Page 66 - Chemistry ICSE Class X
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54 ICSE Chemistry – 10
Let us say it Again
Valence electrons. The electrons in the outermost shell of an atom are called valence electrons.
Valency. The number of electrons which an atom loses, gains or shares with other atoms to attain noble gas configuration
is called its valency.
Noble gases form no compounds because their atoms have stable electronic configurations, i.e., their outermost shells
are completely filled.
Atoms having incompletely-filled outermost shell show chemical reactivity, and form compounds with other atoms.
During any chemical reaction, atoms of all the elements tend to gain stability by acquiring an electronic configuration of
the nearest noble gas element.
Bond formation. Atoms of the reacting elements gain stability by either losing, gaining or by mutual sharing of the
valence electrons.
Octet rule. During chemical reactions, atoms of all elements (except hydrogen) tend to achieve eight electrons in their
outermost shell.
Hydrogen, however, tends to achieve two electrons in its outermost shell. This is called duplet rule.
Oxidation. The process which involves loss of electrons is called oxidation.
Reduction. The process which involves gain of electrons is called reduction.
Electrovalent bond. The bond formed due to transfer of electrons from the atom of an element to that of another
element is called electrovalent (or ionic) bond. The compound formed due to the transfer of electrons from the atom of
an element to that of another is called an electrovalent (or ionic) compound.
Covalent bond. The bond formed due to mutual sharing of electrons is called a covalent bond. The compounds formed
through the process of sharing of electrons between the combining atoms are called covalent compounds. The covalent
compounds do not conduct electricity.
Single bond. The covalent bond which is formed by mutual sharing of one pair of electrons (two electrons) is called a
single (–) covalent bond.
Double bond. The covalent bond which is formed by mutual sharing of two pairs of electrons (four electrons) is called a
double (=) covalent bond.
Triple bond. The covalent bond which is formed by mutual sharing of three pairs of electrons (six electrons) is called a
triple ({) covalent bond.
Polar covalent bond. A covalent bond formed between two atoms of the elements differing in their electronegativities
is called a polar covalent bond.
Some highly polar compounds when dissolved in water give ions. Therefore, the aqueous solutions of highly polar
compounds conduct electricity.
Electrovalent compounds. The compounds formed due to transfer of electrons from the atom of an element to that
of another is called an electrovalent (or ionic) compound. Sodium chloride (NaCl) is a typical electrovalent compound.
Electrovalent compound when dissolved in water or when melted, furnish free ions.
Covalent compounds. The compounds formed due to mutual sharing of electrons between the constituent atoms is
called a covalent compound. Water, Ammonia, Methane etc., are common covalent compounds.
Coordinate bond. The bond formed when one-sided sharing of electrons takes place is called coordinate bond. The
+
+
coordinate bond is also known as dative bond. Formation of H O , NH ions involve the formation of coordinate bond.
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