A dot and cross diagram shows only the outer shell electrons. The electrons from one atom are drawn as dots and the electrons from the other atom as crosses, so you can see where they came from. Inner shells are not shown.
For a binary ionic compound (made from two elements), draw each ion in square brackets with its charge. In sodium chloride the sodium atom gives one electron to chlorine, so the sodium ion is drawn with its full outer shell of 8 electrons and a positive charge (1+), and the chloride ion also has a full outer shell of 8 electrons and a 1- charge. In magnesium oxide, magnesium gives away 2 electrons and oxygen gains 2 electrons. This makes ions with charges 2+ and 2-, and each has a full outer shell.
For a simple covalent molecule, draw the outer shells overlapping and put a shared pair of electrons in each overlap. A hydrogen molecule has one shared pair. In water, oxygen shares one pair with each hydrogen atom, so it has two shared pairs and two non-bonding pairs, giving it 8 outer electrons. Methane has four shared pairs between carbon and hydrogen atoms.
