Limitations of models of ions and ionic compounds
Dot and cross diagrams
A dot and cross diagram shows:
- how the Ionic bonding forms between two atoms when an electron is transferred from one atom to the other, forming a positive-negative ion pair. are formed
- the A ratio is a way to compare amounts of something. It is usually written in the form a:b. in which the The smallest part of an element that can exist. react
However, it does not show how the Electrically charged particle, formed when an atom or molecule gains or loses electrons. are arranged in space.
A dot and cross diagram for sodium chloride suggests that it is made up of pairs of sodium and chloride ions. It is not.
Sodium chloride has a The structure of ionic compounds formed from millions of oppositely charged ions in a 3 dimensional arrangement. structure with equal numbers of sodium and chloride ions.
The ionic lattice structure of an An ionic compound occurs when a negative ion (an atom that has gained an electron) joins with a positive ion (an atom that has lost an electron). needs a 3D model, or a 2D representation of a 3D model.
3D construction models of ionic compounds
A 3D construction model shows how the ions are arranged in a lattice structure. These models usually use coloured balls to represent the ions. Some use sticks to show the ionic bonds.
A 3D construction model still has limitations:
- it is not to scale
- it gives no information about the forces of attraction between the ions, or the movement of Subatomic particle, with a negative charge and a negligible mass relative to protons and neutrons. to form the ions
3D diagrams of ionic compounds
A 3D model may be drawn as a 3D diagram. The limitations are the same as the 3D construction model.
On a computer it may be possible to rotate this diagram to look at it from different angles. This helps to visualise the structure.
2D representations of ionic compounds
A 2D representation is the easiest model to draw.
A 2D model shows the arrangement of one layer of ions but it does have a major additional limitation - it does not show where the ions are located on the other layers. This is important because there are different possible arrangements of ions.
Question
Use the 3D model and diagrams to help you draw the next row of sodium chloride in a 2D model.
In sodium chloride the ions are arranged alternately so, in the next row, chloride ions are located above the sodium ions.