Covalent bonding: The covalent bonds in molecules and giant structures can be represented in the following forms:

The idea

Covalent bonds can be shown in several ways, and you must recognise and evaluate each representation. A dot-and-cross diagram shows outer-shell electrons, with shared pairs in the overlap between atoms (e.g. H₂O has two bonding pairs and two lone pairs). A displayed formula shows every atom and bond as a line in 2D (e.g. H–O–H). A structural formula shows atom arrangement without drawing all bonds (e.g. CH₃CH₂OH). 3D models, including ball-and-stick models, represent the spatial arrangement of atoms and bonds. While 2D formulae are useful for simple molecules, they cannot show the 3D shape. Giant covalent structures are best represented by 3D ball-and-stick models showing a small section of the continuous network, as structural formulae cannot represent their infinite lattices.

What earns the marks

• Dot-and-cross diagrams show outer-shell electrons, using different symbols for electrons from different atoms to highlight shared pairs.
• Displayed formulae show every atom and every covalent bond as a line in 2D, for example H–O–H for water.
• Structural formulae show the arrangement of atoms without drawing every bond, for example CH₃CH₂OH for ethanol.
• 3D models and ball-and-stick models show the spatial arrangement of atoms and bonds, which is essential for visualising molecular shapes.
• Giant covalent structures are represented by drawing a 3D section of the network, as 2D structural formulae cannot show the continuous lattice.
• Evaluate the limitations of models, such as 2D diagrams not showing 3D shape, and dot-and-cross diagrams not showing relative atom sizes.

Common mistakes and how to put them right

• Drawing dot-and-cross diagrams with the wrong number of outer electrons.

  Put it right: Count outer electrons from the group number, such as four for carbon and six for oxygen.

• Using a structural formula to represent a giant covalent structure.

  Put it right: Giant structures form continuous lattices, so they are represented using 3D ball-and-stick models of a section, not discrete structural formulae.

• Assuming a 2D displayed formula shows the actual shape of a molecule.

  Put it right: Displayed formulae only show connectivity; 3D models or ball-and-stick diagrams are needed to show the true spatial arrangement.

Exam tips

• Label shared pairs clearly in dot-and-cross diagrams and keep the symbols for each atom's electrons distinct.
• When drawing displayed formulae, include every bond as a line and check each atom's bonding total.
• Be prepared to evaluate the advantages and limitations of different models, such as 2D vs 3D representations.