64 5 · MORE ABOUT BONDING
temperature, but the central atoms do not have a stable octet of electrons surround-
ing them.
For example, BF 3 is a covalent substance and its formula may be written:In this compound, boron is surrounded by only six electrons. The compound readily
reacts with any substance that will give it the extra electrons it needs to obtain a stable
octet. Two examples of such reactions are shown in the following equations:and
Lithium
Beryllium and boron do not
react by losing electrons,
whereas lithium (Li, 2.1)
does. Can you suggest a
reason for this?Exercise 5A
Compounds which contain an atom with an expanded
octet
The other class of compounds in which the octet rule is not obeyed is covalent com-
pounds in which the central atom can accommodate more than eight electrons in its
outer shell.
Remember that the first electron shell of an atom can hold up to two electrons
(1s^2 ), the second shell can hold eight electrons (2s^2 2p^6 ) and the third shell can hold
18 electrons (3s^2 3p^6 3d^10 ). If the central atom in a covalent compound has electrons
in the third (or a higher) shell, then it can use its empty d orbitals to hold extra
electrons. In theory such elements can share up to nine pairs of electrons (making a
total of 18), but the number of atoms around the central atom is restricted by the size
of the central atom, it gets too ‘crowded’. It is rare to find the central atom
sharing more than six pairs of electrons.More about beryllium compounds
(i) Draw a Lewis structure and also a structural formula for covalent BeCl 2.
(ii)In BeCl 2 , how many electrons are in the outer shell of the Be atom?
(iii)BeCl 2 reacts with Cl ions to form the polyatomic ion [BeCl 4 ]^2 . Draw the structure of this
ion.
(iv)How many electrons are in the outer shell of the Be atom in [BeCl 4 ]^2 ?
(v)Which do you think is the more stable compound, BeCl 2 or [BeCl 4 ]^2 ? Why?Exercise 5B