Limitations of Models
Specification Point 1.41:
Describe the limitations of particular representations and models, to include dot and cross, ball and stick models and two- and three-dimensional representations.
- There are different ways of representing atoms and molecules depending on the type of substance in question.
- Each method has its advantages and disadvantages.
- The main limitation is that although it is relatively easy to represent smaller molecules, larger and more complex molecules are difficult to show graphically.
Dot and Cross Diagrams
- Useful for illustrating covalent compounds as they can show the sharing of electrons.
- Indicate from which atom the bonding electrons come from.
- Fail to illustrate the 3D arrangements of the atoms and electron shells.
- Cannot indicate the relative sizes of the atoms.
Dot and Cross model of ammonia showing how the electrons are shared between the N and H atoms
Ball and Stick Model
- Useful for illustrating the arrangement of atoms in 3D space.
- Very useful for visualizing the shape of a molecule.
- Fail at indicating the sharing or 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
- 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.
- 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
- 3D drawings and models depict the arrangement in space of the ions.
- Also show the repeating pattern in giant lattice structures.
- 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
Edexcel GCSE Chemistry Notes
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