Angle Strain Definition:

Angle strain refers to the increase in energy associated with a bond angle that has deviated from the preferred angle of 109.5°.

Angle Strain of Cycloalkanes:

The ideal angle for cycloalkanes is 109.5°. As shown in the diagram above, many cycloalkanes have different angles, making them unstable. The table below shows the heat combustion per CH2 group: