Chemistry Foundations

4.3.5 - Dot and Cross Diagrams of Molecules

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Dot and Cross Diagrams of Molecules

We have previously learnt how to draw dot and cross diagrams for atoms and ions. Now we will learn how to draw them for molecules.

When drawing dot and cross diagrams for molecules, it is common to only draw the outer shell. That is what we will do here. We don't really need to draw the other shells, because we know that all of the shells before the outer shell are full. Also, it is only the electrons in the outer shell that take part in covalent bonding.

To show a covalent bond in a dot and cross diagram, the outer shells of the two bonded atoms are drawn overlapping and the shared pair of electrons are drawn inside the overlapping area. For example, the dot and cross diagram below shows the molecule chlorine monofluoride, which is made up of a chlorine atom and a fluorine atom joined by a single covalent bond:

A dot and cross diagram of chlorine monofluoride. There is a chlorine atom of the left and a fluorine atom of the right. The nucleus of each atom is represented by its chemical symbol. Only the outer shell of each atom is shown. The outer shells are overlapping. The chlorine atom has 7 electrons, represented by crosses. The fluorine atom has 7 electrons, represented by dots. One of the crosses and one of the dots are in area where the two outer shells overlap. These represent the shared pair of electrons.

Dot and cross diagram of chlorine monofluoride.

Note that the electrons from one atom are shown as dots and the electrons from the other atom are shown as crosses. This makes it clear that each atom has contributed one electron to the shared pair.

Double and triple bonds are drawn in the same way, but for a double bond there are four electrons (two pairs) in the overlapping area, and for a triple bond there are six electrons (three pairs) in the overlapping area.

For example, the dot and cross diagrams below are for an oxygen molecule and a nitrogen molecule. An oxygen molecule is made up of two oxygen atoms joined by a double bond and a nitrogen molecule is made up of two nitrogen atoms joined by a triple bond.

A dot and cross diagram of an oxygen molecule. It is made up of two oxygen atoms joined by a double covalent bond. The atom on the left has crosses to represent its electrons and the atom on the right has dots to represent its electrons. In the region where the outer shells overlap there are two crosses and two dots to represent two shared pairs of electrons.

Dot and cross diagram of an oxygen molecule.

A dot and cross diagram of a nitrogen molecule. It is made up of two nitrogen atoms joined by a triple covalent bond. The atom on the left has crosses to represent its electrons and the atom on the right has dots to represent its electrons. In the region where the outer shells overlap there are three crosses and three dots to represent three shared pairs of electrons.

Dot and cross diagram of a nitrogen molecule.

We can also draw dot and cross diagrams for molecules that have more than two atoms. For example, the dot and cross diagram below is for the molecule ethane (C2H6):

A dot and cross diagram of an ethane molecule. Two carbon atoms are joined by a single covalent bond. In the area where their outer shells overlap, there is one cross and one dot. Each carbon atom is bonded to three hydrogen atoms in the same way.

Dot and cross diagram of an ethane molecule.

Flashcards

Flashcards help you memorise information quickly. Copy each question onto its own flashcard and then write the answer on the other side. Testing yourself on these regularly will enable you to learn much more quickly than just reading and making notes.

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How do you draw a dot and cross diagram of a molecule?

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