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Classification of Elements — GK Questions with Answers

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36 questions
  1. In Döbereiner’s triad Li, Na, K, the atomic weight of Na is nearly equal to —
    1. the sum of Li and K
    2. the average of Li and K
    3. half of K
    4. three times Li
    Show answer

    Answer: (B) the average of Li and K

    (7 + 39)/2 = 23 — the middle one’s atomic weight equals the average of the first and third.

  2. Up to which element did Newlands’ law of octaves work well?
    1. Sodium
    2. Argon
    3. Iron
    4. Calcium
    Show answer

    Answer: (D) Calcium

    After calcium, every eighth heavier element was no longer similar.

  3. Mendeleev’s periodic law was based on —
    1. atomic weight
    2. atomic number
    3. electronic configuration
    4. valency
    Show answer

    Answer: (A) atomic weight

    In 1869 Mendeleev called properties periodic functions of atomic weight; the basis of atomic number came from Moseley (1913).

  4. What did Mendeleev’s "eka-aluminium" later turn out to be?
    1. Scandium
    2. Germanium
    3. Gallium
    4. Indium
    Show answer

    Answer: (C) Gallium

    The properties of gallium (1875) nearly matched Mendeleev’s prediction; eka-boron = Sc, eka-silicon = Ge.

  5. Placing argon before potassium in Mendeleev’s table is an example of which defect?
    1. Position of hydrogen
    2. Isotopes
    3. Anomalous pair
    4. Lanthanoids
    Show answer

    Answer: (C) Anomalous pair

    Ar (39.9), though heavier, comes before K (39.1) — breaking the atomic-weight order; atomic number corrects it by itself.

  6. How many groups are there in the modern periodic table?
    1. 18
    2. 9
    3. 8
    4. 7
    Show answer

    Answer: (A) 18

    IUPAC’s modern long form table has 18 groups and 7 periods.

  7. How many elements are there in period 4?
    1. 8
    2. 2
    3. 32
    4. 18
    Show answer

    Answer: (D) 18

    4s, 3d and 4p have 9 orbitals in all → 18 elements (K to Kr).

  8. In which group is the element with configuration 1s² 2s² 2p⁶ 3s² 3p⁵?
    1. 5
    2. 17
    3. 15
    4. 7
    Show answer

    Answer: (B) 17

    p-block group = 10 + (2 + 5) = 17 — this is chlorine.

  9. Which non-metal is a liquid at room temperature?
    1. Bromine
    2. Mercury
    3. Iodine
    4. Gallium
    Show answer

    Answer: (A) Bromine

    Bromine is the only liquid non-metal; mercury is a liquid metal.

  10. In which group is element 118 (oganesson)?
    1. Group 1
    2. Group 17
    3. Group 18
    4. f-block
    Show answer

    Answer: (C) Group 18

    Og is the last element of period 7 and belongs to group 18 of the noble gases.

  1. Which block has metals, non-metals and metalloids all three?
    1. s
    2. p
    3. d
    4. f
    Show answer

    Answer: (B) p

    The p-block (groups 13–18) has metalloids like B-Si, metals like Al-Pb and non-metals like N-O-halogens.

  2. Atomic radius falls from left to right in a period because —
    1. shells increase
    2. shielding increases
    3. neutrons decrease
    4. effective nuclear charge increases
    Show answer

    Answer: (D) effective nuclear charge increases

    Electrons enter the same shell but nuclear charge rises, so outer electrons are pulled more.

  3. Which of these ions is the smallest?
    1. O²⁻
    2. F⁻
    3. Na⁺
    4. Al³⁺
    Show answer

    Answer: (D) Al³⁺

    All four are isoelectronic (10 e⁻); Al³⁺ has the most protons (13), so it is the smallest.

  4. Why is the first ionisation enthalpy of nitrogen higher than that of oxygen?
    1. N is larger
    2. The half-filled 2p³ subshell of N is more stable
    3. O is a non-metal
    4. N has more protons
    Show answer

    Answer: (B) The half-filled 2p³ subshell of N is more stable

    Extra stability of a half-filled subshell; the paired electron of O (2p⁴) leaves easily because of repulsion.

  5. Which has the most negative electron gain enthalpy?
    1. F
    2. O
    3. Cl
    4. Br
    Show answer

    Answer: (C) Cl

    Cl (−349 kJ/mol); F is small, so the incoming electron faces repulsion — hence −328.

  6. The most electronegative element on the Pauling scale?
    1. Fluorine
    2. Chlorine
    3. Oxygen
    4. Nitrogen
    Show answer

    Answer: (A) Fluorine

    Fluorine (4.0), then oxygen (3.5), then N and Cl (3.0).

  7. Which oxide of period 3 is amphoteric?
    1. Na₂O
    2. Al₂O₃
    3. MgO
    4. SO₃
    Show answer

    Answer: (B) Al₂O₃

    Al₂O₃ forms salts with both acids and bases; Na₂O-MgO are basic, SO₃ acidic.

  8. Lithium has a diagonal relationship with —
    1. sodium
    2. beryllium
    3. aluminium
    4. magnesium
    Show answer

    Answer: (D) magnesium

    Li and Mg have similar charge/radius ratios — both form nitrides and their carbonates break on heating.

  9. Which isotope of hydrogen is radioactive?
    1. Tritium
    2. Deuterium
    3. Protium
    4. All three
    Show answer

    Answer: (A) Tritium

    Tritium (³H) is a β-emitter with a half-life of ≈ 12.3 years.

  10. Which catalyst is used in making hydrogen by steam reforming in industry?
    1. Iron
    2. Platinum
    3. Nickel
    4. V₂O₅
    Show answer

    Answer: (C) Nickel

    CH₄ + H₂O → CO + 3H₂ over a nickel catalyst at ≈ 1270 K.

  1. The process of making vanaspati ghee from vegetable oil?
    1. Oxidation
    2. Hydrolysis
    3. Hydrogenation
    4. Polymerisation
    Show answer

    Answer: (C) Hydrogenation

    Adding hydrogen to unsaturated oil over a nickel catalyst gives solid fat.

  2. Temporary hardness of water is caused by which salts?
    1. Bicarbonates of Ca and Mg
    2. Chlorides of Ca and Mg
    3. Sulphates of Ca and Mg
    4. Sodium chloride
    Show answer

    Answer: (A) Bicarbonates of Ca and Mg

    Bicarbonates form insoluble CaCO₃ and Mg(OH)₂ on boiling and settle — so this hardness is temporary; chlorides-sulphates cause permanent hardness.

  3. Main use of heavy water (D₂O) in a nuclear reactor?
    1. Fuel
    2. Shield
    3. Control rod
    4. Moderator
    Show answer

    Answer: (D) Moderator

    D₂O slows down fast neutrons so that fission of U-235 continues.

  4. Approximate percentage (w/v) of "20 volume" H₂O₂?
    1. 3%
    2. 6%
    3. 10%
    4. 20%
    Show answer

    Answer: (B) 6%

    20 × 68/22.4 ≈ 60.7 g/L ≈ 6%; "10 volume" ≈ 3%.

  5. Which alkali metals are less dense than sodium?
    1. Lithium and potassium
    2. Only rubidium
    3. Caesium
    4. None
    Show answer

    Answer: (A) Lithium and potassium

    Density rises down the group, but K (0.86) is lighter than Na (0.97); Li (0.53) is the lightest metal of all.

  6. Flame colour of a sodium salt?
    1. Red
    2. Green
    3. Golden yellow
    4. Violet
    Show answer

    Answer: (C) Golden yellow

    Sodium gives yellow light of 589 nm — sodium vapour lamps have the same colour.

  7. Which compound is given to a patient for an X-ray of the stomach?
    1. CaSO₄
    2. BaSO₄
    3. MgSO₄
    4. BaCl₂
    Show answer

    Answer: (B) BaSO₄

    BaSO₄ is insoluble and non-poisonous and stops X-rays — "barium meal"; BaCl₂ is soluble and poisonous.

  8. The central metal ion of chlorophyll is —
    1. Fe²⁺
    2. Cu²⁺
    3. Ca²⁺
    4. Mg²⁺
    Show answer

    Answer: (D) Mg²⁺

    Chlorophyll has Mg²⁺; haemoglobin has Fe²⁺.

  9. Lead being more stable in the +2 state than +4 is due to —
    1. lanthanoid contraction
    2. hydrogen bonding
    3. diagonal relationship
    4. inert pair effect
    Show answer

    Answer: (D) inert pair effect

    The 6s² electrons do not take part in bonding because of poor shielding — the inert pair effect.

  10. The strongest of the halogen acids?
    1. HF
    2. HI
    3. HBr
    4. HCl
    Show answer

    Answer: (B) HI

    The H–I bond is the weakest, so HI gives H⁺ most easily; HF is weak.

  1. Which gas is filled in electric bulbs to protect the filament?
    1. Neon
    2. Helium
    3. Argon
    4. Oxygen
    Show answer

    Answer: (C) Argon

    Argon is inert and cheap (0.93% of air) — it does not let the hot tungsten oxidise.

  2. Who made the first compound of a noble gas?
    1. Neil Bartlett
    2. Ramsay
    3. Moseley
    4. Pauling
    Show answer

    Answer: (A) Neil Bartlett

    In 1962 Bartlett made Xe⁺[PtF₆]⁻ — reasoning that Xe and O₂ have nearly equal ionisation energies.

  3. Metal with the highest melting point?
    1. Iron
    2. Tungsten
    3. Platinum
    4. Osmium
    Show answer

    Answer: (B) Tungsten

    Tungsten melts at ≈ 3422 °C, so it is used for bulb filaments.

  4. Why are transition metal ions coloured?
    1. s-s transition
    2. Hydrogen bonding
    3. Nuclear radiation
    4. d-d transition in an incomplete d-subshell
    Show answer

    Answer: (D) d-d transition in an incomplete d-subshell

    d-electrons absorb part of visible light and move to a higher d-level; the remaining complementary colour is seen.

  5. Spin-only magnetic moment of Fe³⁺ (3d⁵)?
    1. 5.92 BM
    2. 3.87 BM
    3. 4.90 BM
    4. 1.73 BM
    Show answer

    Answer: (A) 5.92 BM

    n = 5 unpaired: √(5 × 7) = √35 ≈ 5.92 BM.

  6. Because of the lanthanoid contraction, which two elements are nearly equal in size?
    1. Ti and Zr
    2. Fe and Co
    3. Zr and Hf
    4. Cu and Ag
    Show answer

    Answer: (C) Zr and Hf

    The contraction of the 14 lanthanoids before Hf makes Hf as small as Zr (≈ 160 pm).

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