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The position of hydrogen: the element with three possible seats

Your guide: Miss SanaCounts atoms the way you count rupees: carefully.

The problem

Every element in the table has one fixed seat. Hydrogen is the guest at a wedding who knows both families: it could sit with the alkali metals, with the halogens, or with carbon. Each table would make room, and none would be a perfect fit.

With only one electron, hydrogen can lose it like sodium, gain one more like chlorine, or share it like carbon. This topic lists the evidence for each seat and against it.

Hydrogen is a unique element whose properties do not match exactly with any group. Because of its partial resemblance to the alkali metals and its monovalent nature, it is usually placed at the top of group IA.

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Notes, short questions and MCQs

Read the full notes: key terms, model answers and MCQs with answers

The problem

Every element in the table has one fixed seat. Hydrogen is the guest at a wedding who knows both families: it could sit with the alkali metals, with the halogens, or with carbon. Each table would make room, and none would be a perfect fit.

With only one electron, hydrogen can lose it like sodium, gain one more like chlorine, or share it like carbon. This topic lists the evidence for each seat and against it.

Hydrogen is a unique element whose properties do not match exactly with any group. Because of its partial resemblance to the alkali metals and its monovalent nature, it is usually placed at the top of group IA.

Key terms

Group IA
Like the alkali metals, hydrogen combines strongly with electronegative elements such as the halogens, and forms ionic compounds that dissociate in water. But hydrogen is a true non-metal and does not lose its electron as easily.
Group VIIA
Like most halogens, hydrogen is a diatomic gas and needs one electron to complete its shell. It forms H−, like F−, Cl− and Br−. But it also forms H+, which halogens never do, and it forms very stable oxides.
Group IVA
Hydrogen's valence shell is half filled, like carbon's. Both bond covalently, and like carbon, hydrogen is a remarkable reducing agent. But carbon and silicon form long chains and bond to several atoms at once; hydrogen bonds to only one.
The verdict
Hydrogen matches no group exactly. It could be placed at the top of IA, IVA or VIIA. It is usually placed on group IA, because it is monovalent and partly resembles the alkali metals.

Short questions with model answers

  1. Q1. How do you justify the position of hydrogen at the top of various groups?

    • IA: combines with halogens; forms ionic compounds that dissociate in water.
    • VIIA: diatomic gas; needs one electron; forms H−.
    • IVA: half-filled valence shell; covalent bonding; reducing agent.

    It partly resembles IA, IVA and VIIA, so it can sit atop any; it is usually placed on IA.

  2. Q2. Show that hydrogen, like carbon, is a reducing agent.

    • CuO + H2 → Cu + H2O
    • SnO2 + C → Sn + CO2

    Both remove oxygen from metal oxides, leaving the metal.

  3. Q3. Hydrogen forms a uninegative ion like the halogens. Show it, and give one way it differs.

    • H + e− → H− · Cl + e− → Cl−
    • H → H+ + e−

    Both form uninegative ions, but only hydrogen also forms a positive ion, H+.

Common mistakes

  • ✗ Copying the reducing reaction as printed: “CuO + H2O → Cu + H2O”.

    ✓ Water cannot reduce copper oxide; hydrogen gas does. The correct equation is CuO + H2 → Cu + H2O.

  • ✗ Writing that hydrogen is a metal because it sits on top of group IA.

    ✓ Hydrogen is a non-metal in the true sense. It sits on IA by convention, for its monovalent nature and partial resemblance.

  • ✗ Saying halogens form positive ions just as hydrogen forms H+.

    ✓ That is exactly a difference: hydrogen forms H+, but halogens do not form positive ions.

MCQs

  1. 1. Hydrogen can be placed above which groups of the periodic table?

    1. (a) IA only
    2. (b) IA, IVA and VIIA
    3. (c) IIA and VIIIA
    4. (d) IB and IIB
    Show answer

    (b) IA, IVA and VIIA: it partly resembles all three, as the book's key points say.

  2. 2. True or false: hydrogen forms a uninegative ion like the halogens.

    1. (a) True
    2. (b) False
    Show answer

    (a) True: H−, the hydride ion, like F−, Cl− and Br−.

  3. 3. Hydrogen is usually placed at the top of group:

    1. (a) IA
    2. (b) IVA
    3. (c) VIIA
    4. (d) VIIIA
    Show answer

    (a) Group IA, because of its partial resemblance to the alkali metals and its monovalent nature.

  4. 4. Which property does hydrogen share with group IVA elements?

    1. (a) it forms long chains
    2. (b) its valence shell is half filled
    3. (c) it bonds to four atoms at once
    4. (d) it is a metal
    Show answer

    (b) A half-filled valence shell. Long chains and bonding to several atoms at once are differences, not likenesses.

  5. 5. Which of these shows hydrogen is NOT a halogen?

    1. (a) it is a diatomic gas
    2. (b) it needs one electron
    3. (c) it forms H+
    4. (d) it forms ionic compounds with alkali metals
    Show answer

    (c) Forming H+ is a difference: halogens do not form positive ions. The other three are likenesses.

Quick revision

  • Hydrogen partly resembles IA (combines with halogens), VIIA (diatomic gas, forms H−) and IVA (half-filled shell, reducing agent).
  • It also differs from each: a true non-metal, it forms H+ and stable oxides, and it bonds to only one atom at a time.
  • Verdict: hydrogen is unique, and it is usually placed at the top of group IA.