Modelling molecules
Dot and cross diagrams
A dot and cross diagram can show the bonding in a small A collection of two or more atoms held together by chemical bonds.:
- the outer shell of each The smallest part of an element that can exist. is drawn as a circle
- circles overlap where there is a A bond between atoms formed when atoms share electrons to achieve a full outer shell of electrons.
- Subatomic particle, with a negative charge and a negligible mass relative to protons and neutrons. from one atom are drawn as dots, and electrons from another atom as crosses
For example, a hydrogen molecule, H2, forms when two hydrogen atoms each share their outer electron.
An ammonia molecule, NH3, forms when one nitrogen atom shares its outer electrons with three hydrogen atoms. There are two types of dot and cross diagram - one without circles, and one with.
Structural formulae
A structural formula shows the bonds between the atoms in a molecule. In these formulae:
- each atom is shown by its chemical symbol
- each covalent bond is shown as a straight line
For example, an ammonia molecule has three covalent bonds. The diagram shows its structure.
Three-dimensional structures
The structure of a small molecule can also be shown as a three-dimensional ball and stick model. These models show how the atoms and bonds are arranged in space. The diagram shows a ball and stick model of ammonia.
Question
Give one advantage and one limitation of using a dot and cross diagram to represent a molecule.
A dot and cross diagram shows the pairs of outer electrons, and which atoms they have come from. It does not show how the atoms are arranged in space.
Question
Give one advantage and one limitation of using a structural formula to represent a molecule.
The structural formula shows the bonds between the atoms, but it does not show which atoms the electrons in the bonds have come from.