with an inert-gas conÜguration [Ar].
The bonding in calcium chloride is
the electrostatic attraction between
the ions.
Covalentbonds are formed by shar-
ing of valence electrons rather than
by transfer. For instance, hydrogen
atoms have one outer electron (1s^1 ).
In the hydrogen molecule, H 2 , each
atom contributes 1 electron to the
bond. Consequently, each hydrogen
atom has control of 2 electrons – one
of its own and the second from the
other atom – giving it the electron
conÜguration of an inert gas [He]. In
the water molecule, H 2 O, the oxygen
atom, with six outer electrons, gains
control of an extra two electrons sup-
plied by the two hydrogen atoms.
This gives it the conÜguration [Ne].
Similarly, each hydrogen atom gains
control of an extra electron from the
oxygen, and has the [He] electron
conÜguration.
A particular type of covalent bond
is one in which one of the atoms sup-
plies both the electrons. These are
known as coordinate (semipolaror
dative) bonds, and written A→B,
where the direction of the arrow de-
notes the direction in which elec-
trons are donated.
Covalent or coordinate bonds in
which one pair of electrons is shared
are electron-pair bonds and are
known as single bonds. Atoms can
also share two pairs of electrons to
form double bonds or three pairs in
triple bonds. See orbital.
In a compound such as sodium
chloride, Na+Cl–, there is probably
complete transfer of electrons in
forming the ionic bond (the bond is
said to be heteropolar). Alternatively,
in the hydrogen molecule H–H, the
pair of electrons is equally shared be-
tween the two atoms (the bond is ho-
mopolar). Between these two
extremes, there is a whole range of
intermediate bonds, which have bothionic and covalent contributions.
Thus, in hydrogen chloride, H–Cl,
the bonding is predominantly cova-
lent with one pair of electrons shared
between the two atoms. However,
the chlorine atom is more elec-
tronegative than the hydrogen and
has more control over the electron
pair; i.e. the molecule is polarized
with a positive charge on the hydro-
gen and a negative charge on the
chlorine, forming a *dipole. See also
banana bond; hydrogen bond;
metallic bond; multicentre bond;
multiple bond.chemical cellSee cell.chemical combinationThe com-
bination of elements to give com-
pounds. There are three laws of
chemical combination.
(1) The law of constant composition
states that the proportions of the el-
ements in a compound are always
the same, no matter how the com-
pound is made. It is also called the
law of constant proportions or
deÜnite proportions.
(2) The law of multiple proportions
states that when two elements A and
B combine to form more than one
compound, then the masses of B that
combine with aÜxed mass of A are
in simple ratio to one another. For
example, carbon forms two oxides.
In one, 12 grams of carbon is com-
bined with 16 grams of oxygen (CO);
in the other 12 g of carbon is com-
bined with 32 grams of oxygen (CO 2 ).
The oxygen masses combining with a
Üxed mass of carbon are in the ratio
16:32, i.e. 1:2.
(3) The law of equivalent proportions
states that if two elements A and B
each form a compound with a third
element C, then a compound of A
and B will contain A and B in the rel-
ative proportions in which they react
with C. For example, sulphur and
carbon both form compounds withchemical cell 116c