10th SSC Board · Science & Technology Part-1

Periodic Classification
of Elements

Lesson No. 2 · Lesson Explanation · Pages 16–29

Prepared by: Anurag Santosh Maurya

⚠ Disclaimer: This document has been prepared by Anurag Santosh Maurya for personal study and educational purposes only. Unauthorized sharing, reproduction, copying, or distribution of this material — in whole or in part — without prior written permission is strictly prohibited. All rights reserved © Anurag Santosh Maurya.

1. Introduction to the Lesson

🌍 Why Study This Lesson?

Today we know about 118 elements. Studying all of them separately would be impossible without any order. Scientists spent many years finding the best way to arrange (classify) elements based on their properties. This lesson tells you about that journey — from Dobereiner's simple Triads in 1817, all the way to the Modern Periodic Table with 118 elements today!

💡 What You Will Learn

• How elements were classified over different time periods
• Dobereiner's Triads, Newlands' Law of Octaves, Mendeleev's Periodic Table
• Modern Periodic Law, structure of Modern Periodic Table
• Periodic trends: Valency, Atomic Size, Metallic/Non-metallic character

📅 How Knowledge of Elements Grew Over Time

From just 30 elements in 1800 to 118 elements today — here is the amazing journey!

~1800

30 elements known. Only basic metals and non-metals were known — Gold, Silver, Copper, Sulfur, Oxygen etc. No system of classification yet.

1817

~35 elements. Döbereiner grouped elements into "Triads". Each triad had 3 elements with similar properties where the middle element's mass = average of the other two.

1866

~56 elements. Newlands arranged all known elements by atomic mass. Every 8th element showed similar properties to the 1st — he called this the "Law of Octaves".

1869–72

63 elements. Mendeleev created his famous Periodic Table based on atomic mass. He predicted properties of undiscovered elements and left blank spaces for them.

~1890s

~75 elements. Noble gases (He, Ne, Ar) were discovered. Mendeleev added a "Zero Group" without disturbing his original table.

1913

92 natural elements known. Henry Moseley showed that Atomic Number (not mass) is the true basis for classification → Modern Periodic Law stated.

1940s–80s

Up to ~108 elements. Scientists created artificial (man-made) radioactive elements in labs. All are unstable and have very short life.

2016

118 elements — Complete! All 118 elements are now confirmed and the Modern Periodic Table is completely filled.

2. Classification of Elements

📖 Definition

Classification of elements means arranging all elements into groups based on their similar properties, so that they are easier to study, compare, and remember.

The first (and simplest) classification was into just two groups. Later, a third was added:

Type	Properties	Examples
Metals	Shiny, good conductors of heat & electricity, malleable, ductile, lose electrons	Iron (Fe), Gold (Au), Copper (Cu), Sodium (Na), Potassium (K)
Non-metals	Poor conductors, dull, brittle (if solid), gain electrons	Oxygen (O), Carbon (C), Nitrogen (N), Sulfur (S), Chlorine (Cl)
Metalloids	Show properties of BOTH metals and non-metals	Silicon (Si), Arsenic (As), Germanium (Ge), Boron (B)

🤔 Why was a better system needed?

As more elements were discovered, just three groups were not enough. Scientists needed a system that could show relationships between all elements AND help predict properties of undiscovered ones.

3. Dobereiner's Triads (1817)

🧑‍🔬 Johann Wolfgang Döbereiner (1780–1849)

Nationality:	German
Year of Work:	1817
Basis:	Atomic Mass
Contribution:	Grouped elements into sets of three (Triads). Middle element's mass = average of first and third.

📖 Story — The Three Brothers!

Imagine three brothers. The youngest weighs 7 kg and the oldest weighs 39 kg. Döbereiner noticed that the middle brother's weight would be close to the average: (7 + 39) ÷ 2 = 23 kg. In chemistry, this was exactly what happened with Lithium (7), Sodium (23), and Potassium (39)! Döbereiner found this for several groups of three elements — he called them "Triads".

📖 Definition — Dobereiner's Triad

A Triad is a group of three elements with similar chemical properties, arranged in increasing order of atomic mass. The atomic mass of the middle element is approximately equal to the average (mean) of the atomic masses of the first and third elements.

Formula: Middle element's mass ≈ (First element's mass + Third element's mass) ÷ 2

Dobereiner's Triads — Table 2.1 (from Textbook)

Triad	Element 1 (a)	Mean = (a+c)÷2	Element 2 (actual mass)	Element 3 (c)
Li, Na, K	Lithium (Li) = 6.9	(6.9 + 39.1) ÷ 2 = 23.0	Sodium (Na) = 23.0 ✓	Potassium (K) = 39.1
Ca, Sr, Ba	Calcium (Ca) = 40.1	(40.1 + 137.3) ÷ 2 = 88.7	Strontium (Sr) = 87.6 ✓	Barium (Ba) = 137.3
Cl, Br, I	Chlorine (Cl) = 35.5	(35.5 + 126.9) ÷ 2 = 81.2	Bromine (Br) = 79.9 ✓	Iodine (I) = 126.9

⚠ Limitations of Dobereiner's Triads

ALL known elements could NOT be classified into triads.
Only a few groups of three elements followed this pattern.
This method was incomplete and limited.

4. Newlands' Law of Octaves (1866)

🧑‍🔬 John Alexander Reina Newlands (1837–1898)

Nationality:	English (British)
Year of Work:	1866
Basis:	Atomic Mass (arranged in increasing order)
Key Idea:	Every 8th element has properties similar to the 1st element — just like musical octaves!

🎵 Story — Music and Chemistry Come Together!

Newlands noticed something amazing. In Indian classical music, we have 7 notes: Sa Re Ga Ma Pa Dha Ni. After the 7th note, the 8th note "Sa" comes again — at a higher pitch. This is called a "Saptak".

Newlands found the SAME pattern in elements! When arranged by atomic mass, every 8th element showed properties similar to the 1st element. He named this the Law of Octaves — because an octave in music also has 8 notes.

Example: Lithium (Li) is 1st. Sodium (Na) is the 8th element. Both Li and Na are soft metals that react vigorously with water!

📖 Definition — Newlands' Law of Octaves

When elements are arranged in increasing order of atomic mass, every 8th element has properties similar to the 1st element. This is called the Law of Octaves.

Newlands' Octave Table 2.2 (from Textbook)

Musical Note	Do (Sa)	Re	Mi (Ga)	Fa (Ma)	So (Pa)	La (Dha)	Ti (Ni)
Row 1	H	Li	Be	B	C	N	O
Row 2	F	Na	Mg	Al	Si	P	S
Row 3	Cl	K	Ca	Cr	Ti	Mn	Fe
Row 4	Co & Ni	Cu	Zn	Y	In	As	Se
Row 5	Br	Rb	Sr	Ce & La	Zr	—	—

Note: Li and Na both come under 'Re' (2nd position). Both have similar properties — the Law of Octaves works here!

⚠ Limitations of Newlands' Law of Octaves

The law worked only up to Calcium (Ca). After that, every 8th element did NOT have similar properties.
Newlands fitted all elements in a 7 × 8 table (56 boxes). He had to put 2 elements in the same box (e.g., Co & Ni, Ce & La). This is not scientifically correct.
Some elements with different properties were placed in the same column. E.g., metals Co & Ni were placed with halogens under the same note.
Iron (Fe) was placed far from Co and Ni, even though all three are very similar metals.
No space was left for newly discovered elements — they didn't fit the pattern.

5. Mendeleev's Periodic Table (1869–1872)

🧑‍🔬 Dmitri Ivanovich Mendeleev (1834–1907)

Nationality:	Russian
University:	Professor at St. Petersburg University, Russia
Year of Work:	1869–1872
Basis:	Atomic Mass (in increasing order)
Greatest Achievement:	Created Periodic Table of 63 elements AND predicted properties of undiscovered elements!

📖 Story — The Card Game That Changed Chemistry!

Mendeleev made a separate card for every known element, writing its atomic mass and properties on it. He then spread all 63 cards on a table and kept arranging them, like solving a huge jigsaw puzzle.

He noticed that elements with similar properties appeared at regular intervals when arranged by atomic mass. When he saw a gap in his arrangement, instead of forcing an element there, he LEFT IT EMPTY — and said: "An element with these properties will be discovered someday."

He was right! Gallium (1875), Scandium (1879), and Germanium (1886) were discovered later — their properties matched his predictions almost perfectly!

📖 Mendeleev's Periodic Law (Definition)

"Properties of elements are a periodic function of their atomic masses."

This means: When arranged in increasing order of atomic mass, elements with similar properties appear repeatedly at regular intervals (periodically).

Structure of Mendeleev's Table

Vertical columns = Groups (originally 8 groups: Group I to Group VIII)
Horizontal rows = Periods (Series) (12 series in Mendeleev's table)
Basis of arrangement: Increasing order of Atomic Mass
He used molecular formulae of hydrides and oxides as a guide (R₂O, RH₄, etc.)

✅ Merits of Mendeleev's Periodic Table

Merit 1 — Correction of Atomic Masses

Mendeleev corrected wrong atomic masses so elements fell in the right place. E.g., Beryllium's mass was corrected from 14.09 → 9.4, so it was placed before Boron.

Merit 2 — Prediction of Undiscovered Elements ⭐

Left gaps for undiscovered elements and predicted their properties. 'Eka-aluminum' (→ Gallium), 'Eka-silicon' (→ Germanium), 'Eka-boron' (→ Scandium) — all predicted accurately!

Merit 3 — Noble Gases Added

When noble gases (He, Ne, Ar) were discovered in the late 19th century, Mendeleev created a 'Zero Group' without disturbing the rest of the table.

Merit 4 — Systematic Study

For the first time, all known elements were arranged in one table, making it easy to study, compare, and predict chemical behaviour.

Mendeleev's Prediction vs Gallium (Table 2.4 from Textbook)

Property	Eka-Aluminum (Mendeleev's Prediction, 1871)	Gallium — Ga (Actual, Discovered 1875)
Atomic Mass	68	69.7
Density (g/cm³)	5.9	5.94
Melting Point	Low	30.2°C (melts in your hand!)
Formula of Chloride	ECl₃	GaCl₃
Formula of Oxide	E₂O₃	Ga₂O₃
Nature of Oxide	Amphoteric oxide	Amphoteric oxide

❌ Demerits of Mendeleev's Periodic Table

Demerit 1 — Co and Ni Problem

Cobalt (Co) and Nickel (Ni) have the same whole-number atomic mass (58). So which comes first? This created confusion in Mendeleev's table.

Demerit 2 — Isotopes Problem

Isotopes (same element, different masses) were discovered later. Since Mendeleev arranged by mass, where should isotopes be placed? This was a major problem.

Demerit 3 — Non-uniform Atomic Mass Rise

The increase in atomic mass was not regular, so it was impossible to predict how many elements existed between two heavy elements.

Demerit 4 — Position of Hydrogen

Hydrogen behaves like alkali metals (Group I) in some ways and like halogens (Group VII) in others. Mendeleev could NOT fix its position. This problem is still not fully solved!

6. Modern Periodic Law (1913) — Henry Moseley

🧑‍🔬 Henry Gwyn Jeffreys Moseley (1887–1915)

Nationality:	English (British)
Year of Work:	1913
Experiment:	Used X-ray tube experiments on elements. Found that Atomic Number (Z) = number of protons in nucleus = true fundamental property.
Key Contribution:	Replaced Atomic Mass with Atomic Number as the basis for classification.
Sad Fact:	Moseley was only 27 years old when he died in World War I. He could have won the Nobel Prize.

📖 Modern Periodic Law (Definition)

"Properties of elements are a periodic function of their atomic numbers."

Atomic Number (Z) = Number of protons in the nucleus of an atom.

This solved the Co-Ni problem! Co has Z=27, Ni has Z=28 → Co comes before Ni. ✓

Isotopes have same Z → placed in the SAME group. ✓

Mendeleev's Law vs Modern Periodic Law

Point	Mendeleev's Law	Modern Periodic Law
Basis	Atomic Mass	Atomic Number (Z)
Year	1869	1913
Scientist	Mendeleev (Russia)	Moseley (England)
Co, Ni Problem	Could NOT resolve	Solved ✓ (Co Z=27 before Ni Z=28)
Isotopes	Problem remained	No problem (same Z = same group) ✓

7. The Modern Periodic Table (Long Form)

📌 What is the Modern Periodic Table?

The Modern Periodic Table (also called the Long Form of the Periodic Table) arranges all 118 elements in increasing order of atomic number. It has 7 horizontal rows (periods) and 18 vertical columns (groups).

Structure of the Modern Periodic Table

Feature	Details
Periods (Rows)	7 horizontal rows. Period 1 to Period 7. Going left to right, electrons fill the same shell.
Groups (Columns)	18 vertical columns. Group 1 to Group 18. Same group = same number of valence electrons.
Total Elements	118 elements — all 118 boxes are now filled!
Lanthanide & Actinide Series	Shown separately at the bottom (f-block). Lanthanides are Period 6, Actinides are Period 7.
s-block	Groups 1 and 2. Alkali metals and Alkaline Earth metals. 1 or 2 valence electrons.
p-block	Groups 13–18. Contains metals, non-metals, metalloids, noble gases. Zig-zag line here.
d-block	Groups 3–12. Transition elements (Fe, Cu, Zn, Cr, Mn etc.).
f-block	Lanthanide & Actinide series. All actinides (Z=90–103) are radioactive.

💡 Did You Know?

Uranium (U) has atomic number 92. All elements beyond uranium — from Z=93 to 118 — are man-made in laboratories. They are all radioactive, unstable, and exist for only a very short time!

Groups and Electronic Configuration

🔑 Key Rule

All elements in the same group have the same number of valence electrons. This is why they have similar chemical properties!

Group	Family Name	Valence Electrons	Example Elements & Configuration
Group 1	Alkali Metals	1	Li (2,1) Na (2,8,1) K (2,8,8,1)
Group 2	Alkaline Earth Metals	2	Be (2,2) Mg (2,8,2) Ca (2,8,8,2)
Group 17	Halogens	7	F (2,7) Cl (2,8,7) Br (2,8,18,7)
Group 18	Noble Gases	8 (He = 2)	He (2) Ne (2,8) Ar (2,8,8)

Periods and Electronic Configuration

🔑 Key Rule

All elements in the same period have the same number of electron shells. A new period starts when a new electron shell begins filling.

Period	Shells Filled	Elements	Number of Elements
Period 1	K only	H, He	2 (K shell capacity = 2)
Period 2	K, L	Li, Be, B, C, N, O, F, Ne	8 (L shell capacity = 8)
Period 3	K, L, M	Na, Mg, Al, Si, P, S, Cl, Ar	8 (due to octet rule, not 18)
Period 4	K, L, M, N	K to Kr (includes d-block)	18

Electron Capacity of Shells (Table 2.9 from Textbook)

Shell	n	Formula: 2n²	Max Electrons
K	1	2 × 1² = 2	2
L	2	2 × 2² = 8	8
M	3	2 × 3² = 18	18
N	4	2 × 4² = 32	32

8. Periodic Trends in the Modern Periodic Table

When properties of elements are compared along a period or down a group, they change in a regular (periodic) pattern. These are called Periodic Trends. We study three main trends:

8.1 Valency

📖 Definition

Valency is the combining capacity of an element. It is determined by the number of valence electrons (electrons in the outermost shell).

Valency = Valence electrons (if ≤ 4) OR 8 − Valence electrons (if > 4) | Noble gases: Valency = 0

Direction	Trend in Valency	Example
Left → Right in Period	Increases from 1 to 4, then decreases (4 → 0)	Period 3: Na(1), Mg(2), Al(3), Si(4), P(3), S(2), Cl(1), Ar(0)
Top → Bottom in Group	Valency remains THE SAME	Group 1: Li(1), Na(1), K(1), Rb(1) — all valency 1

8.2 Atomic Size (Atomic Radius)

📖 Definition

Atomic Radius is the distance between the nucleus of an atom and its outermost electron shell. Measured in picometers (pm). (1 pm = 10⁻¹² m)

→ Across a Period (Left to Right)

Decreases ↓

More protons → stronger pull on electrons → electrons pulled closer → smaller atom.

↓ Down a Group (Top to Bottom)

Increases ↑

New electron shell added → electrons farther from nucleus → bigger atom.

📏 Real Example from Textbook

Period 2 elements (left → right): Li = 152 pm, Be = 111 pm, B = 88 pm, C = 77 pm, N = 74 pm, O = 66 pm
Li (leftmost) is biggest, O is smallest — proves the trend! ✓

Group 1 elements (top → bottom): Li=152, Na=186, K=231, Rb=244, Cs=262 pm
Goes on increasing as we go down the group. ✓

8.3 Metallic and Non-Metallic Character

📖 Key Definitions

Electropositivity (Metallic Character): Tendency of an atom to LOSE electrons and form a positive ion (cation). This is metallic character.

Electronegativity (Non-Metallic Character): Tendency of an atom to GAIN electrons and form a negative ion (anion). This is non-metallic character.

Direction	Metallic Character	Non-Metallic Character	Reason
Left → Right in Period	Decreases ↓	Increases ↑	Nuclear charge increases → harder to lose electrons → easier to gain electrons.
Top → Bottom in Group	Increases ↑	Decreases ↓	New shell added → valence electrons far from nucleus → easier to lose them.

📌 Zig-Zag Line in Periodic Table

A zig-zag line in the p-block separates metals from non-metals:
Left of line → Metals | On the line → Metalloids | Right of line → Non-metals

Gradation in Halogen Family (Group 17) — from Textbook

Element	Symbol	Physical State at Room Temp.	Colour
Fluorine	F₂	Gas	Pale yellow
Chlorine	Cl₂	Gas	Greenish-yellow
Bromine	Br₂	Liquid	Reddish-brown
Iodine	I₂	Solid	Violet/Dark grey

As you go down Group 17: Gas → Gas → Liquid → Solid. This is gradation!

Gradation in Alkaline Earth Metals (Group 2) — Reactivity with Water

M + 2H₂O → M(OH)₂ + H₂↑

Element	Reaction with Water
Beryllium (Be)	Does NOT react with water
Magnesium (Mg)	Reacts slowly with STEAM only (not cold water)
Calcium (Ca)	Reacts with cold water at room temperature
Strontium (Sr)	Reacts faster with cold water
Barium (Ba)	Reacts most vigorously with cold water

Reactivity increases going DOWN the group (Be → Mg → Ca → Sr → Ba). This is gradation!

Revision Notes — SSC Board 10th

Periodic Classification
of Elements

Lesson No. 2 · Quick Revision · Anurag Santosh Maurya

⚠ Disclaimer: This document has been prepared by Anurag Santosh Maurya for personal study only. Unauthorized sharing, copying, or distribution without prior written permission is strictly prohibited. © Anurag Santosh Maurya.

📊 Quick Summary of All Scientists

Scientist	Year	Country	Basis	Contribution	Main Limitation
Dobereiner	1817	Germany	Atomic Mass	Triads — groups of 3 similar elements. Middle mass ≈ average of 1st & 3rd.	Not all elements fit into triads.
Newlands	1866	England	Atomic Mass	Law of Octaves — every 8th element similar to 1st.	Works only up to Calcium. 2 elements in 1 box.
Mendeleev	1869–72	Russia	Atomic Mass	Periodic Table of 63 elements. Predicted undiscovered elements.	H position unclear. Co-Ni problem. Isotope problem.
Moseley	1913	England	Atomic Number	Modern Periodic Law — Z is the true basis. Solved Co-Ni & isotope problems.	H position still debated.

📖 Key Definitions (One Liner Each)

Term	One-Line Definition
Element	Pure substance made of only one type of atom.
Atomic Mass	Average mass of one atom of an element (in amu).
Atomic Number (Z)	Number of protons in the nucleus. (Z = electrons in neutral atom)
Triad	Group of 3 elements where middle mass ≈ average of 1st and 3rd. (Dobereiner)
Law of Octaves	Every 8th element similar to the 1st when arranged by atomic mass. (Newlands, 1866)
Mendeleev's Periodic Law	Properties are periodic function of ATOMIC MASSES.
Modern Periodic Law	Properties are periodic function of ATOMIC NUMBERS.
Period	Horizontal row. Same period = same number of electron shells.
Group	Vertical column. Same group = same valence electrons → similar properties.
Valency	Combining capacity; determined by number of valence electrons.
Atomic Radius	Distance from nucleus to outermost shell. Measured in picometers (pm).
Electropositivity	Tendency to LOSE electrons → metallic character.
Electronegativity	Tendency to GAIN electrons → non-metallic character.
Isotopes	Same element, same Z, but different atomic mass. Same group in modern table.
Noble Gases	Group 18. Full valence shell. Valency = 0. Very unreactive.
Transition Elements	d-block elements (Groups 3–12). Variable valency. E.g., Fe, Cu, Zn.

📈 Periodic Trends — Quick Reference

Property	→ Across Period (Left to Right)	↓ Down Group (Top to Bottom)
Atomic Radius	Decreases ↓	Increases ↑
Valency	1→2→3→4→3→2→1→0	Remains SAME
Metallic Character	Decreases ↓	Increases ↑
Non-Metallic Character	Increases ↑	Decreases ↓
Electropositivity	Decreases ↓	Increases ↑
Electronegativity	Increases ↑	Decreases ↓
Number of Shells	Same (stays constant)	Increases (new shell each period)
Valence Electrons	Increases by 1 each time	Same (same group = same valence e⁻)

⭐ Important Points to Remember for Board Exam

Dobereiner (1817): Triads — 3 elements; middle mass ≈ average of 1st & 3rd. Basis = Atomic Mass.
Newlands (1866): Law of Octaves — 8th element similar to 1st. Works only up to Calcium.
Mendeleev (1869–72): Periodic Law on Atomic Mass. Table of 63 elements. Predicted Ga, Sc, Ge accurately.
Moseley (1913): Modern Periodic Law on Atomic Number (Z = protons in nucleus).
Modern Table: 7 periods, 18 groups, 118 elements. 4 blocks: s, p, d, f.
Period number = number of electron shells of elements in that period.
Group number (main groups) = number of valence electrons.
Atomic radius DECREASES left → right in period. (More protons → stronger pull → smaller)
Atomic radius INCREASES top → bottom in group. (New shell added → bigger atom)
Metallic character INCREASES down a group. (Easier to lose electrons with more shells)
Metallic character DECREASES left → right in period. (Harder to lose electrons as Z increases)
Electronegativity INCREASES left → right in period; DECREASES down a group.
Noble gases (Group 18) have valency = 0 (full outermost shell → no tendency to react).
Halogens (Group 17): F₂(gas) → Cl₂(gas) → Br₂(liquid) → I₂(solid). Physical state gradation down group.
Alkaline earth metals (Group 2) reactivity: Be < Mg < Ca < Sr < Ba with water.
Isotopes: same Z → placed in SAME group. (Modern table solves isotope placement problem)
Position of Hydrogen is still DEBATED — shows similarity to both Group 1 and Group 17.
Elements Z = 93 to 118 are man-made, radioactive, unstable. (Uranium Z=92 is last natural element)
Zig-zag line in p-block: Metals (left) | Metalloids (on line) | Non-metals (right).
Valency in Period 3: Na(1) Mg(2) Al(3) Si(4) P(3) S(2) Cl(1) Ar(0)

🧪 Practice — Check if it's a Dobereiner's Triad

Formula: Is middle element's mass ≈ (1st mass + 3rd mass) ÷ 2?

Group	Element 1 (a)	Element 2 (actual)	Element 3 (c)	(a+c)÷2	Is it a Triad?
Li, Na, K	Li = 6.9	Na = 23.0	K = 39.1	(6.9+39.1)÷2 = 23.0	Yes ✓
Ca, Sr, Ba	Ca = 40.1	Sr = 87.6	Ba = 137.3	(40.1+137.3)÷2 = 88.7	Yes ✓ (≈87.6)
Cl, Br, I	Cl = 35.5	Br = 79.9	I = 126.9	(35.5+126.9)÷2 = 81.2	Yes ✓ (≈79.9)
Mg, Ca, Sr	Mg = 24.3	Ca = 40.1	Sr = 87.6	(24.3+87.6)÷2 = 55.95	No ✗ (55.95 ≠ 40.1)
S, Se, Te	S = 32.1	Se = 79.0	Te = 127.6	(32.1+127.6)÷2 = 79.85	Yes ✓ (≈79.0)
Be, Mg, Ca	Be = 9.0	Mg = 24.3	Ca = 40.1	(9.0+40.1)÷2 = 24.55	Yes ✓ (≈24.3)

⚡ Interesting Element Facts (Good for Exams)

Element	Z	Interesting Fact
Gallium (Ga)	31	Melts in your palm! Melting point = 30.2°C. Predicted by Mendeleev as 'Eka-Aluminum' (1871). Discovered 1875.
Fluorine (F)	9	Most electronegative element in the entire periodic table. Most reactive non-metal. Used in toothpaste!
Cesium (Cs)	55	Largest atomic radius in Group 1 (stable elements). Most electropositive. Melts at just 28.5°C.
Uranium (U)	92	Heaviest naturally occurring element. All elements Z=93 to 118 are man-made in labs!
Helium (He)	2	Smallest atom. Noble gas. Valency = 0. Used in balloons (lighter than air and non-flammable).
Scandium (Sc)	21	Mendeleev predicted it as 'Eka-boron' (mass ≈ 44). Discovered 1879. Properties matched perfectly!
Germanium (Ge)	32	Predicted as 'Eka-silicon'. Discovered 1886. This was Mendeleev's most accurate prediction.

✅ Mendeleev's Merits & ❌ Demerits — Quick Notes

Merits

1. Corrected atomic masses (e.g., Be: 14.09 → 9.4)
2. Left gaps for undiscovered elements + predicted their properties (Ga, Sc, Ge all matched!)
3. Added Zero Group for noble gases without disturbing the table

Demerits

1. Co and Ni problem (same atomic mass → sequence unclear)
2. Isotopes problem (different mass, same element → where to place?)
3. Non-uniform rise in mass (can't predict how many elements between two heavy elements)
4. Hydrogen position unclear (similar to Group I AND Group VII)

Periodic Classificationof Elements

Periodic Classificationof Elements

Periodic Classification
of Elements

Periodic Classification
of Elements