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.

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 examples🎯 6 practice⚖️ 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 Law★ Board fav

Properties = f (atomic mass)

Modern Periodic Law (Moseley, 1913)★ Board fav

Properties = f (atomic number Z)

Maximum Electrons in Shell (Bohr-Bury)

Max = 2 n^{2}

Valency Rule★ Board fav

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

Atomic Radius Trend★ Board fav

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.