The stability of a nucleus depends on the balance between the strong nuclear force that holds the nucleus together and the repulsive electromagnetic force between the positively charged protons. When the number of neutrons in a nucleus is decreased, the balance between these forces is disrupted, which can lead to a less stable nucleus. This is because the strong nuclear force between the protons and neutrons is weakened, and the repulsive electromagnetic force between the protons becomes stronger, making the nucleus more likely to decay into a more stable configuration.