Ch 03: Periodic Classification of Elements

📋 Chapter At a Glance

118

Elements in Modern Table

Periods

Groups (Modern)

Mendeleev

Father of Periodic Table

History: Dobereiner (1829) → Newlands (1866) → Mendeleev (1869) → Moseley/Modern (1913)
Modern table based on Atomic Number (not atomic mass)
4 Blocks: s, p, d, f | Groups 1–18
Most electronegative: Fluorine | Highest electron affinity: Chlorine
Largest element: Cs (Caesium) | Lightest metal: Li (Lithium) | Highest MP: W (Tungsten)

📊 Periodic Tables — Comparison

Feature	Mendeleev (1869)	Modern / Moseley (1913)
Basis	Atomic Mass	Atomic Number
Periods	7	7
Groups	8	18
Total Elements	63 (known then)	118
Inert/Noble Gas	Absent (Zero group)	Group 18 present
Proposed by	Dmitri Mendeleev	Henry Moseley
H & Isotopes	Position not fixed	Fixed positions

📜 Evolution of Periodic Classification

1829

Dobereiner's Triads

Wolfgang Dobereiner — Germany

Grouped elements into triads of 3. Middle element's mass = average of outer two. e.g., Li (7), Na (23), K (39) → Na = (7+39)/2 = 23.

1866

Newlands' Law of Octaves

John Newlands — England

Elements in increasing atomic mass order. Properties of every 8th element repeat. Based on musical notes (Sa Re Ga Ma...). Applied only up to Calcium. Failed after noble gas discovery.

1869

Mendeleev's Periodic Table

Dmitri Mendeleev — Russia

Based on atomic mass. 7 periods, 8 traditional groups (I–VIII), 63 elements. Predicted missing elements (Eka-Boron → Scandium, Eka-Aluminium → Gallium, Eka-Silicon → Germanium). No noble gas group.

1913

Modern Periodic Table

Henry Moseley — England

Moseley established atomic number as the periodic basis (1913). The current table has 7 periods, 18 groups and 118 elements; isotopes share a position, while hydrogen remains chemically exceptional.

🔺 Dobereiner's Triads (1829)

📘 Rule

Three elements with similar properties arranged in ascending order of atomic weight. Middle element's mass = Average of other two.

Mass of Middle = (Mass of 1st + Mass of 3rd) / 2Dobereiner's Triad Formula

Triad Group	Element 1	Element 2 (Middle)	Element 3	Verification
Group I (Alkali)	Li (7)	Na (23)	K (39)	(7+39)/2 = 23 ✓
Group II (Alkaline)	Ca (40)	Sr (87.5)	Ba (137)	(40+137)/2 = 88.5 ≈ ✓
Group III (Halogen)	Cl (35.5)	Br (80)	I (127)	(35.5+127)/2 = 81.25 ≈ ✓

⚠️ Exam Trap — Dobereiner

Proposed by Wolfgang Dobereiner in 1829
Only groups of 3 elements (triads), not all elements
Li-Na-K: Na = (7+39)/2 = 23 — most asked calculation
Failed because not all elements fit into triads

🎵 Newlands' Law of Octaves (1866)

📘 Law

Elements arranged in increasing order of atomic mass. Properties of every 8th element are similar to the first. Based on musical notes (Sa, Re, Ga, Ma, Pa, Da, Ni).

Proposed by

John Newlands

Year

1866

Country

England

Applied up to

Calcium (Ca)

Elements known

OCTAVE PATTERN — Row 1 properties repeat at Row 2 (every 8th element):

Highlighted = same properties; 8th element (Cl) repeats properties of 1st (F)

⚠️ Exam Trap — Newlands

Every 8th element repeats (not every 7th or 9th)
Applied only up to Calcium — failed beyond
Failed when Inert/Noble gases were discovered (had to squeeze 2 elements in one box)
Based on atomic mass (NOT atomic number)
56 elements were known at that time

📊 Mendeleev's Periodic Table (1869)

📘 Mendeleev's Periodic Law

Physical and chemical properties of elements are a periodic function of their atomic mass.

1869

Year

Known Elements

Periods

Groups

Based on Atomic Mass
No Inert/Noble gas group (Zero group absent)
Position of Hydrogen and Isotopes not fixed
Left blank spaces for undiscovered elements (Eka-elements)
Father of Periodic Table = Dmitri Mendeleev

🔮 Mendeleev's Predicted Elements (Eka-Elements)

Predicted Name (Eka-)	Modern Name (Discovered)	Symbol
Eka-Boron	Scandium	Sc (21)
Eka-Aluminium	Gallium	Ga (31)
Eka-Silicon	Germanium	Ge (32)

⚠️ Exam Trap — Eka Elements

Eka-Boron → Scandium (Sc)
Eka-Aluminium → Gallium (Ga) — most asked!
Eka-Silicon → Germanium (Ge)
"Eka" = Sanskrit word meaning "one" (one step below in the group)

⚗️ Modern Periodic Table (1913)

📘 Modern Periodic Law

Physical and chemical properties of elements are a periodic function of their atomic number. Proposed by Henry Moseley (1913).

118

Total Elements

Groups

Periods

Gaseous Elements at STP

📂 Groups in Modern Periodic Table

Group 1

Alkali Metals (s-block)

Group 2

Alkaline Earth Metals (s-block)

Groups 3–12

d-block elements; strict transition definition has exceptions

Group 15

Pnictogens (p-block)

Group 16

Chalcogens (p-block)

Group 17

Halogens (p-block)

Group 18

Noble gases; He is s-block by configuration, others p-block

Lanthanoids

Atomic No. 58–71

Period 6, f-block
Inner transition elements
Also called Rare Earth elements

Actinoids

Atomic No. 90–103

Period 7, f-block
Inner transition elements
All actinoids are radioactive; many heavier ones are synthetic

📅 Periods in Modern Periodic Table

Period	No. of Elements	Orbitals Filled	Type
1	2	1s	Very short period
2	8	2s, 2p	Short period
3	8	3s, 3p	Short period
4	18	4s, 3d, 4p	Long period
5	18	5s, 4d, 5p	Long period
6	32	6s, 4f, 5d, 6p	Very long period (Lanthanoids)
7	32	7s, 5f, 6d, 7p	Complete: 32 elements (includes actinoids)

⚠️ Exam Trap — Periods

Periods 1, 2, 3 = Short (2, 8, 8 elements) | Periods 4, 5 = Long (18 each) | Periods 6, 7 = Very long (32 each)
Ti (Titanium, Z=22) placed in Group 4, Period 4
11 elements exist as gases at normal atmospheric conditions

🧱 Block Classification of Elements

Block	Groups	Orbital	Type of Elements	Examples
s-block	1, 2	s	Alkali metals (Gr.1), Alkaline earth metals (Gr.2)	Li, Na, K, Mg, Ca
p-block	13–18	p	Representative elements: Halogens, Noble gases, Pnictogens, Chalcogens	C, N, O, F, Cl, Ne
d-block	3–12	d	d-block; strict transition-element definition has exceptions	Fe, Cu, Zn, Cr, Ni
f-block	Lanthanoids & Actinoids	f	Inner transition elements (Period 6 & 7)	Ce, Nd, U, Th

⚠️ Exam Trap — Blocks

s-block by configuration: Groups 1–2 plus helium (1s²), which is placed in Group 18
p-block: Groups 13–18 except He, which is 1s² and s-block by configuration
d-block: Groups 3–12; many, but not all, satisfy the strict transition-element definition
f-block = Lanthanoids (58–71) and Actinoids (90–103)
Noble gases: Group 18; He is s-block by configuration, the others are p-block
Hydrogen belongs to s-block (Group 1) but is a non-metal

📈 Periodic Properties — Trends

⚡ Electronegativity

Ability to attract electrons. Highest: Fluorine (F)

Group ↓ Decreases

Period → Increases

🔋 Electron Affinity

Energy released on adding e⁻. Highest: Chlorine (Cl)

Group ↓ Decreases

Period → Increases

🔵 Atomic Size

Size of atom. Largest: Cs | Smallest: F

Group ↓ Increases

Period → Decreases

🔢 Valency

No. of electrons for bonding

Group: Same

Period → Increases

⚡ Ionisation Energy

Energy to remove outermost e⁻

Group ↓ Decreases

Period → Increases

🔩 Metallic Properties

Tendency to lose electrons

Group ↓ Increases

Period → Decreases

⚠️ Exam Trap — Periodic Properties

Most electronegative = Fluorine (F) — NOT Chlorine
Highest electron affinity = Chlorine (Cl) — NOT Fluorine (F has small size, e⁻ repulsion)
Largest atom = Caesium (Cs) | Smallest non-noble = Fluorine (F)
Groups 2, 15 and 18 are exceptions to simple electron-affinity trends; electron addition is relatively unfavourable, especially for noble gases
Metallic properties increase going down a group (more electrons, easier to lose outermost)
Noble gases (Group 18): zero valency, very high ionisation energy

⭐ Special Important Facts

Lightest Element

H (Hydrogen)

Largest Element

Cs (Caesium)

Lightest Metal

Li (Lithium)

Highest MP among Metals

W (Tungsten)

Most Electroneg.

F (Fluorine)

Highest e⁻ Affinity

Cl (Chlorine)

🎯 High-Yield Exam Points

Dobereiner Triads: Wolfgang Dobereiner (1829); middle = average of outer two; Li-Na-K
Newlands' Octaves: John Newlands (1866), England; every 8th element repeats; up to Ca; failed on noble gas discovery
Mendeleev: 1869; based on atomic mass; 7 periods, 8 traditional groups (I–VIII), 63 elements; NO noble gas group
Modern Table: Henry Moseley (1913); based on atomic number; 7 periods, 18 groups, 118 elements
Eka-Boron → Scandium | Eka-Aluminium → Gallium | Eka-Silicon → Germanium
Group 1 = Alkali metals | Group 2 = Alkaline earth metals
Groups 3–12 = Transition (d-block) | Group 15 = Pnictogens
Group 16 = Chalcogens | Group 17 = Halogens | Group 18 = Noble gases
Lanthanoids: Z = 58–71, Period 6, f-block
Actinoids: Z = 90–103, Period 7, f-block
Electronegativity: Group ↓ decreases; Period → increases; Highest = Fluorine
Electron Affinity: Highest = Chlorine (NOT Fluorine)
Atomic Size: Group ↓ increases; Period → decreases; Largest = Cs; Smallest = F
Ionisation Energy: Group ↓ decreases; Period → increases
Metallic Properties: Group ↓ increases; Period → decreases
Lightest element: hydrogen; atomic-size comparisons depend on the radius convention
Caesium: among the largest stable atomic radii in common textbook conventions
Lightest metal = Lithium (Li)
Highest melting point among metals: tungsten
Father of Periodic Table = Dmitri Mendeleev

📋 Tables Comparison Summary

Person	Year	Basis	Groups
Dobereiner	1829	Atomic Mass (triads)	—
Newlands	1866	Atomic Mass (octaves)	—
Mendeleev	1869	Atomic Mass	8
Moseley	1913	Atomic Number	18

⚠️ Most Common Exam Traps

Mendeleev = 8 traditional groups (I–VIII); Modern = 18 groups
Mendeleev based on atomic mass; Moseley on atomic number
Newlands failed at Calcium & noble gas discovery
Most electronegative = F; Highest electron affinity = Cl
Eka-Aluminium = Gallium (NOT Germanium)
Eka-Silicon = Germanium (NOT Gallium)
f-block: Lanthanoids (58–71) + Actinoids (90–103)
Lightest metal = Li (NOT Al or H)
Group 18: He has 1s² and is s-block by configuration; other noble gases are p-block

📋 Periodic Classification of Elements