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Chemistry : Elements with Variable Valencies

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Do Some Elements Have Variable Valencies?

Valency is the number of hydrogen atoms that can combine with (or displace) one atom of the element to form a compound. For example, two atoms of hydrogen combine with one atom of oxygen to form water (H2O); so, the valency of oxygen is two.

For elements that do not combine with hydrogen, the valency is the combining power of the element with another element whose valency is known. Valency may also be defined as the number of electrons that an atom donates or accepts to form the duplet state (i.e., 2 electrons in outermost shell) or octet state (i.e., 8 electrons in outermost shell).

The valency of an element is always a whole number. Some elements exhibit more than one valency, i.e., they have variable valency. In Iron [II] sulfate or ferrous sulfate, i.e., FeSO4, the valency of iron is two. In Iron [III] sulfate or ferric sulfate, i.e., Fe2(SO4)3, the valency of iron is three. Generally, the Latin / Greek name for the element (e.g., Ferrun) is modified to end in 'ous' for the lower valency (e.g., Ferrous) and to end in 'ic' for the higher valency (e.g., Ferric). The table below lists some elements that exhibit variable valencies.

Element Symbol Valency Ions Nomenclature
Copper Cu 1, 2 Cu1+
Copper [I] or Cuprous
Copper [II] or Cupric
Silver Ag 1, 2 Ag1+
Silver [I] or Argentous
Silver [II] or Argentic
Mercury Hg 1, 2 Hg1+
Mercury [I] or Mercurous
Mercury [II] or Mercuric
Iron Fe 2, 3 Fe2+
Iron [II] or Ferrous
Iron [III] or Ferric
Tin Sn 2, 4 Sn2+
Tin [II] or Stannous
Tin [IV] or Stannic
Lead Pb 2, 4 Pb2+
Lead [II] or Plumbous
Lead [IV] or Plumbic
Phosphorus P 3, 5    
Sulfur S 4, 6    

An ion is any atom or group of atoms with a positive or negative charge due to loss or gain of electrons. Positively charged ions are called cations [e.g., Fe3+], whereas negatively charged ions are called anions [e.g., Cl1-].


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