VALENCY – COMBINING CAPACITY OR COMBINING POWER
Every element has a definite capacity to combine with other elements.
The combining capacity of an element is called its valency.”
In normal course, hydrogen has 1, oxygen has 2, nitrogen has 3 and carbon has 4 valency. This will depend upon the nature of the element.
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Valency in term of Hydrogen combination –
The number of hydrogen atoms which combine with one atom of the element is called the combining number or valency of the element.
For example –
- One hydrogen atom combines with one chlorine atom to form hydrogen chloride (HCl) and thus the valency of chlorine atom is one. Elements with valency of one are called as univalent.
- Two hydrogen atoms combine with one oxygen atom to form water (H2O) and thus the valency of oxygen atom is two. Elements with valency of two are called as divalent.
- Three hydrogen atoms combine with one nitrogen atom to form ammonia (NH3) and thus the valency of nitrogen atom is three. Elements with valency of three are called as trivalent.
- Four hydrogen atoms combine with one carbon atom to form methane (CH4) and thus the valency of carbon atom is four. Elements with valency of four are called as tetravalent.
- Some elements (noble gases or inert gases e.g. helium, neon, argon etc.) do not generally combine with other elements and thus they have zero valency. Such elements are known as zerovalent.
Valency in term of Valence Electrons –
The number of electrons present in the outermost shell of an element are known as valence electrons.
Every element tries to acquire a stable electronic configuration of the nearest noble gas either by losing / gaining or sharing of electrons.
The number of electrons required for losing/gaining or sharing to acquire a stable electronic configuration of the nearest noble gas give valency of the element.
- From Li to B (period 2) and Na to Al (period 3) elements can easily lose their electrons. Here, valency is equal to number of valence electrons.
- Valency of metal = Number of valence electrons
- In C and Si, four valence electrons are there and four more electrons are needed to complete the octet. Since large amount of energy is needed for gaining or losing four electrons, such elements prefer sharing of four electrons. Thus, the valency is four for these elements.
- From N and P onwards except noble gases, these elements prefer gaining of electrons as they can not lose so many electrons due to energy consideration. Thus, the valency for such elements can be determined by-
Valency of non-metal = 8 – number of valence electrons
- Noble gases have completely filled octet. So, they do not form bonds. Thus, the valency for such elements is zero.
- Hydrogen has only one valence electron in its orbit. Hence, it shows a tendency to either donate its electron or share it. Therefore, its valency is one.
Sometimes an element shows more than one valency. It means that element has variable valency. For example, nitrogen forms several oxides – N2O, N2O2, N2O3, N2O4 and N2O5. If we take valency of oxygen 2 then valency of nitrogen in these oxides will be 1,2,3,4 and 5 respectively.
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Valency of Ions –
- The charge on an ion represents the valency of the ion. A cation has a positive charge, so it has positive valency and an anion has a negative charge, so it has negative valency.
- If an ion has one unit charge, its valency is 1 and that ion is called monovalent ion. A monovalent cation has +1 valency and a monovalent anion has -1 valency.
- Monovalent cation –
H+ hydrogen ion
Na+ sodium ion
K+ potassium ion
Ag+ silver ion
Cu+ copper (I) ion or cuprous ion
Li+ lithium ion
NH4+ ammonium ion
Hg+ mercury (I) ion or mercurous ion
- Monovalent anion –
HCO3– hydrogen carbonate (bicarbonate)
HSO4– hydrogen sulphate (bisulphate)
HSO3– hydrogen sulphite (bisulphite)
- If an ion has two unit charge, its valency is 2 and that ion is called divalent ion. A divalent cation has +2 valency and a divalent anion has -2 valency.
Divalent cation –
Mg+2 magnesium ion
Ca+2 calcium ion
Ba+2 barium ion
Zn+2 zinc ion
Cu+2 copper (II) ion or cupric ion
Fe+2 iron (II) or ferrous ion
Hg+2 mercury (II) ion or mercuric ion
Divalent anion –
O-2 oxide ion
CO3-2 carbonate ion
SO3-2 sulphite ion
S-2 sulphide ion
HPO4-2 hydrogen phosphate ion
SO4-2 sulphate ion
- If an ion has three unit charge, its valency is 3 and that ion is called trivalent ion. A trivalent cation has +3 valency and a trivalent anion has -3 valency.
Trivalent cation –
Al+3 aluminium ion
Fe+3 iron (III) or ferric ion
Cr+3 chromium ion
Trivalent anion –
N-3 nitride ion
PO4-3 phosphate ion
P-3 phosphide ion
- If an ion has four unit charge, its valency is 4 and that ion is called tetravalent ion. A tetravalent cation has +4 valency and a tetravalent anion has -4 valency.
Pb+4 plumbic ion
Pt+4 platinum ion
Tetravalent anion –
C–4 carbide ion
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