Shapes of covalent molecules (SQA National 5 Chemistry): Revision Note

Shapes of simple covalent molecules

• Covalent molecules are not flat; they have specific three-dimensional (3D) shapes

• The shape of a simple molecule is determined by:

  • The number of atoms bonded to a central atom

  • The way the bonds are arranged in space

• The pairs of electrons in the bonds repel each other and try to get as far apart in space as possible

• This repulsion forces the molecule into its most stable, and therefore correct, shape

• For the National 5 course, the four common shapes are:

  1. Linear

  2. Angular

  3. Trigonal pyramidal

  4. Tetrahedral

• 3D drawing of molecules often include wedges and dashes

• These show how bonds extend in space from the central atom:

  • Solid wedge: The bond is coming out of the page, towards you

  • Dashed wedge: The bond is going into the page, away from you

  • Straight line: The bond lies in the plane of the page

Linear

• The atoms are arranged in a straight line

  • There are two atoms bonded to one central atom

• For example, carbon dioxide:

Angular

• The atoms are arranged in a bent or 'V' shape

  • There are two atoms bonded to one central atom

• For example, water:

Trigonal pyramidal

• The atoms are arranged like a shallow pyramid with the central atom at the top

  • There are three atoms bonded to one central atom

• For example, ammonia

Tetrahedral

• A central atom with four bonds pointing to the corners of a tetrahedron

  • There are four atoms bonded to the central atom

• For example, methane