Instead of trying to remember lots of different dot and cross diagrams, it may help to understand how to draw them.
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.
To work out how many circles to draw for a simple molecular 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-bonding outer electrons. Nitrogen atoms have five outer electrons. Three of these are shared, which leaves two electrons that do not take part in bonding.
Draw a dot and cross diagram for methane, CH4.