Dot-and-cross diagrams — practice questions

Identify valence electrons: Magnesium (Group 2) has 2 valence electrons. Chlorine (Group 17) has 7 valence electrons.

Determine ion formation: Mg loses 2 electrons to form Mg²⁺. Each Cl atom gains 1 electron to form Cl⁻. To balance the charges, one Mg atom reacts with two Cl atoms.

Draw the diagram:

• Draw the magnesium ion: Show the Mg symbol in the centre. Place it in square brackets with a 2+ charge. The outer shell is now the one below the original valence shell, which is full (but we often don't draw the electrons for the cation, just the symbol and charge, as the shell is now empty). Syllabus convention is to show an empty outer shell.
• Draw the chloride ions: Draw two Cl ions. For each, show the original 7 electrons (e.g., as crosses) and the 1 electron gained from magnesium (e.g., as a dot). This makes 8 electrons in the outer shell. Place each ion in square brackets with a -1 charge.

Final Diagram: One [Mg]²⁺ ion shown alongside two [Cl]⁻ ions. Each chloride ion's bracket should contain the 'Cl' symbol and 8 outer electrons (e.g., 7 crosses and 1 dot).

• Draw the two carbon atoms overlapping. Place two electrons from one carbon (e.g., dots) and two from the other (e.g., crosses) in the overlapping region between them. This is the C=C double bond (4 shared electrons in total).
• Each carbon atom now has 2 remaining valence electrons.
• Draw two hydrogen atoms around each carbon. Each C-H bond is formed by one electron from carbon and one from hydrogen. Place one shared pair in each C-H overlap.
• Check: Each carbon now has 8 electrons in its outer shell (2 from the other C, 2 of its own in the double bond, plus 2x1 from H atoms and 2x1 of its own in single bonds). Each hydrogen has 2 electrons. All atoms are stable.

• Place the sulfur atom in the centre. Draw its 6 valence electrons (e.g., as crosses) arranged singly around it.
• Draw six fluorine atoms surrounding the sulfur. For each S-F bond, show one electron from sulfur and one from a fluorine atom (e.g., a dot) forming a shared pair.
• Complete the outer shell of each fluorine atom by drawing its remaining 6 non-bonding electrons.
• Check: The central sulfur atom has 6 shared pairs, totalling 12 electrons in its outer shell (an expanded octet). Each fluorine atom has 1 shared pair and 3 lone pairs, totalling 8 electrons. This is a valid structure.