Page 85 - ICSE Chemistry 8

In general:
• The valency of metals is the same as the number of electrons in the valence shell. For example,
the valency of sodium is 1, with 1 valence electron and that of magnesium is 2, with 2 valence
electrons.

• The valency of non-metals is determined using the formula (8 – the valence electron(s)). For
example, the valency of nitrogen is (8 – 5) = 3, oxygen is (8 – 6) = 2 and that of chlorine is (8 – 7) = 1.
• The valency of noble or inert gases is zero as they have eight electrons in their valence shells and
are therefore chemically inert.

Elements with valency one are called monovalent elements, for example, sodium and chlorine. Elements
with valency two are called divalent elements, for example, magnesium and calcium. Elements with
valency three are called trivalent elements, for example, lithium and phosphorus.

When an atom donates electrons to acquire a stable electronic confi gura on, it becomes a posi vely
charged ion and is said to be an electroposi ve ion.

During chemical reac ons, metals donate electrons, resul ng in a greater number of protons and lesser
number of electrons le with them. As a result, they become posi vely charged thus forming ca ons.
Examples of Formation of Electropositive Ions

+
• Sodium (Na) donates 1 electron to become sodium ion (Na ).
–1 electron
+
(11 electrons) Na Na (10 electrons) Thirst for Knowledge
All metals and hydrogen have posi ve
Electronic confi gura on:
valency, since their valence shells
+
Na 2(K), 8(L), 1(M) Na 2(K), 8(L) have 1 to 3 electrons.
unstable stable

2+
• Magnesium (Mg) donates 2 electrons to become magnesium ion (Mg ).
–2 electrons
2+
(12 electrons) Mg Mg (10 electrons)
Electronic confi gura on:
2+
Mg 2(K), 8(L), 2(M) Mg 2(K), 8(L)

3+
• Aluminium (Al) donates 3 electrons to become aluminium ion (Al ).
–3 electrons
3+
(13 electrons) Al Al (10 electrons)
Electronic confi gura on:

3+
Al 2(K), 8(L), 3(M) Al (K), 8(L)

+
2+
Depending upon the number of electrons donated, the elements are monovalent (Na ), divalent (Mg )
3+
or trivalent (Al ).

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