Chemistry · Ch 03

📋 Periodic Classification of Elements

BPSC / BSSC · Exam Fusion Prep  |  Dobereiner · Newland · Mendeleev · Modern Table · Blocks · Periodic Properties

📋 Chapter At a Glance
118
Elements in Modern Table
7
Periods
18
Groups (Modern)
Mendeleev
Father of Periodic Table
  • History: Dobereiner (1829) → Newland (1864) → 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 9 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.

1864
Newland's Law of Octaves
John Newland — 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, 9 groups, 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

Based on atomic number (Modern Periodic Law). 7 periods, 18 groups, 118 elements. All anomalies of Mendeleev's table resolved. Noble gases placed in Group 18.

🔺 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
🎵 Newland's Law of Octaves (1864)
📘 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 Newland
Year
1864
Country
England
Applied up to
Calcium (Ca)
Elements known
56

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

H
Li
Be
B
C
N
O
F
F
Na
Mg
Al
Si
P
S
Cl

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

⚠️ Exam Trap — Newland
  • 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
63
Known Elements
7
Periods
9
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
18
Groups
7
Periods
11
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
Transition Elements (d-block)
Group 15
Pnictogens (p-block)
Group 16
Chalcogens (p-block)
Group 17
Halogens (p-block)
Group 18
Noble/Inert Gases (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
  • Most are radioactive (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 Incomplete (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 Transition elements 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 = Groups 1 and 2 only
  • p-block = Groups 13 to 18
  • d-block = Groups 3 to 12 (Transition metals)
  • f-block = Lanthanoids (58–71) and Actinoids (90–103)
  • Noble gases (Group 18) are in p-block (NOT a separate 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, 18 elements have zero or positive electron affinity
  • 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
Smallest Element
H (Hydrogen)
Largest Element
Cs (Caesium)
Lightest Metal
Li (Lithium)
Highest Melting Pt
W (Tungsten)
Most Electroneg.
F (Fluorine)
Highest e⁻ Affinity
Cl (Chlorine)
🎯 High-Frequency BPSC/BSSC Exam Points
  • Dobereiner Triads: Wolfgang Dobereiner (1829); middle = average of outer two; Li-Na-K
  • Newland's Octaves: John Newland (1864), England; every 8th element repeats; up to Ca; failed on noble gas discovery
  • Mendeleev: 1869; based on atomic mass; 7 periods, 9 groups, 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
  • Smallest element = Hydrogen (H)
  • Largest element = Caesium (Cs)
  • Lightest metal = Lithium (Li)
  • Highest melting point = Tungsten (W)
  • Father of Periodic Table = Dmitri Mendeleev
📋 Tables Comparison Summary
Person Year Basis Groups
Dobereiner 1829 Atomic Mass (triads) —
Newland 1864 Atomic Mass (octaves) —
Mendeleev 1869 Atomic Mass 9
Moseley 1913 Atomic Number 18
⚠️ Most Common Exam Traps
  • Mendeleev = 9 groups; Modern = 18 groups
  • Mendeleev based on atomic mass; Moseley on atomic number
  • Newland 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)
  • Noble gases in p-block (Group 18), NOT separate block
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