Dot-and-cross diagrams

AS Level Chemistry · Chemical bonding. A short explanation of the idea, the rules to remember, the mistake to avoid, a worked example and practice questions with answers.

Test this topic freeAll AS Level Chemistry topics

The idea

A dot-and-cross diagram shows only the outer-shell electrons. Dots are used for the electrons of one atom and crosses for the electrons of the other, so you can see where each electron came from.

In an ionic compound electrons are transferred, and each ion is drawn in square brackets with its charge. In a covalent bond a pair of electrons is shared: one dot and one cross. In a coordinate (dative) bond both shared electrons come from the same atom, so the pair is drawn as two dots or two crosses.

Not every atom ends with eight outer electrons. Some have fewer, some have more (an expanded octet), and a few molecules have an odd number of electrons.

Rules to remember

Common mistake

Students force every atom to have eight outer electrons. Count the electrons each atom really has: boron in BF3 has only 6, and sulfur in SF6 has 12.

Worked example

In a dot-and-cross diagram of sulfur hexafluoride, SF6, how many electrons are around the sulfur atom, and how many lone pairs are shown in the whole molecule?

  1. Sulfur has 6 outer electrons and uses each one to form a single bond with a fluorine atom.
  2. 6 single bonds = 6 shared pairs, so sulfur has 6 × 2 = 12 electrons around it, and no lone pair.
  3. Each fluorine has 7 outer electrons: 1 is shared, so 6 remain as 3 lone pairs.
  4. Total lone pairs = 6 × 3 = 18.

Answer: 12 electrons around sulfur; 18 lone pairs, all on the fluorine atoms.

Practice questions

Try each one, then open the answer.

1. Nitrogen monoxide, NO, has 11 outer electrons. Its dot-and-cross diagram shows an N=O double bond, and the oxygen atom has a full octet. How many electrons surround the nitrogen atom?

  1. A
    6
  2. B
    7
  3. C
    8
  4. D
    9
Show answer

Answer: B. N contributes 5 and O 6 electrons. With oxygen holding two bonding pairs and two lone pairs, nitrogen has 4 bonding electrons, one lone pair and one unpaired electron: 7 in total, so NO is a free radical.

2. In a dot-and-cross diagram of carbon dioxide, how many lone pairs of electrons are shown in total?

  1. A
    0
  2. B
    2
  3. C
    4
  4. D
    8
Show answer

Answer: C. Each oxygen shares two pairs with carbon and keeps two lone pairs; carbon has no lone pairs. Two oxygens × two lone pairs = 4 lone pairs.

3. In a dot-and-cross diagram of magnesium oxide, how many electrons are shown in the outer shell of the oxide ion, and how many of these came from magnesium?

  1. A
    6 and 0
  2. B
    8 and 2
  3. C
    8 and 1
  4. D
    10 and 2
Show answer

Answer: B. Oxygen has six outer electrons and gains two from magnesium, so the O2− ion is drawn with eight outer electrons, two of them shown with magnesium's symbol.

More questions on this topic

Read the full notes

Keep going

← Intermolecular forces, electronegativity and bond propertiesThe gaseous state: ideal and real gases and pV = nRT →All rules on one pageStuck? Ask a question