The enclosed graph shows the number of Protons on the x-axis (Z), and the number of Neutrons on the y-axis for all elements (N). Stable combinations are marked by black squares, whereas unstable ones are marked by grey squares. I understand why there is a grey "line" located above the black "line", as adding electrically positive, mutually repelling, protons would make the nucleus less stable. However, I do not understand why there is a grey "line" to the right of the black "line": isn't adding neutrons supposed to make a nucleus more stable (as a result of the additional Strong Force and greater distance between the protons)?
Can anyone help out?