Valency

Valency is the combining capacity of an atom. It is equal to the number of electrons the atom loses, or gains or shares when it combines with one or more atoms.

Depending on whether the atom loses, gains or shares electrons, the valency may be positive or negative or neutral respectively.

Example: Valency of sodium is + 1, because it can lose one electron. It is represented as Na⁺. Valency of oxygen may be - 2 if it accepts 2 electrons i.e., O^2-.

If two or more atoms combine by sharing electrons i.e., not losing or gaining electrons, then the valency numbers are neither positive nor negative. In the case of carbon dioxide, carbon and oxygen atoms, do not, lose or gain electrons, and their valencies are 4 and 2 respectively.

Certain atoms exhibit different kinds of valencies. This is because such elements can loose more than one electron from their outer most shell depending on the available conditions. When such atoms exhibit variable valency, the name of the atom with the lower valency ends as -ous, and the one with the higher valency, ends as -ic.

Lower Valency		Higher Valency
Cuprous	Cu+	Cupric	Cu++
Ferrous	Fe++	Ferric	Fe+++
Plumbous	Pb++	Plumbic	Pb++++
Mercurous	Hg+	Mercuric	Hg++
Aurous	Au+	Auric	Au+++

Lower ValencyHigher

Remember :

Atoms with variable valencies have their symbols same but are named differently.

Example 1: Copper (Cu) has a variable valency of 1 and 2. It is shown as Cu⁺ and Cu⁺⁺. Cu⁺ is read as Cuprous and Cu⁺⁺ is read as Cupric.

Example 2: Ferrous (Fe⁺⁺) and Ferric (Fe⁺⁺⁺).

Example 3: Plumbous (Pb⁺⁺) and Plumbic (Pb⁺⁺⁺⁺).

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Variable valencies are also represented, using the symbols along with the respective valency number in roman numerals in bracket.

Cuprous Cu(I)	Cupric Cu(II)
Ferrous Fe(II)	Ferric Fe(III)
Plumbous Pb(II)	Plumbic Pb(IV)

Atoms also combine in a group when the atoms of different elements combine in a group they are called radicals. They behave as a single unit and the valency of radicals is the overall net charge of the group. The radicals maintain their identity in the chemical changes but are incapable of independent existence.

Remember :

Radicals Formulae

Hydroxide - OH^-

Sulphate - SO₄^-

Nitrate - NO₃^-

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Positive Radicals

Valency 1		Valency 2		Valency 3		Valency 4		Valency 5		Valency 6
Potassium	K+	Barium	Ba2+	Aluminium	Al3+	Plumbic	Pb4+
Sodium	Na+	Calcium	Ca2+	Chromium	Cr3+	Manganic	Mn4+
Cuprous	Cu+	Ferrous	Fe2+	Ferric	Fe3+	Platinum	Pt4+
Mercurous	Hg+	Cupric	Cu2+	Auric	Au3+
Silver	Ag+	Mercuric	Hg2+
Hydrogen	H+	Plumbous	Pb2+
		Zinc	Zn2+
Ammonium	NH4+	Nickel	Ni2+
		Magnesium	Mg2+
		Manganese	Mn2+
Hydrogen	H	Carbon	C	Phosphorus	P	Carbon	C	Phosphorus	P
Nitrogen	N	Nitrogen	N	Nitrogen	N	Nitrogen	N	Nitrogen	N
		Sulphur	S			Sulphur	S			Sulphur	S
						Silicon	Si

 

Negative Radicals

Valency 1		Valency 2		Valency 3		Valency 4
Fluoride	F -
Chloride	Cl -	Carbonate	CO32 -	Nitride	N3 -	Carbide	C4 -
Bromide	Br -	Sulphide	S2 -	Phosphide	P3 -
Iodide	I -
Bisulphite	HSO4 -	Sulphite	SO32 -	Phosphate	PO43 -
Bisulphate	HSO4-	Sulphate	SO4 2 -
Bicarbonate	HCO3 -
Hydroxide	OH -	Oxide	O2 -
Nitrate	NO3 -	Chromate	CrO42 -
Nitrite	NO2-	Dichromate	Cr2O72 -
Hydride	H -
Permanganate	MnO4 -
Chlorate	ClO3 -	Manganate	MnO42 -

Table of Symbols, Atomic Number and Relative Atomic Masses

 

Element	Symbol	Atomic Number	Relative Atomic mass
Actinium	Ac	89	227
Aluminium	Al	13	27
Americium	Am	95	243
Antimony	Sb	51	122
Argon	Ar	19	40
Arsenic	As	38	75
Astatine	At	95	210
Barium	Ba	56	137
Berkelium	Bk	97	247
Beryllium	Be	4	9
Bismuth	Bi	83	209
Boron	B	5	11
Bromine	Br	35	80
Cadmium	Cd	49	112
Caesium	Cs	55	133
Calcium	Ca	20	40
Californium	Cf	98	251
Carbon	C	6	12
Cerium	Ce	58	140
Chlorine	Cl	17	35.5
Chromium	Cr	24	52
Cobalt	Co	27	59
Copper	Cu	29	63.5
Curium	Cm	96	247
Dysprosium	Dy	66	162.5
Einsteinium	Es	99	254
Erbium	Er	68	167
Europium	Eu	63	152
Fermium	Fm	100	253
Fluorine	F	9	19
Francium	Fr	87	223
Gadolinium	Gd	64	157
Gallium	Ga	31	70
Germanium	Ge	32	72.5
Gold	Au	79	197
Hafnium	Hf	72	178
Helium	He	2	4
Holmium	Ho	67	164
Hydrogen	H	1	1
Indium	In	49	115
Iodine	I	53	127
Iridium	Ir	77	192
Iron	Fe	26	56
Krypton	Kr	36	84
Lanthanum	La	57	139
Lawrencium	Lr	103	257
Lead	Pb	82	207
Lithium	Li	3	7
Lutecium	Lu	71	175
Magnesium	Mg	12	24
Manganese	Mn	25	55
Mendelevium	Md	101	256
Mercury	Hg	80	201
Molybdenum	Mo	42	96
Neodymium	Nd	60	144
Neon	Ne	10	20
Neptunium	Np	93	237
Nickel	Ni	26	59
Niobium	Nb	41	93
Nitrogen	N	7	14
Nobelium	No	102	254
Osmium	Os	76	190
Oxygen	O	8	16
Palladium	Pb	46	106
Phosphorus	P	15	31
Platinum	Pt	76	195
Plutonium	Pu	94	242
Polonium	Po	64	210
Potassium	K	19	39
Praesodymium	Pr	59	141
Promethium	Pm	61	147
Protactinium	Pa	91	231
Radium	Ra	66	226
Radon	Rn	66	222
Rhenium	Re	75	166
Rhodium	Rh	45	103
Rubidium	Rb	37	85
Ruthenium	Ru	44	101
Samarium	Sm	62	150
Scandium	Sc	21	45
Selenium	Se	34	79
Silicon	Si	14	26
Silver	Ag	47	108
Sodium	Na	11	23
Strontium	Sr	38	87
Sulphur	S	16	32
Tantalum	Ta	73	181
Technetium	Tc	43	99
Tellurium	Te	52	127
Terbium	Tb	65	159
Thallium	TI	81	204
Thorium	Th	90	232
Thulium	Tm	69	169
Tin	Sn	50	119
Titanium	Ti	22	48
Tungsten	W	74	184
Uranium	U	92	238
Vanadium	V	23	51
Xenon	Xe	54	131
Ytterbium	Yb	70	173
Yttrium	V	39	89
Zinc	Zn	30	65
Zirconium	Zr	40	91

 

(i) The number of protons or electrons in an atom constitutes its atomic number.

(ii) The relative atomic mass of an element is the mass of its atom expressed in atomic mass unit, which is exactly 1/12^th the mass of one atom of carbon-12.

Framing of Formulae of compounds with the Help of Valency

A compound is a substance formed by the combination of atoms of two or more elements. To be able to frame formulae, one should be thorough with the valencies.

1) Frame the formula of the compound formed when sodium and chlorine combine.

Write down the symbol of sodium and chlorine and draw a short line next to each as shown. The short lines represent the valency (just the number and not the charge) of each atom.

Connect each valency, with the valency of the other.

Now there are no free valencies. So the formula is NaCl.

2) Frame the formula of the compound formed when calcium and chlorine combine.

Write down the symbol of calcium and chlorine, and draw 2 small lines next to calcium (valency= 2) and 1 small the next to chlorine.

Join the valencies of calcium with that of chlorine.

Here one valency of calcium is free. So take one more chlorine.

Join them.

So we get the formula CaCl₂.

3) Similarly, frame the formula of the compound formed between aluminium and chlorine. (Al, valency= 3, Cl, valency = 1).

Draw 3 small lines next to aluminium (valency= 3) and 1 small the next to chlorine. Here two valency of aluminium is free. So take two more chlorine and join them.

4) Formula of compound formed between magnesium (valency = 2) and nitrogen (valency = 3).

Draw 2 lines next to Mg and 3 lines next to N.

Connect 2 valencies of Mg with 2 of the 3 valencies of N, as shown.

Now N has 1 valency free. So take one more Mg, and join one valency of Mg to the free valency of N.

That leaves Mg with 1 free valency. So take another N.

That leaves N with 2 free valencies. So take one more Mg.

Now all the valencies are connected. So we get the formula Mg3N2.

Remember:-

Now this formula can be derived by an easier method. The valency of Mg = 2 and that of

N = 3 and the formula is Mg₃N₂. So the number of magnesium atoms is equal to the valency of Nitrogen (3), and the number of nitrogen atoms is equal is equal to the valency of Mg (2). So, to frame this formula, write Mg and N and write their respective valencies just above them as:

Then place 3 near Mg and 2 near N. In other words, interchange their valency numbers.

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More Examples: