Is it correct to say: Saying Mendeleev arranged elements by atomic number?

Mendeleev used atomic MASS. Atomic NUMBER (Moseley) is the modern criterion.

Is it correct to say: Confusing period and group?

Row = period (horizontal, shell number); column = group (vertical, valence electrons).

Is it correct to say: Believing hydrogen belongs only to Group 1?

H shows properties of BOTH Group 1 (alkali) and Group 17 (halogen) — its exact position is debated.

Is it correct to say: Atomic radius increases across a period?

It DECREASES because nuclear charge increases while shell number stays the same.

Is it correct to say: Placing noble gases in Group 0 in the modern table?

In modern IUPAC table, noble gases are Group 18. 'Group 0' was Mendeleev-era.

Is it correct to say: Assuming atomic mass always increases with Z?

Isotope effects can invert order: Ar (Z=18, mass 40) is heavier than K (Z=19, mass 39.1). Modern table places them by Z.

Formula sheet (PDF) →

Chemistry · Chapter 2

Periodic Classification of Elements

Dobereiner's triads, Newlands' octaves, Mendeleev's and modern periodic table, trends — Maharashtra SSC Science Part 1 Ch 2

10 formulas · 3 solved examples · 6 practice questions · Board weight: 4-5 marks · Maharashtra State Board Class 10 · 2025–26 syllabus

Board Exam Tips

→Historical timeline (Dobereiner → Newlands → Mendeleev → Modern) — dates and names must match. Frequent 3-mark question.
→Draw Mendeleev's table with gaps for eka-Si (Ge), eka-Al (Ga); this is a 3-mark diagram.
→Marathi: गट (group), आवर्त (period), अणुत्रिज्या (atomic radius), विद्युत् ऋणता (electronegativity).
→Trends along period (L→R): atomic radius decreases, metallic character decreases, electronegativity increases. Down group: opposite.
→State ONE limitation of Mendeleev vs Modern table — position of H, isotopes, Ar-K order.

Formulas(10)

Dobereiner's Law of Triads

Atomic mass of middle \approx \frac{m _{1} + m _{3}}{2}

Symbol	Meaning
$m_{1}, m_{3}$	Atomic masses of first and third elements

Newlands' Law of Octaves

Property of n^{th} element \approx (n + 8)^{th}

Mendeleev's Periodic LawBoard favourite

Properties = f (atomic mass)

Modern Periodic Law (Moseley, 1913)Board favourite

Properties = f (atomic number Z)

Maximum Electrons in Shell (Bohr-Bury)

Max = 2 n^{2}

Valency RuleBoard favourite

Valency = ve if ve \leq 4; Valency = 8 - ve if ve > 4

Atomic Radius TrendBoard favourite

Across period ↓; Down group ↑

Electronegativity Trend

Across period ↑; Down group ↓

Ionisation Energy Trend

Across period ↑; Down group ↓

Atomic Number and Mass Number

A = Z + N

Solved Examples

1Solved Exampleeasy3 steps

Write the electronic configuration of element with atomic number 17 and predict its group and period.

Distribute 17 electrons using 2n^2 rule

2, 8, 7

2Solved Exampleboard5 steps

Which is more metallic — Na or Mg — and why? State two properties supporting your answer.

Both are in Period 3; Na is Group 1, Mg is Group 2

3Solved ExampleHOTS5 steps

Element X has electronic configuration 2, 8, 4 and element Y has 2, 8, 7. Predict the formula of the compound between them and the type of bond formed. Also predict their positions in the periodic table.

X: 2,8,4 → Period 3, Group 14 → tetravalent (like C, Si) → Silicon

Traps & Common Mistakes

Common Mistakes6

1
Saying Mendeleev arranged elements by atomic number
✓Mendeleev used atomic MASS. Atomic NUMBER (Moseley) is the modern criterion.
2
Confusing period and group
✓Row = period (horizontal, shell number); column = group (vertical, valence electrons).
3
Believing hydrogen belongs only to Group 1
✓H shows properties of BOTH Group 1 (alkali) and Group 17 (halogen) — its exact position is debated.
4
Atomic radius increases across a period
✓It DECREASES because nuclear charge increases while shell number stays the same.
5
Placing noble gases in Group 0 in the modern table
✓In modern IUPAC table, noble gases are Group 18. 'Group 0' was Mendeleev-era.
6
Assuming atomic mass always increases with Z
✓Isotope effects can invert order: Ar (Z=18, mass 40) is heavier than K (Z=19, mass 39.1). Modern table places them by Z.

Practice Yourself

Practice Yourself6 questions

Q1
State Mendeleev's periodic law and one merit of his table.
Q2
Give the group and period of element with Z = 20.
Q3
Which element has smaller atomic radius: Li or F?
Q4
Write two limitations of Mendeleev's periodic table.
Q5
Which family is called alkaline earth metals? Name any two.
Q6
State Modern Periodic Law.

Notes

Periodic Classification of Elements

Chemistry becomes manageable only after we sort ~118 elements into a periodic pattern. This chapter traces how the pattern was discovered — and how to read it.

SSC angle

Chapter 2 of Science Part 1 typically carries:

1-mark: name Mendeleev/Newlands/Moseley, or a group name.
3-mark: difference between Mendeleev's and Modern tables, or an atomic-number → group/period problem.
HOTS: deduce formula & bond type from two electron configurations.

Timeline of classification

Year	Scientist	Basis	Limitation
1817	Döbereiner	Triads (mass average)	Only a few worked
1866	Newlands	Octaves (every 8th similar)	Failed after Ca
1869	Mendeleev	Atomic mass + valency	H position, isotopes
1913	Moseley	Atomic number Z	Modern law

Periodic trends — memorise the direction

Across a period (L → R):

Atomic radius ↓
Ionisation energy ↑
Electronegativity ↑
Metallic character ↓
Non-metallic character ↑

Down a group (top → bottom):

Atomic radius ↑
Ionisation energy ↓
Electronegativity ↓
Metallic character ↑

Special families

Group 1 — Alkali metals (Li, Na, K, Rb, Cs, Fr) — most reactive metals.
Group 2 — Alkaline earth metals (Be, Mg, Ca, Sr, Ba, Ra).
Group 17 — Halogens (F, Cl, Br, I, At) — most reactive non-metals.
Group 18 — Noble gases (He, Ne, Ar, Kr, Xe, Rn) — inert due to full valence shells.

Board vs CBSE

CBSE covers the same content but often skips deep trend explanations. Maharashtra SSC asks reasoning-based short answers on why radius, IE and EN change the way they do — memorise the reasons, not just the direction.