Periodic Classification of Elements Questions for CTET PAPER II

Historical Development: From Dobereiner to Moseley

Scientists arranged elements in increasing order of properties over time. Dobereiner grouped elements in triads where the middle element's atomic mass was the average of the other two. Newlands arranged 56 elements and found every eighth element had similar properties — his Law of Octaves. Mendeleev arranged 63 known elements by increasing atomic mass and similar chemical properties, leaving gaps for undiscovered elements. Finally, Henry Moseley showed atomic number (not mass) is the fundamental property, leading to the Modern Periodic Law.

• Dobereiner's Triads: Li-Na-K, Ca-Sr-Ba, Cl-Br-I — middle element is average of two extremes
• Newlands' Law of Octaves: every 8th element repeats properties (worked only up to calcium)
• Mendeleev: arranged by atomic mass, predicted Eka-Boron (Sc), Eka-Aluminium (Ga), Eka-Silicon (Ge)
• Mendeleev's limitation: could not explain position of isotopes and hydrogen
• Modern Periodic Law (Moseley): properties are periodic functions of ATOMIC NUMBER
• Modern table has 18 groups (vertical) and 7 periods (horizontal)

Structure of the Modern Periodic Table

The Modern Periodic Table has 118 elements arranged in 7 periods and 18 groups. Periods represent the number of electron shells. Groups represent similar valence electron configuration, hence similar chemical properties. The table is divided into s-block (Groups 1-2), p-block (Groups 13-18), d-block (Groups 3-12, transition metals), and f-block (lanthanides and actinides at the bottom).

• Period 1: 2 elements (H, He) — shortest period
• Periods 2 and 3: 8 elements each — short periods
• Periods 4 and 5: 18 elements each — long periods
• Periods 6 and 7: 32 elements each — very long periods
• Group 1 = Alkali metals, Group 17 = Halogens, Group 18 = Noble gases
• Period number = number of electron shells; Group number = valence electrons (for main group)

Periodic Trends: Atomic Radius

Atomic radius is the size of an atom. Across a period (left to right), nuclear charge increases but electrons are added to the same shell, so the nucleus pulls electrons closer — atomic radius DECREASES. Down a group (top to bottom), new shells are added, so atomic radius INCREASES. Think of it like this: across a period the nucleus becomes a stronger magnet shrinking the cloud; down a group you add floors to a building making it taller.

• Across period: atomic radius decreases (more protons, same shell)
• Down group: atomic radius increases (more shells added)
• Largest atom in periodic table: Francium (Fr) — bottom-left corner
• Smallest atom: Helium or Fluorine depending on context (Fluorine among reactive elements)
• Cations are smaller than parent atoms; anions are larger than parent atoms

Periodic Trends: Ionization Energy and Electronegativity

Ionization Energy (IE) is the energy needed to remove the outermost electron from a gaseous atom. More nuclear charge and smaller size means it is harder to remove electrons. Electronegativity is the tendency of an atom to attract shared electrons in a bond. Both IE and electronegativity follow the same trend: increase across a period and decrease down a group. Noble gases are exceptions — they are not typically assigned electronegativity values.

• Ionization Energy: increases left to right across period, decreases top to bottom in group
• Highest IE: Helium (noble gas, very stable); among metals: Cesium has lowest IE
• Electronegativity: increases left to right, decreases top to bottom
• Most electronegative element: Fluorine (F) — remember F is the greediest atom
• Least electronegative (among reactive elements): Francium or Cesium
• Metallic character decreases across a period and increases down a group

Valency and Chemical Properties Across Periods

Valency is the combining capacity of an element. For Period 2 and 3 elements, valency first increases from 1 to 4 then decreases back to 0 (for noble gas). Metals are on the left side of the periodic table with 1-3 valence electrons; non-metals are on the right with 4-7 valence electrons. Metalloids (Si, Ge, As, Sb, Te) lie on the staircase boundary and show mixed properties.

• Valency pattern across Period 3: Na=1, Mg=2, Al=3, Si=4, P=3, S=2, Cl=1, Ar=0
• Non-metals increase in reactivity down Group 17 is reversed — Fluorine is most reactive halogen
• Metals increase in reactivity down Group 1 — Caesium is most reactive alkali metal
• Oxides of metals are basic; oxides of non-metals are acidic
• Amphoteric oxides: Al2O3 and ZnO — react with both acid and base

Mendeleev's Contributions and Limitations (CTET Favourite)

CTET specifically tests Mendeleev because it is in Class 10 NCERT which teachers of Class 6-8 must know as background knowledge. Mendeleev is celebrated for predicting undiscovered elements and their properties. His limitations are equally tested. The key anomaly is that some elements are placed out of atomic mass order (Cobalt before Nickel, Tellurium before Iodine) to maintain similar group properties.

• Mendeleev predicted: Eka-Boron = Scandium, Eka-Aluminium = Gallium, Eka-Silicon = Germanium
• Limitation 1: Position of Hydrogen not clearly justified (resembles alkali metals AND halogens)
• Limitation 2: Isotopes of same element have different atomic masses but one position
• Limitation 3: Wrong order for Co-Ni and Te-I pairs based on atomic mass
• Limitation 4: No place for noble gases (discovered later)

• Confusing atomic radius trend: many aspirants say it increases across a period — it actually DECREASES because nuclear charge pulls electrons inward
• Mixing up Ionization Energy and Electron Affinity — IE is energy to REMOVE an electron; Electron Affinity is energy released when an electron is ADDED
• Saying Mendeleev arranged elements by atomic number — he used atomic MASS; Moseley introduced atomic number
• Forgetting that metallic character increases DOWN a group and decreases across a period — opposite to IE trend
• Confusing groups and periods: Groups are vertical columns (same properties); Periods are horizontal rows (same number of shells)

• DNAM for historical order: Dobereiner, Newlands, mEndeleev (M), mOseley (O) — D-N-M-O like Do Not Miss Out
• For trends across a period — IDEA: Ionization energy, Distance (radius) decreases, Electronegativity increases, metallic character decreAses
• Fluorine is the GREEDIEST — highest electronegativity, highest ionization energy among non-noble elements
• Eka-clue: Eka means ONE in Sanskrit — Mendeleev named undiscovered elements ONE step below known ones in the group

• CTET Paper II Science section asks 1-3 questions from this topic; mostly from Class 8 NCERT Chapter 5 — read that chapter once before exam
• Questions are almost always direct and factual: Who proposed, what is the trend, which element has highest electronegativity — no calculations required
• Mendeleev's achievements and limitations is a guaranteed hot spot — expect at least one question framed as a situation (a teacher explains to students that...)
• Periodic trends questions often give a scenario with 4 elements from same period or group and ask which has largest atomic radius or highest IE — apply the left-right, top-bottom rule
• Pedagogical angle may appear: how to teach the concept of periodicity using activity or analogy — connect it to constructivist methods from CDP for combined questions