There are different ways of representing small covalent molecules, including dot and cross diagrams, structural formulae and three-dimensional structures.
For example, an ammonia molecule has three covalent bonds. The diagram shows its structure.
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.
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.
A dot and cross diagram can show the bonding in a small molecule:
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.
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.
Atoms form covalent bonds by sharing electrons to get a full outer shell. This means that the number of covalent bonds an atom can form is the same as the number of electrons needed to get a full outer shell. For most elements, a full outer shell is eight electrons.
The table below shows the number of bonds formed by elements in groups 4 to 7.
Hydrogen atoms only have one electron and form one covalent bond as they only need one more for a full outer shell.
Draw a dot and cross diagram for methane, CH4.