ChemistryICSE Class 9 · The Periodic Table

The Modern Periodic Table: Groups, Periods, Configurations and Families (ICSE Class 9)

Updated 17 October 2026 · Fact-checked

The long form of the modern periodic table has 18 groups and 7 periods. The period number equals the number of shells; for groups 1, 2 the group number equals the valence electrons and for groups 13 to 17 it is valence electrons + 10 (noble gases: 8, helium 2). Elements in a group have similar properties because they have the same number of valence electrons.

Understand The Modern Periodic Table: Groups, Periods, Configurations and Families

Layout. 18 vertical groups and 7 horizontal periods.

Period	Number of elements
1	2 (H, He)
2, 3	8 each
4, 5	18 each
6, 7	32 each (including the lanthanides and actinides)

From configuration to position.

Period $=$ number of shells.
Group: groups 1 and 2 have 1 and 2 valence electrons; groups 13 to 17 have 3 to 7 valence electrons (group $=$ valence electrons $+ 10$ ); group 18 has 8 (helium 2).

Element	Z	Configuration	Period	Group
Na	11	2, 8, 1	3	1
Al	13	2, 8, 3	3	13
S	16	2, 8, 6	3	16
K	19	2, 8, 8, 1	4	1

Regions. Main-group elements: groups 1, 2 and 13–17. Transition elements: groups 3–12 (periods 4–7). Inner transition elements: lanthanides (Z = 57–71) and actinides (Z = 89–103) in two rows at the bottom. Noble gases: group 18.

Families.

*Alkali metals* (group 1: Li, Na, K, Rb, Cs, Fr): 1 valence electron; soft, very reactive; lose 1 electron (M⁺); react with cold water ( $2 N a + 2 H_{2} O \to 2 N a O H + H_{2}$ ); stored in kerosene.
*Alkaline earth metals* (group 2: Be, Mg, Ca, Sr, Ba, Ra): 2 valence electrons; lose 2 (M²⁺); less reactive than group 1; basic oxides and hydroxides ( $C a + 2 H_{2} O \to C a (O H)_{2} + H_{2}$ ).
*Halogens* (group 17: F, Cl, Br, I, At): 7 valence electrons; gain 1 (X⁻); diatomic; $C l_{2}$ displaces bromine and iodine from their salts: $C l_{2} + 2 K B r \to 2 K C l + B r_{2}$ .
*Noble gases* (group 18: He, Ne, Ar, Kr, Xe, Rn): complete outer shell, inert, monatomic gases.

Reasoning about trends. Same group: same valence electrons, so similar chemical properties. Same period: the number of valence electrons changes by one from element to element, so properties change gradually.

Compounds from positions. A group 1 element and a group 17 element give an ionic compound of ratio 1 : 1 (NaCl); group 2 with group 17 gives 1 : 2 (MgCl₂); group 13 with 17 gives 1 : 3 (AlCl₃).

The Modern Periodic Table: Groups, Periods, Configurations and Families: formulas and key facts

Period: $period = number of shells$; From the configuration.
Group (main elements): $group = v (v = 1, 2); group = v + 10 (v = 3 to 7)$; v = valence electrons.
Alkali metal with water: $2 N a + 2 H_{2} O \to 2 N a O H + H_{2}$; Hydrogen and an alkali.
Halogen displacement: $C l_{2} + 2 K B r \to 2 K C l + B r_{2}$; More reactive halogen wins.

How to solve The Modern Periodic Table: Groups, Periods, Configurations and Families questions

Write the configuration first; everything else follows.

1Write the configuration with the 2, 8, 8, 18 rule.
2Count shells for the period.
3Count outer electrons for the group (add 10 if 3 to 7 valence electrons).
4Name the family and the likely ion.
5Use the group to write the formula of its compounds.

The Modern Periodic Table: Groups, Periods, Configurations and Families in real life

Each concept, met in an everyday situation. Read the situation, try the problem, then check the solution. The one-line hook is what to remember.

Why alkali metals live in kerosene

Suppose a laboratory stores sodium and potassium under kerosene.

Explain using their position in the periodic table.

Try it first, then show the solutionSolution

They are in group 1 with one valence electron, easily lost.
They react vigorously with air and water, so they are kept in kerosene.

Answer: Group 1 metals are highly reactive.

Memory hook: One electron, lost at once.

Tricks to remember The Modern Periodic Table: Groups, Periods, Configurations and Families

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

Shells give the period, valence electrons give the group
Period = shells; group from outer electrons.
Use it when: Positions.
Group number − 10
Groups 13–17: valence electrons = group number − 10.
Use it when: Reverse questions.
Same group, same behaviour
Same valence electrons gives similar chemistry.
Use it when: Reasoning.

The Modern Periodic Table: Groups, Periods, Configurations and Families: solved examples

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

Example 1: Number of elements in a period · 1 mark · MCQBoard level

Period 4 of the modern periodic table contains how many elements?
(a) 8
(b) 18
(c) 32
(d) 2

Try it first, then show the solutionSolution

Periods 4 and 5 are long periods with 18 elements each.

Answer: (b) 18

Example 2: From atomic number to position · 2 marks · Short answerBoard level

An element has atomic number 19. Write its electronic configuration, group, period and family.

Try it first, then show the solutionSolution

Configuration 2, 8, 8, 1. (½)
Four shells: period 4. (½)
One valence electron: group 1; alkali metal. (1)

Answer: 2,8,8,1; group 1; period 4; alkali metal

Example 3: An element in group 13, period 3 · 3 marks · Short answerHarder board level

An element lies in group 13 and period 3. Find its atomic number, valence electrons, the ion it forms and the formulae of its oxide and chloride.

Try it first, then show the solutionSolution

Three shells and 3 valence electrons: 2, 8, 3; atomic number 13. (1)
It loses 3 electrons: $A l^{3 +}$ . (1)
Oxide $A l_{2} O_{3}$ and chloride $A l C l_{3}$ (valency 3 with O 2 and Cl 1). (1)

Answer: Z = 13; 3 valence electrons; Al³⁺; Al₂O₃; AlCl₃

Example 4: Balancing family reactions · 3 marks · Short answerHarder board level

Write balanced equations for (a) potassium with water, (b) calcium with water, (c) chlorine with potassium iodide, and name the family of the first metal in each.

Try it first, then show the solutionSolution

(a) $2 K + 2 H_{2} O \to 2 K O H + H_{2}$ (alkali metal). (1)
(b) $C a + 2 H_{2} O \to C a (O H)_{2} + H_{2}$ (alkaline earth metal). (1)
(c) $C l_{2} + 2 K I \to 2 K C l + I_{2}$ (a halogen displaces a less reactive halogen). (1)

Answer: See working

Example 5: Four cards · 4 marks · Case-basedHOTS / competency

Elements A, B, C and D have atomic numbers 9, 10, 11 and 12.
(a) Write their configurations, periods and groups.
(b) Name the family of each.
(c) Write the formula of the compound of C with A.
(d) Write the formula of the compound of D with A and say which has the higher valency.

Try it first, then show the solutionSolution

(a) A: 2, 7 (period 2, group 17); B: 2, 8 (period 2, group 18); C: 2, 8, 1 (period 3, group 1); D: 2, 8, 2 (period 3, group 2). (1½)
(b) Halogen; noble gas; alkali metal; alkaline earth metal. (1)
(c) $C^{+}$ with $A^{-}$ : CA (NaF). (½)
(d) $D^{2 +}$ with $A^{-}$ : DA₂ (MgF₂); D (valency 2) has the higher valency than C (1). (1)

Answer: (a) as given (b) halogen, noble gas, alkali, alkaline earth (c) CA (d) DA₂; D

Where students lose marks in The Modern Periodic Table: Groups, Periods, Configurations and Families

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

Mistake: Period taken as number of valence electrons
Period = number of shells.
Wrong working
2, 8, 1: period 1.
Right working
2, 8, 1: period 3.
Do this: Count shells.
Usually costs: 1 mark.
Mistake: Group for 3–7 valence electrons not corrected
Add 10.
Wrong working
Seven valence electrons: group 7.
Right working
Group 17.
Do this: Group = v + 10.
Usually costs: 1 mark.
Mistake: Transition elements placed in groups 1 and 2
They are groups 3–12.
Wrong working
Fe in group 8 of main elements.
Right working
Fe is a transition element (groups 3–12).
Do this: Name the block.
Usually costs: 1 mark.

Exam tips

Always write the configuration, then the period and group.
Quote the family name with the group number.
Give an equation for each family property you cite.
Mention that elements of a group have the same number of valence electrons.

Practice questions on The Modern Periodic Table: Groups, Periods, Configurations and Families

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

Q1 · 1 mark · MCQBoard level
An element with the configuration 2, 8, 7 belongs to:
(a) group 7, period 3
(b) group 17, period 3
(c) group 17, period 7
(d) group 3, period 17
Hint
Add 10 to the valence electrons.
Answer
(b) group 17, period 3
Q2 · 1 mark · Very short answerBoard level
How many groups are there in the long form of the modern periodic table?
Hint
Count vertical columns.
Answer
18.
Q3 · 2 marks · Short answerHarder board level
State the number of valence electrons in elements of groups 1, 2, 13, 15 and 17.
Hint
Group − 10 for 13 to 17.
Answer
1, 2, 3, 5 and 7.
Q4 · 3 marks · Short answerHarder board level
An element has three shells and six valence electrons. Find its atomic number, group, period, the ion it forms and the formula of its compound with sodium.
Hint
2, 8, 6.
Answer
Z = 16; group 16; period 3; gains 2 electrons to form X²⁻; compound with sodium Na₂X (Na₂S).
Q5 · 2 marks · Assertion-reasonHarder board level
Assertion (A): Sodium and potassium show similar chemical properties.
Reason (R): Both have one valence electron.
(a) Both true, R explains A.
(b) Both true, R does not explain A.
(c) A true, R false.
(d) A false, R true.
Hint
Same group.
Answer
(a)
Q6 · 3 marks · Short answerHOTS / competency
Bromine water (Br₂) is added separately to potassium iodide solution and to potassium chloride solution. Predict what happens in each, write the equation where a reaction occurs and explain using the position of the halogens in group 17.
Hint
Reactivity falls down the group.
Answer
With KI: $B r_{2} + 2 K I \to 2 K B r + I_{2}$ (iodine liberated), because bromine is more reactive than iodine. With KCl: no reaction, because bromine is less reactive than chlorine (reactivity decreases down group 17).

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1.Number of groups and periods?Show answer
18 groups and 7 periods.
2.Period number equals?Show answer
The number of shells.
3.Group number for 7 valence electrons?Show answer
17.
4.Halogens are in?Show answer
Group 17.
5.Noble gases are in?Show answer
Group 18.

The Modern Periodic Table: Groups, Periods, Configurations and Families: frequently asked questions

How is the position of an element decided from its configuration?

The number of shells gives the period; the number of valence electrons gives the group (add 10 for 3 to 7 valence electrons).

Why do elements of the same group have similar properties?

They have the same number of valence electrons.

What are alkali metals?

The group 1 elements Li, Na, K, Rb, Cs and Fr; soft, very reactive metals with one valence electron.