Valency, Radicals & Chemn

VALENCY

  • Definition: The number of hydrogen atoms which can combine with or displace one atom of the element or radical to form a compound.

  • Valency of metals and hydrogen is considered positive.

  • Valency of non-metals/groups of non-metals is taken as negative.

Combining AtomProduct FormedValency of Element
One atom of hydrogen--
One atom of chlorineHydrogen chlorideChlorine = 11
One atom of oxygenWaterOxygen = 22
Three atoms of hydrogenAmmoniaNitrogen = 33

  • Valency of a metal: Number of electrons lost per atom of the metal.

    • K−1e−→K1+K - 1e^- \rightarrow K^{1+} (univalent)
    • Mg−2e−→Mg2+Mg - 2e^- \rightarrow Mg^{2+} (divalent)
    • Al−3e−→Al3+Al - 3e^- \rightarrow Al^{3+} (trivalent)

  • Valency of a non-metal: Number of electrons gained per atom of the non-metal.

    • S+2e−→S2−S + 2e^- \rightarrow S^{2-}(divalent)
    • Cl+1e−→Cl1−Cl + 1e^- \rightarrow Cl^{1-}(univalent)
    • N+3e−→N3−N + 3e^- \rightarrow N^{3-}(trivalent)

VARIABLE VALENCY

  • Certain metals exhibit more than one valency, hence showing variable valency.

  • Examples:

    • Cu1+Cu^{1+}, Cu2+Cu^{2+} (Copper)
    • HgHg (Mercury)
    • AgAg (Silver)
    • AuAu (Gold)
    • Fe2+Fe^{2+}, Fe3+Fe^{3+} (Iron)
    • PbPb (Lead)
    • SnSn (Tin)
    • MnMn (Manganese)
    • PtPt (Platinum)

  • Reasons for exhibiting variable valency:

    • Loss from penultimate (last but one) shell.
      • Cuprous (Cu1+Cu^{1+}) & Cupric (Cu2+Cu^{2+}).
      • Cu:29(2,8,18,1)Cu: 29 (2, 8, 18, 1): The outermost shell has one electron, and the penultimate shell contains 18 electrons. However, the penultimate shell has not attained stability, and one or more electrons sometimes jumps to the outermost shell, thereby increasing the valency electron. Hence, the atom has a new configuration (2,8,17,2)(2, 8, 17, 2). Therefore, copper exhibits Cu1+Cu^{1+} & Cu2+Cu^{2+} valency.
    • An atom of an element can sometimes lose more electrons than are present in its valence shell.