Drawing structures
Instead of trying to remember lots of different dot and cross diagrams, it may help to understand how to draw them using given information.
How many bonds?
The smallest part of an element that can exist. form A bond between atoms formed when atoms share electrons to achieve a full outer shell of electrons. by sharing Subatomic particle, with a negative charge and a negligible mass relative to protons and neutrons., 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 A vertical column in the periodic table containing elements with similar chemical properties. 4 to 7 (IUPAC groups 14 to 17).
| Group number | IUPAC Group number | Example | Number of outer electrons | Number of covalent bonds |
| 4 | 14 | Carbon, C | 4 | 4 |
| 5 | 15 | Nitrogen, N | 5 | 3 |
| 6 | 16 | Oxygen, O | 6 | 2 |
| 7 | 17 | Chlorine, Cl | 7 | 1 |
| Group number | 4 |
|---|---|
| IUPAC Group number | 14 |
| Example | Carbon, C |
| Number of outer electrons | 4 |
| Number of covalent bonds | 4 |
| Group number | 5 |
|---|---|
| IUPAC Group number | 15 |
| Example | Nitrogen, N |
| Number of outer electrons | 5 |
| Number of covalent bonds | 3 |
| Group number | 6 |
|---|---|
| IUPAC Group number | 16 |
| Example | Oxygen, O |
| Number of outer electrons | 6 |
| Number of covalent bonds | 2 |
| Group number | 7 |
|---|---|
| IUPAC Group number | 17 |
| Example | Chlorine, Cl |
| Number of outer electrons | 7 |
| Number of covalent bonds | 1 |
Note: Hydrogen atoms only have one electron. They form one covalent bond as they only need one more electron to complete their outer shell. This is because the first shell is filled by only two electrons.
How many circles?
To work out how many circles to draw for a simple Refers to a substance made up of molecules, so it contains small groups of atoms, of a set size, held together by covalent bonds. substance and how to label them, look at the formula. For example, the formula for ammonia is NH3. For this, draw four circles, one labelled N and three labelled H. Each of the three H circles overlaps the N circle.
Nitrogen is in group 5 so it forms three covalent bonds. There are three shared spaces between the circles, so add a dot and cross to each one.
Finally add in the non-The chemical link that holds molecules together. outer electrons. Nitrogen atoms have five outer electrons, three are shared, leaving two non-bonding. Add these to complete the diagram (hydrogen atoms only have one electron, so they have no non-bonding electrons).
The number of non-bonding electrons is almost always a multiple of two. It is usually much clearer to arrange the non-bonding electrons in pairs, to match the pairs of electrons in the covalent bonds.
Double and triple bonds
Some A collection of two or more atoms held together by chemical bonds. contain a A covalent bond resulting from the sharing of four electrons (two pairs) between two atoms., which consists of two shared pairs of electrons. For example, oxygen molecules consist of two oxygen atoms joined together. Oxygen atoms can form two covalent bonds, so to link the two oxygen atoms together, a double bond forms.
Nitrogen molecules consist of two nitrogen atoms joined together. Nitrogen atoms can form three covalent bonds, so a triple bond forms between them. The structure of nitrogen is N鈮, showing that it has three shared pairs of electrons.