Chapter 4: Chemical bonding
a Oxygen
QUESTION
O
O
+
O
two oxygen atoms (2,6)
O
O
3 Draw dot-and-cross diagrams for the following covalently bonded molecules; show only the outer electron shells:
O
oxygen molecule
a ethene, CH2
CH2
b carbon disulfide, CS2.
b Carbon dioxide
O
+
two oxygen atoms (2,6)
C
C
O
carbon atom (2,4)
O
O
C
Co-ordinate bonding (dative covalent bonding)
O
A co-ordinate bond (or dative covalent bond) is formed when one atom provides both the electrons needed for a covalent bond. For dative covalent bonding we need:
carbon dioxide molecule
c Ethene H
■
C H
H
+
H C
H
C
H
C
H
H C H
C
■
H H
H four hydrogen atoms (1)
two carbon atoms (2,4)
ethene molecule
Figure 4.11 Dot-and-cross diagrams for a oxygen, O2, b carbon dioxide, CO2, and c ethene, C2H4. ■
In ethene, two hydrogen atoms share a pair of electrons with each carbon atom. This leaves each carbon atom with two outer shell electrons for bonding with each other. A double bond is formed.
Atoms can also bond together by sharing three pairs of electrons. We call this a triple covalent bond. Figure 4.12 shows a dot-and-cross diagram for the triple-bonded nitrogen molecule. In order to form a nitrogen molecule, each nitrogen atom needs to gain three electrons to complete its outer shell. So three pairs of electrons are shared and three covalent bonds are formed.
one atom having a lone pair of electrons a second atom having an unfilled orbital to accept the lone pair; in other words, an electron-deficient compound.
An example of this is the ammonium ion, NH4+, formed when ammonia combines with a hydrogen ion, H+. The hydrogen ion is electron deficient; it has space for two electrons in its shell. The nitrogen atom in the ammonia molecule has a lone pair of electrons. The lone pair on the nitrogen atom provides both electrons for the bond (Figure 4.13). In a displayed formula (which shows all atoms and bonds), a co-ordinate bond is represented by an arrow. The head of the arrow points away from the lone pair that forms the bond. Another molecule that has co-ordinate bonds is aluminium chloride. At high temperatures aluminium chloride exists as molecules with the formula AlCl3. This molecule is electron deficient; it still needs two electrons to complete the outer shell of the aluminium atom. At lower +
H
H + H
N
+
H
N
H
H
Nitrogen H
N
+ two nitrogen atoms (2,5)
N
N
N
nitrogen molecule
Figure 4.12 Dot-and-cross diagram for a nitrogen molecule, N2.
N
H
N
+
H H
N
H
H
Figure 4.13 The formation of a co-ordinate bond in the ammonium ion.
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