ChemistryICSE Class 9 · Chapter 5

The Periodic Table ICSE Class 9: From Dobereiner to the Modern Periodic Table

Updated 15 October 2026 · Fact-checked

The periodic table arranges elements so that similar ones fall in the same column. Dobereiner grouped triads, Newlands found octaves and Mendeleev ordered elements by atomic mass. The modern periodic law orders them by atomic number, giving 18 groups and 7 periods, with alkali metals in group 1 and noble gases in group 18.

What The Periodic Table is about

By the 1800s chemists knew dozens of elements and needed a way to organise them. Each attempt found a pattern (periodicity): properties repeat at regular intervals.

The CISCE 2028 syllabus (Class IX, topic 5) covers: Dobereiner's triads, Newlands' law of octaves and Mendeleev's periodic law, limited to principles, examples, merits and limitations; the discovery of atomic number and its use as the basis of the modern periodic law; the modern periodic table with groups 1 to 18 and periods 1 to 7; the placement of transition and inner transition elements (to be mentioned); and special reference to alkali metals (group 1), alkaline earth metals (group 2), halogens (group 17) and the zero group (group 18).

Weightage: CISCE publishes no chapter weightage for ICSE Chemistry. The Periodic Table is topic 5 of the Class IX part of the 2028 syllabus; the Class IX paper (80 marks, 2 hours) is set and marked by the school.Source: CISCE ICSE Chemistry (Science Paper 2) syllabus, examination year 2028 (Class IX)

Key concepts

Dobereiner's triads

Principle (Johann Dobereiner, 1817-1829): similar elements occur in groups of three (triads); when arranged in increasing atomic mass, the atomic mass of the middle element is about the average of the other two, and its properties are in between.

Triad	Atomic masses	Mean of 1st and 3rd
Li, Na, K	7, 23, 39	(7 + 39)/2 = 23
Ca, Sr, Ba	40, 88, 137	(40 + 137)/2 = 88.5
Cl, Br, I	35.5, 80, 127	(35.5 + 127)/2 = 81.25

Merit: the first attempt to link the properties of elements with their atomic masses.
Limitations: only a few triads could be found; the rule did not work for all known elements (e.g. N, P, As fail); very light and very heavy elements did not fit.

Worked example

S (32), Se (79) and Te (128) are similar. Do they form a Dobereiner triad?

Try it first, then show the solutionSolution

Mean of S and Te = (32 + 128)/2 = 80.
Se = 79, very close to 80.

Answer: Yes: Se's atomic mass is about the mean.

Newlands' law of octaves

Principle (John Newlands, 1865): when elements are arranged in increasing order of atomic mass, every eighth element has properties similar to the first, like the eighth note of a musical scale (sa re ga ma pa dha ni sa).

Example: starting at Li, the eighth element is Na, and the eighth from Na is K; starting at F, the eighth is Cl.

Merits: first to show periodicity clearly; arranged all known elements in a table by atomic mass.
Limitations:

Worked well only up to calcium (light elements).
He put two elements in one slot (cobalt and nickel) and placed unlike elements together (e.g. iron with oxygen and sulphur).
Left no room for elements still to be discovered; the later discovery of noble gases spoilt the octaves.

Worked example

In Newlands' arrangement (H, Li, Be, B, C, N, O, F, Na, Mg, Al, Si, P, S, Cl, ...), which element resembles nitrogen?

Try it first, then show the solutionSolution

Count eight from N: N (1), O, F, Na, Mg, Al, Si, P (8).
P is the eighth element.

Answer: Phosphorus.

Mendeleev's periodic law and table

Mendeleev's periodic law (1869): the physical and chemical properties of elements are periodic functions of their atomic masses.

He arranged the 63 known elements by increasing atomic mass in vertical groups and horizontal periods, putting elements with similar properties (and similar formulae of oxides and hydrides) together.

Merits:

He left gaps for undiscovered elements and predicted their properties: eka-aluminium (gallium), eka-silicon (germanium) and eka-boron (scandium) were later found to match.
He corrected doubtful atomic masses (e.g. beryllium).
The noble gases, found later, fitted in as a new group without disturbing the table.

Limitations:

No fixed position for hydrogen (it resembles both alkali metals and halogens).
Isotopes of an element have different masses but would need the same place.
Anomalous pairs: a heavier element placed before a lighter one, e.g. argon (39.9) before potassium (39.1), cobalt (58.9) before nickel (58.7), tellurium (127.6) before iodine (126.9).
Some unlike elements were put together (e.g. copper and silver with alkali metals).

Worked example

Why did Mendeleev place cobalt (58.9) before nickel (58.7) even though it is heavier?

Try it first, then show the solutionSolution

He gave more importance to similarity of properties than to strict mass order.
Cobalt's properties matched the group before nickel's.

Answer: To keep elements with similar properties in the same group.

Atomic number and the modern periodic law

In 1913 Henry Moseley studied the X-rays given off by elements and found that a property of the nucleus (its positive charge), the atomic number, increases by one from each element to the next. Atomic number, not atomic mass, is the fundamental property of an element.

Modern periodic law: the properties of elements are periodic functions of their atomic numbers.

This removed Mendeleev's problems: isotopes have the same atomic number, so one place; anomalous pairs fall into order (Ar 18 before K 19; Co 27 before Ni 28; Te 52 before I 53).

Why properties repeat: elements with the same number of valence electrons behave alike, and the outer configuration repeats after every shell fills.

Worked example

Explain how the modern periodic law solves the argon-potassium problem.

Try it first, then show the solutionSolution

By mass, argon (39.9) should come after potassium (39.1).
By atomic number, argon is 18 and potassium 19.
So argon correctly comes first, with the noble gases, and potassium with the alkali metals.

Answer: Ordering by atomic number puts both in their natural groups.

The modern periodic table: groups and periods

The long form of the table has 18 vertical groups and 7 horizontal periods.

Periods: the period number = number of shells in the atom.

Period	Number of elements	Type
1	2 (H, He)	shortest
2, 3	8 each	short
4, 5	18 each	long
6, 7	32 each	very long (include inner transition elements)

Groups: elements in a group have the same number of valence electrons, so similar properties.

Groups 1, 2: group number = valence electrons.
Groups 13-17: valence electrons = group number - 10.
Group 18: 8 valence electrons (helium 2).

Blocks of elements:

Normal (main) elements: groups 1, 2 and 13-17.
Transition elements: groups 3 to 12, in the middle (periods 4-7), e.g. Fe, Cu, Zn.
Inner transition elements: the lanthanides (Z = 57-71, period 6) and actinides (Z = 89-103, period 7) are placed in two separate rows at the bottom of the table, belonging to group 3.
Noble gases: group 18 (zero group).

Worked example

Find the group and period of an element with configuration 2, 8, 6.

Try it first, then show the solutionSolution

Three shells: period 3.
Six valence electrons: group 6 + 10 = 16.

Answer: Group 16, period 3 (sulphur).

Alkali metals, alkaline earth metals, halogens and noble gases

Family	Group	Members	Valence electrons	Typical behaviour
Alkali metals	1	Li, Na, K, Rb, Cs, Fr	1	soft, very reactive metals; lose 1 electron → M⁺; react with cold water giving an alkali and hydrogen; stored in kerosene
Alkaline earth metals	2	Be, Mg, Ca, Sr, Ba, Ra	2	reactive metals (less than group 1); lose 2 electrons → M²⁺; oxides and hydroxides are basic
Halogens	17	F, Cl, Br, I, At	7	reactive non-metals; gain 1 electron → X⁻; diatomic (F₂, Cl₂, Br₂, I₂); form salts with metals ('halogen' = salt-former); F₂ and Cl₂ gases, Br₂ liquid, I₂ solid
Noble gases (zero group)	18	He, Ne, Ar, Kr, Xe, Rn	8 (He 2)	complete outer shell; very unreactive; monatomic gases

Equations:

$2 N a + 2 H_{2} O \to 2 N a O H + H_{2}$
$C a + 2 H_{2} O \to C a (O H)_{2} + H_{2}$
$2 N a + C l_{2} \to 2 N a C l$
$C l_{2} + 2 K B r \to 2 K C l + B r_{2}$ (a more reactive halogen displaces a less reactive one)

Worked example

Why are alkali metals stored under kerosene?

Try it first, then show the solutionSolution

They react very quickly with oxygen and moisture in air.
Sodium and potassium may even catch fire with water.
Kerosene keeps air and water away and does not react with them.

Answer: To prevent reaction with air and moisture.

Chemistry in real life: The Periodic Table

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.

Predicting a new element

Suppose a chemist in 1870 is told by Mendeleev that a missing element below aluminium should be a metal with atomic mass near 68, forming an oxide with formula E₂O₃.

Explain how Mendeleev could predict this and what element was later found.

Try it first, then show the solutionSolution

Elements in a group have similar properties and the same oxide formula: Al₂O₃ → E₂O₃.
Its mass lies between those of its neighbours (zinc 65 and arsenic 75 in the period).
Gallium, discovered in 1875, matched closely.

Answer: From group similarity and mass trends; gallium.

Memory hook: A gap is a prediction waiting to be found.

Choosing a gas for light bulbs

Suppose a bulb manufacturer must fill bulbs with a gas that will not react with the hot tungsten filament.

Choose a group of the periodic table and explain.

Try it first, then show the solutionSolution

Group 18 noble gases have full outer shells.
They do not react even with a white-hot filament.
Argon is the cheapest and commonest (about 1% of air).

Answer: Argon, a group 18 noble gas.

Memory hook: Full shell, no reaction.

Iodised salt and halogens

Suppose a student reads that salt works are told to add potassium iodate, not elemental iodine, to table salt.

Using properties of halogens, explain why a compound is used instead of the element.

Try it first, then show the solutionSolution

Iodine, a halogen, is a reactive non-metal and a solid that slowly vaporises (sublimes).
As a compound (iodide or iodate ion), it is stable and stays in the salt.

Answer: The element is reactive and volatile; the ion is stable.

Memory hook: Halogens settle down as ions.

Topics in this chapter

The Periodic Table formula sheet

Dobereiner's mean: $A_{mi dd l e} \approx \frac{A _{1} + A _{3}}{2}$; Atomic masses of a triad.
Group from valence electrons: $Group = v (v = 1, 2); Group = v + 10 (v = 3 to 7)$; Main-group elements.
Period from shells: $Period = number of shells$; E.g. 2, 8, 1 → period 3.
Sodium with water: $2 N a + 2 H_{2} O \to 2 N a O H + H_{2}$; Group 1 behaviour.
Halogen displacement: $C l_{2} + 2 K B r \to 2 K C l + B r_{2}$; Cl more reactive than Br.

The Periodic Table: diagrams to practise

Practise each one on paper: draw it from the checklist, then tick off every label.

Outline of the modern periodic table

Sketch the outline of the long form of the periodic table showing the main regions.

Draw

Seven rows; row 1 with a box at each end (H and He).
Rows 2-3: 2 boxes on the left, 6 on the right.
Rows 4-7: full width of 18 columns, the middle 10 shaded as transition elements.
Two separate rows below the table for lanthanides and actinides, with an arrow to group 3 in periods 6 and 7.

Label

groups 1-18
periods 1-7
alkali metals (1)
alkaline earth metals (2)
transition elements (3-12)
halogens (17)
noble gases (18)
lanthanides
actinides

Watch out: Number groups along the top and periods down the side.

How the school exam asks The Periodic Table

The question patterns to expect, and the method that works for each.

Question type	How it is asked	Method
Early classifications	State Dobereiner's, Newlands' or Mendeleev's law; give an example; one merit and one limitation.	Learn each as principle - example - merit - limitation.
Triad calculation	Check whether three elements form a triad; find the missing atomic mass.	Mean of the first and third.
Modern periodic law	State it; explain how it removed Mendeleev's anomalies.	Atomic number; isotopes and anomalous pairs.
Position from configuration	Find the group and period of an element; identify the family.	Shells → period; valence electrons → group.
Families	Name members and properties of groups 1, 2, 17, 18; placement of transition and inner transition elements.	Use the family table.

Tricks to remember The Periodic Table

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

Triad means: 'middle is the mean'
Li-Na-K: 7 and 39 average to 23.
Use it when: Dobereiner examples.
Octave = sa to sa
Eight notes, first and eighth sound alike; Li and Na, Na and K.
Use it when: Newlands' law.
Mendeleev's gaps: 'Ga Ge Sc'
Eka-aluminium = gallium, eka-silicon = germanium, eka-boron = scandium.
Use it when: Merits of Mendeleev.
Period lengths 2, 8, 8, 18, 18, 32, 32
'Two eights, two eighteens, two thirty-twos' after the first two.
Use it when: Number of elements in periods.
Group 1: 'Little Naughty Kids Rub Cats Fur'
Li, Na, K, Rb, Cs, Fr.
Use it when: Alkali metals.

The Periodic Table: exam-level solved questions

Ordered from exam level to HOTS. Cover the solution, write your own, then compare step by step.

Example 1: Modern law · 1 mark · MCQExam level

The modern periodic law is based on:
(a) atomic mass
(b) atomic number
(c) number of neutrons
(d) density

Try it first, then show the solutionSolution

Moseley showed atomic number is the fundamental property.

Answer: (b)

Example 2: Missing triad mass · 2 marks · Short answerExam level

In a Dobereiner triad P, Q, R, the atomic masses of P and R are 40 and 137. Estimate the atomic mass of Q and identify the triad.

Try it first, then show the solutionSolution

Q ≈ (40 + 137)/2 = 88.5.
Masses 40 and 137 are calcium and barium; Q is strontium (88).

Answer: About 88.5; Ca, Sr, Ba.

Example 3: Limitations compared · 3 marks · Short answerExam level

State one limitation each of Dobereiner's triads, Newlands' octaves and Mendeleev's table.

Try it first, then show the solutionSolution

Dobereiner: only a few triads could be found.
Newlands: worked only up to calcium.
Mendeleev: no fixed position for hydrogen (or isotopes, anomalous pairs).

Answer: As above.

Example 4: Position from atomic number · 4 marks · Long answerHarder exam level

Elements A, B and C have atomic numbers 9, 12 and 19. For each, give the configuration, group and period, and name the family.

Try it first, then show the solutionSolution

A: 2, 7 → period 2, group 17: halogen (F).
B: 2, 8, 2 → period 3, group 2: alkaline earth metal (Mg).
C: 2, 8, 8, 1 → period 4, group 1: alkali metal (K).

Answer: As above.

Example 5: Anomalous pairs · 1 mark · Assertion-reasonHarder exam level

Assertion (A): In the modern periodic table, tellurium (127.6) comes before iodine (126.9).
Reason (R): Elements in the modern table are arranged in increasing atomic mass.
(a) Both (A) and (R) are true, and (R) is the correct explanation for (A).
(b) Both (A) and (R) are true, but (R) is not the correct explanation for (A).
(c) (A) is true and (R) is false.
(d) (A) is false and (R) is true.

Try it first, then show the solutionSolution

Te (52) comes before I (53) by atomic number: A true.
The modern table uses atomic number, not mass: R false.

Answer: (c)

Example 6: Hydrogen's problem · 4 marks · Long answerHarder exam level

Give two reasons why hydrogen could be placed with the alkali metals and two why it could be placed with the halogens.

Try it first, then show the solutionSolution

Alkali metals: one valence electron; loses it to form H⁺ (like Na⁺).
Halogens: needs one electron to complete its shell; forms H⁻ (like Cl⁻) and a diatomic molecule H₂ (like Cl₂).

Answer: As above.

Example 7: Same group or same period? · 3 marks · Short answerHOTS / competency

X has 3 shells with 7 electrons in its outer shell; Y has 2 shells with 7 outer electrons; Z has 3 shells with 1 outer electron. (a) Which two are in the same group? (b) Which two are in the same period? (c) Write the formula of the compound of X and Z and give the type of bonding.

Try it first, then show the solutionSolution

(a) X and Y: both 7 valence electrons (group 17).
(b) X and Z: both 3 shells (period 3).
(c) Z loses 1 electron, X gains 1: ZX, electrovalent (NaCl).

Answer: X and Y; X and Z; ZX (electrovalent).

Example 8: Predicting a halogen reaction · 3 marks · Short answerHOTS / competency

Chlorine is bubbled through potassium iodide solution and separately through potassium fluoride solution. Predict what happens in each and write any equation.

Try it first, then show the solutionSolution

Reactivity of halogens: F > Cl > Br > I.
With KI: chlorine displaces iodine (brown colour): $C l_{2} + 2 K I \to 2 K C l + I_{2}$ .
With KF: no reaction, since fluorine is more reactive than chlorine.

Answer: Iodine freed from KI; no change with KF.

Case-study practice (Section B style)

Read the situation, then answer every part. In a school exam set on the board pattern this is a 5-mark question split into 2 + 1 + 1 + 1 marks.

Building a mini table

Suppose a teacher gives four cards: P (Z = 3), Q (Z = 10), R (Z = 11) and S (Z = 17), and asks students to place them in a blank modern periodic table.

(i) · 2 marks
Write the configuration of each.
Answer
Answer: P 2, 1; Q 2, 8; R 2, 8, 1; S 2, 8, 7.
(ii) · 1 mark
Which two belong to the same group, and what is the group called?
Answer
Answer: P and R: group 1, alkali metals.
(iii) · 1 mark
Which is the least reactive, and why?
Answer
Answer: Q (neon): it has a complete octet.
(iv) · 1 mark
Which element is a halogen, and what ion does it form?
Answer
Answer: S (chlorine); Cl⁻.

Where the marks are

How a typical answer is split into step marks. Show every step: a right final answer alone rarely earns full marks.

State Mendeleev's periodic law and the modern periodic law. (2 marks) (2 marks)

Step	Marks
Mendeleev: properties are periodic functions of atomic masses	1
Modern: properties are periodic functions of atomic numbers	1

Give three merits of Mendeleev's periodic table. (3 marks) (3 marks)

Step	Marks
Gaps left for undiscovered elements	1
Properties of missing elements predicted (Ga, Ge, Sc)	1
Atomic masses corrected / noble gases fitted without disturbing the table	1

An element has atomic number 16. Give its configuration, period, group and valency. (4 marks) (4 marks)

Step	Marks
2, 8, 6	1
Period 3	1
Group 16	1
Valency 2	1

Where students lose marks in The Periodic Table

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

Mistake: Writing 'atomic mass' in the modern law
Students mix up Mendeleev's and the modern law.
Wrong working
Modern periodic law: properties are periodic functions of atomic masses.
Right working
... periodic functions of their atomic numbers.
Do this: Mendeleev = Mass; Modern = Number.
Usually costs: 1 mark.
Mistake: Wrong group number for groups 13-17
Valence electrons are written as the group number for p-block elements.
Wrong working
Chlorine (2, 8, 7) is in group 7.
Right working
Group 17 (7 + 10) in the modern 18-group table.
Do this: Add 10 for 3-7 valence electrons.
Usually costs: 1 mark.
Mistake: Group and period confused
Rows and columns are swapped: the number of shells is given as the group and the valence electrons as the period. A related slip, noted in CISCE examiners' comments on the ICSE Class 10 paper, 2025, is writing the period as the Roman numeral III instead of 3.
Wrong working
Na (2, 8, 1) is in period 1, group 3.
Right working
Period 3 (three shells), group 1 (one valence electron).
Do this: Period = shells (rows); group = valence electrons (columns). Write periods as ordinary numbers.
Usually costs: 1 mark each.
Mistake: Vague limitations
Answers like 'it was not correct' earn nothing.
Wrong working
Newlands' law had many mistakes.
Right working
Newlands' law worked only up to calcium.
Do this: State a specific, named limitation.
Usually costs: 1 mark.

Practice questions on The Periodic Table

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 · MCQExam level
The element placed by Mendeleev with a gap called eka-aluminium is:
(a) germanium
(b) gallium
(c) scandium
(d) aluminium
Hint
Below Al.
Answer
(b)
Q2 · 1 mark · MCQExam level
The number of elements in period 3 is:
(a) 2
(b) 8
(c) 18
(d) 32
Hint
Short period.
Answer
(b)
Q3 · 3 marks · Very short answerExam level
Name: (a) the alkaline earth metal in period 3, (b) a halogen that is a liquid at room temperature, (c) the noble gas with only two electrons.
Hint
Groups 2, 17, 18.
Answer
(a) Magnesium (b) Bromine (c) Helium
Q4 · 2 marks · Short answerExam level
State Newlands' law of octaves. Using Li (7), Be (9), B (11), C (12), N (14), O (16), F (19), Na (23), explain why it is called an 'octave'.
Hint
Count to Na.
Answer
When elements are arranged by increasing atomic mass, every eighth element resembles the first. Counting from Li, the eighth is Na, which is like Li, just as the eighth musical note repeats the first.
Q5 · 2 marks · Short answerHarder exam level
Mendeleev's table could not explain the position of isotopes. How does the modern periodic table solve this?
Hint
Same Z.
Answer
Isotopes have different atomic masses but the same atomic number. Since the modern table uses atomic number, all isotopes of an element share one place.
Q6 · 3 marks · Short answerHarder exam level
An element E is in group 2 and period 4. (a) Write its configuration. (b) Find its atomic number. (c) Write the formula of its chloride and oxide.
Hint
4 shells, 2 outer.
Answer
(a) 2, 8, 8, 2 (b) 20 (calcium) (c) ECl₂, EO
Q7 · 2 marks · Short answerHarder exam level
Why do elements in the same group show similar chemical properties but elements in the same period do not?
Hint
Valence electrons.
Answer
Chemical properties depend on valence electrons. A group has the same number of valence electrons; across a period the number increases by one each time, so properties change.
Q8 · 3 marks · Short answerHarder exam level
Write the equations for the reactions of (a) potassium with water, (b) chlorine with sodium bromide solution. State one observation for each.
Hint
Alkali + H₂; displacement.
Answer
(a) $2 K + 2 H_{2} O \to 2 K O H + H_{2}$ : potassium darts about and the hydrogen catches fire with a lilac flame. (b) $C l_{2} + 2 N a B r \to 2 N a C l + B r_{2}$ : the colourless solution turns orange-brown.
Q9 · 2 marks · Short answerHarder exam level
N (14), P (31) and As (75) are in the same group. Show that they do not form a Dobereiner triad, and state what this shows about the triad rule.
Hint
Mean of 14 and 75.
Answer
Mean = (14 + 75)/2 = 44.5, far from 31. Not every group of similar elements follows the rule, a limitation of Dobereiner's idea.
Q10 · 3 marks · Short answerHOTS / competency
Element X forms a chloride XCl₃ and is in period 3. (a) Which group is it in? (b) Write its configuration and name it. (c) Write the formula of its oxide and sulphate.
Hint
Valency 3.
Answer
(a) Group 13 (b) 2, 8, 3: aluminium (c) X₂O₃; X₂(SO₄)₃
Q11 · 3 marks · Short answerHOTS / competency
If elements were still arranged by atomic mass, where would argon (39.9) and potassium (39.1) go, and what chemical nonsense would result?
Hint
Swap them.
Answer
Potassium would come before argon: potassium would fall in the noble gas group and argon with the alkali metals. A very reactive metal would sit with inert gases, and an inert gas with reactive metals. Ordering by atomic number (Ar 18, K 19) avoids this.

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.Who gave the law of octaves?Show answer
John Newlands.
2.Eka-silicon is now called?Show answer
Germanium.
3.Who showed atomic number is fundamental?Show answer
Henry Moseley.
4.Number of groups and periods in the modern table?Show answer
18 groups, 7 periods.
5.Which groups hold the transition elements?Show answer
Groups 3 to 12.
6.Where are lanthanides and actinides placed?Show answer
In two separate rows at the bottom (group 3, periods 6 and 7).
7.Valence electrons of halogens?Show answer
Seven.
8.Another name for group 18?Show answer
Zero group (noble gases).

Best order to study it

Learn Dobereiner, Newlands and Mendeleev as principle-example-merit-limitation.
Learn Moseley's discovery and the modern periodic law.
Learn the groups and periods and practise placing elements from Z.
Learn the placement of transition and inner transition elements.
Learn the four families with members, valence electrons and behaviour.

Quick revision checklist

The night before the exam: can you do each of these without looking?

Can you give two Dobereiner triads with the mean calculation?
Can you state Newlands' law and two limitations?
Can you give three merits and three limitations of Mendeleev's table?
Can you explain how the modern law fixes isotopes and anomalous pairs?
Can you find group and period for Z = 1-20?
Can you name the members of groups 1, 2, 17 and 18?

The Periodic Table: frequently asked questions

Do I need to know trends like atomic size and ionisation energy?

Not in Class IX. Periodic properties and their trends are in the Class X syllabus (Periodic Table, Periodic Properties and Variations of Properties). In Class IX focus on the history, the modern law, groups, periods and the four families.

Why are they called 'alkali' and 'alkaline earth' metals?

Group 1 metals react with water to form strong alkalis (NaOH, KOH). Group 2 metal oxides were once called 'earths' and they also form alkaline solutions.

How much do I need about transition elements?

Only their placement: groups 3 to 12, and the inner transition elements (lanthanides and actinides) in two rows at the bottom. The syllabus says their placement is to be mentioned.

Why is group 18 called the zero group?

Noble gases have a combining capacity (valency) of zero, because their outer shells are complete.

Based on: CISCE ICSE 2028 Science (52) Chemistry (Science Paper 2) syllabus, Classes IX and X; CISCE Analysis of Pupil Performance, ICSE (Class 10) Chemistry (Science Paper 2), 2025