Limitations of Models
Dot and Cross Diagrams
- Advantages:
- Useful for illustrating the transfer of electrons
- Indicates from which atom the bonding electrons come from
- Disadvantages:
- Fails to illustrate the 3D arrangements of the atoms and electron shells
- Doesn’t indicate the relative sizes of the atoms
A dot & cross diagram of ammonia
Ball and Stick Model
- Advantages:
- Useful for illustrating the arrangement of atoms in 3D space
- Especially useful for visualizing the shape of a molecule
- Disadvantages:
- Fails at indicating the movement of electrons
- The atoms are placed far apart from each other, which in reality is not the case as the gaps between atoms are much smaller
Ball and stick model of ammonia which illustrates the 3D arrangement of the atoms in space and the shape of the molecule
2D Representations of Molecules
- Advantages:
- Displayed formulae are 2D representations and are basically simpler versions of the ball and stick model
- Adequately indicate what atoms are in a molecule and how they are connected
- Disadvantages:
- Fail to illustrate the relative sizes of the atoms and bonds
- Cannot give you an idea of the shape of a molecule and what it looks like in 3D space
Displayed formula of ammonia
3D Representations of Ionic Solids
- Advantages:
- 3D drawings and models depict the arrangement in space of the ions
- Also show the repeating pattern in giant lattice structures
- Disadvantages:
- Only illustrate the outermost layer of the compound
- Are difficult and time-consuming to draw
3D representation of the ionic lattice structure of silver fluoride. The silver atoms are in blue/grey and the fluorine atoms in green
Examiner Tip
You should be able to confidently describe the limitations of particular representations and models, including dot and cross, ball and stick models and two- and three-dimensional representations.
