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Chemistry : Elements & Radicals with Positive Valencies

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What is the Valency of an Element?

Valency is the number of hydrogen atoms that can combine with [or displace] one atom of the element [or radical] 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. Similarly, the valency of chlorine in the compound hydrogen chloride [HCl] is 1, and the valency of the ammonium radical [NH4] in the compound ammonium chloride [NH4Cl] is 1.

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. Elements with valency one are monovalent, those with valency two are divalent, and those with valency three are trivalent. All metals, hydrogen and the ammonium radical have positive valencies as shown in the table below.

Valency Element / Radical Symbol Ion
Monovalent [1] Hydrogen H H1+
Sodium Na Na1+
Potassium K K1+
Ammonium NH4 NH41+
Divalent [2] Magnesium Mg Mg2+
Calcium Ca Ca2+
Zinc Zn Zn2+
Trivalent [3] Aluminum Al Al3+

Note that a radical is a group of atoms of elements, e.g., ammonium radical [NH4]. 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., Na1+], whereas negatively charged ions are called anions [e.g., Cl1-].


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