Which family acts as oxidising agents?

Halogens.

11 · What is the physical state of bromine?

It is a reddish-brown volatile liquid.

12 · What happens to the melting and boiling points of halogens down the group?

They increase.

Type it out — the four definitions

Recall · 13 of 16

What does electropositive mean?

Model answer: An electropositive element readily loses electrons and forms positive ions.

Recall · 14 of 16

What does electronegative mean?

Model answer: An electronegative element has a strong tendency to attract or gain electrons.

Recall · 15 of 16

Define ionisation energy.

Model answer: Ionisation energy is the energy needed to remove an electron from a neutral isolated gaseous atom.

Recall · 16 of 16

Define electron affinity.

Model answer: Electron affinity is connected with the energy released when an isolated gaseous atom gains an electron.

16✗Traps

Mistakes that cost marks

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

They both have valency one, but their behaviour is opposite: an alkali metal loses 1 electron and a halogen gains 1 electron.

Halogens have seven valence electrons and need one more to complete the outermost shell. One is their valency, not their number of valence electrons.

They have low ionisation energy, because their outermost electron is removed easily.

Halogens have high electronegativity and strongly attract electrons. Fluorine has the highest electronegativity of all.

Group 1: reactivity increases. Group 17: reactivity decreases. This is the single most examined difference in the lesson.

Alkali metals lose electrons and act as reducing agents. Halogens gain electrons and act as oxidising agents.

The chapter describes bromine as a poisonous reddish-brown volatile liquid. Fluorine and chlorine are gases, bromine is a liquid and iodine is a solid.

17EExam

Exam notes

Definitions to learn word-for-word

Electropositive

An electropositive element readily loses electrons and forms positive ions.

Electronegative

An electronegative element has a strong tendency to attract or gain electrons.

Ionisation energy

Ionisation energy is the energy needed to remove an electron from a neutral isolated gaseous atom.

Electron affinity

Electron affinity is connected with the energy released when an isolated gaseous atom gains an electron.

The eight “give reason” answers

Say each one aloud before you open it. These are the exact answers to write in the exam.

Why do alkali metals form positive ions?

Alkali metals possess one valence electron. They lose this electron easily and therefore form positive ions carrying a single positive charge.

Why do halogens form negative ions?

Halogens possess seven valence electrons and require one additional electron to complete their outermost shell. They therefore gain one electron and form ions carrying a single negative charge.

Why do alkali metals have low ionisation energies?

Their single valence electron is comparatively easy to remove. Therefore, only a small amount of energy is required to form a positive ion.

Why do halogens have high electron affinities?

Halogens need only one electron to complete their outermost shells. They therefore readily accept an electron and have high electron affinities.

Why does the reactivity of alkali metals increase down the group?

Down Group 1, atomic size increases and ionisation energy decreases. The valence electron is therefore lost more easily, causing reactivity to increase.

Why does the reactivity of halogens decrease down the group?

Down Group 17, atomic size increases and the attraction for an incoming electron decreases. Therefore, the tendency to gain an electron and the reactivity decrease.

Why are alkali metals reducing agents?

Alkali metals readily lose electrons. Elements that lose electrons act as reducing agents.

Why are halogens oxidising agents?

Halogens readily accept electrons. Elements that gain electrons act as oxidising agents.

One full-mark answer

Question: Explain why reactivity increases down Group 1 but decreases down Group 17, even though atomic size increases in both.

Write it like this

Down both groups the atomic size increases, but the two families react in opposite ways. An alkali metal reacts by losing its single valence electron; as the atom becomes larger the valence electron is farther from the nucleus and the ionisation energy decreases, so the electron is lost more easily and reactivity increases. A halogen reacts by gaining one electron; as the atom becomes larger the attraction for an incoming electron becomes weaker, so the electron is gained less readily and reactivity decreases.

Memory tricks worth keeping

Valence electrons

Group 1 → 1 valence electron Group 17 → 7 valence electrons

Electron behaviour

Alkali gives; halogen takes.

Ions

Alkali metal → +1 Halogen → −1

Reactivity

Group 1 down → Reactivity goes UP Group 17 down → Reactivity goes DOWN

Chemical role

Metals reduce; halogens oxidise.

Alkali metals → Reducing agents Halogens → Oxidising agents

18QPractice

Practice questions

Level 1 — easy recall

Name the family present in Group 1.
Answer
The alkali metals — Li, Na, K, Rb, Cs and Fr.
Name the family present in Group 17.
Answer
The halogens — F, Cl, Br, I and At.
State the number of valence electrons in sodium.
Answer
One. Sodium is 2, 8, 1.
State the number of valence electrons in chlorine.
Answer
Seven. Chlorine is 2, 8, 7.
What ion is formed by potassium?
Answer
K⁺ — a positive ion carrying a single positive charge, formed as K → K⁺ + e⁻.
What ion is formed by bromine?
Answer
Br⁻ — a negative ion carrying a single negative charge, formed as Br + e⁻ → Br⁻.
Which halogen is a liquid?
Answer
Bromine — a poisonous reddish-brown volatile liquid.
Which halogen is a dark-grey solid?
Answer
Iodine — a dark-grey crystalline solid.
Which family has low electronegativity?
Answer
The alkali metals.
Which family has high electron affinity?
Answer
The halogens.

Level 2 — comparison

Differentiate between alkali metals and halogens with respect to: (i) valence electrons; (ii) ions formed; (iii) metallic or non-metallic nature; (iv) electrical conduction; (v) ionisation energy; (vi) electron affinity; (vii) electronegativity; (viii) reactivity down the group; (ix) reducing or oxidising nature; (x) compounds formed.

Answer

Point of difference	Alkali metals	Halogens
Valence electrons	1	7
Ions formed	Positive ions, M⁺	Negative ions, X⁻
Nature	Metals	Non-metals
Electrical conduction	Good conductors	Non-conductors
Ionisation energy	Low	High
Electron affinity	Low	High
Electronegativity	Low	High
Reactivity down the group	Increases	Decreases
Chemical role	Reducing agents	Oxidising agents
Compounds formed	Electrovalent compounds with non-metals	Electrovalent with metals; covalent with hydrogen and other non-metals

Level 3 — give reasons

Sodium forms Na⁺, while chlorine forms Cl⁻.
Answer
Sodium (2, 8, 1) has one valence electron, which it loses easily to form a positive ion with a single positive charge. Chlorine (2, 8, 7) has seven valence electrons and needs one more to complete its outermost shell, so it gains one electron and forms an ion with a single negative charge.
Caesium is more reactive than lithium.
Answer
Down Group 1 the atomic size increases and the ionisation energy decreases. Caesium is far lower in the group than lithium, so its valence electron is lost much more easily, making it more reactive.
Fluorine is more reactive than iodine.
Answer
Down Group 17 the atomic size increases and the attraction for an incoming electron becomes weaker. Fluorine is the smallest halogen, so it attracts and gains an electron most readily and is the most reactive; iodine is much larger and gains an electron less readily.
Alkali metals have low ionisation energies.
Answer
Their single valence electron is comparatively easy to remove, so only a small amount of energy is required to form a positive ion.
Halogens have high electronegativities.
Answer
Halogens need only one electron to complete their outermost shells, so they attract shared electrons strongly. Their electronegativity is therefore high, fluorine's being the highest of all.
Reducing power increases down Group 1.
Answer
A reducing agent works by losing electrons. Down Group 1 the atomic size increases and the ionisation energy decreases, so electrons are lost more easily and the reducing power increases.
Oxidising power decreases down Group 17.
Answer
An oxidising agent works by gaining electrons. Down Group 17 the tendency to gain an electron decreases as the atom becomes larger, so the oxidising power decreases.
Bromine is a liquid while iodine is a solid, according to the trends given.
Answer
The melting and boiling points of halogens increase down the group. Bromine lies above iodine, so its melting and boiling points are lower; bromine is a reddish-brown volatile liquid while iodine, further down, is a dark-grey crystalline solid.

ICSE-style questions

Compare alkali metals and halogens with respect to: (i) valence electrons; (ii) nature of ions formed; (iii) ionisation energy; (iv) electronegativity; (v) chemical nature.

Model answer

Property	Alkali metals	Halogens
Valence electrons	1	7
Ions formed	Positive ions with +1 charge	Negative ions with −1 charge
Ionisation energy	Low	High
Electronegativity	Low	High
Chemical nature	Reducing agents	Oxidising agents

State how reactivity changes down (i) Group 1 and (ii) Group 17. Give reasons.
Model answer
The reactivity of Group 1 metals increases down the group because atomic size increases and ionisation energy decreases, making the valence electron easier to remove. The reactivity of Group 17 halogens decreases down the group because atomic size increases and the attraction for an incoming electron decreases.
An element X has electronic configuration 2, 8, 1, while element Y has electronic configuration 2, 8, 7. Answer: (i) Which one is an alkali metal? (ii) Which one is a halogen? (iii) What ions will they form? (iv) What will be the charge on each ion? (v) Which one acts as a reducing agent? (vi) Which one acts as an oxidising agent?
Answer
(i) X is an alkali metal. (ii) Y is a halogen. (iii) X forms X⁺; Y forms Y⁻. (iv) +1 and −1. (v) X acts as a reducing agent. (vi) Y acts as an oxidising agent.
Choose the correct member in each case: (i) more reactive alkali metal — Li or K; (ii) more reactive halogen — Cl or I; (iii) higher electronegativity — Na or Cl; (iv) lower ionisation energy — Li or Cs; (v) stronger oxidising agent — F or I; (vi) stronger reducing agent — Li or Cs.
Answers
(i) K · (ii) Cl · (iii) Cl · (iv) Cs · (v) F · (vi) Cs.

19✓Close

One-page revision

ALKALI METALS — GROUP 1 │ ├── Li, Na, K, Rb, Cs, Fr ├── 1 valence electron ├── Lose 1 electron ├── Form +1 ions ├── Metals and good conductors ├── Low ionisation energy ├── Low electron affinity ├── Low electronegativity ├── Atomic size increases down group ├── Reactivity increases down group ├── Reducing agents └── Form electrovalent compounds

HALOGENS — GROUP 17 │ ├── F, Cl, Br, I, At ├── 7 valence electrons ├── Gain 1 electron ├── Form −1 ions ├── Non-metals and non-conductors ├── High ionisation energy ├── High electron affinity ├── High electronegativity ├── Atomic size increases down group ├── Reactivity decreases down group ├── Oxidising agents └── Form electrovalent and covalent compounds

The whole lesson as one map

ALKALI METALS vs HALOGENS │ ┌────────────────┴────────────────┐ │ │ Group 1 Group 17 │ │ 1 valence electron 7 valence electrons │ │ Loses electron Gains electron │ │ +1 ion −1 ion │ │ Electropositive Electronegative │ │ Low I.E., E.A. and E.N. High I.E., E.A. and E.N. │ │ Reactivity increases Reactivity decreases down the group down the group │ │ Reducing agents Oxidising agents

Mastery check

I can say this aloud, without looking: “Alkali metals are Group 1 elements with one valence electron. They lose this electron, form positive ions and act as reducing agents. Their reactivity increases down the group. Halogens are Group 17 elements with seven valence electrons. They gain one electron, form negative ions and act as oxidising agents. Their reactivity decreases down the group. Alkali metals generally have low ionisation energy, electron affinity and electronegativity, while halogens have high values of these properties.”

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