Chemistry
Ionic compounds
KEY WORDS 3D representation electrostatic empirical formula giant lattice
Learning objectives: • identify ionic compounds from structures • explain the limitations of diagrams and models • work out the empirical formula of an ionic compound.
Positive ions and negative ions attract to form giant lattice structures with the electrostatic forces acting in all directions. Models and diagrams help us to understand this type of bonding and the structures that are formed.
(a)
(b)
Key Na+ Cl−
Giant structures Ionic bonding occurs when oppositely charged ions are held together by strong electrostatic forces of attraction.
–
+
–
+
–
+
–
+
–
Figure 2.9 Models of NaCl (a) ball and stick (b) close packed
An ionic compound is a giant structure of ions in a lattice. One example of an ionic compound is sodium chloride.
Diagram for question 1 (i)
The structure of sodium chloride can be represented in different ways (Figure 2.9).
(ii) –
Type of diagram:
The diagram is helpful to show:
ball and stick (a)
close packed (b)
the structure in 3D
the structure in 3D(i)
the charges on the ions
(i) on the ions the charges
the arrangement of ions in 3D
the arrangement of ions in 2D (look at the front face of the diagram)
the type of ions in all directions
+
(ii)
the ions are actually close together gives a false image of bond direction when it is only electrostatic attraction
This diagram is best to represent: 1
the number and type of ions in 3D
difficult to see the arrangement of ions in 3D
(iii) +
+
–
–
–
–
(iii) (iii)
the way that ions are packed close together
Which one of the three models shows an ionic compound? i, ii or iii?
Empirical formula In magnesium oxide the ions are held together by electrostatic attraction, to form a giant structure, in exactly the same way as in sodium chloride, except that the magnesium ion has a 2+ charge and the oxide ion has a 2– charge.
60
– –
the closeness of ions The diagram is unhelpful:
–
(ii)
DID YOU KNOW? Each atom has either gained or lost the correct number of electrons to achieve a complete outer shell.
AQA GCSE Chemistry for Combined Science: Trilogy: Student Book
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5/24/16 8:50 PM
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