The Stability Zone
The graph has a plot of stable elements, this part is called band of stability.
At the higher end of the band of stability lies alpha decay, below is positron emission or electron capture, above is beta emissions and elements beyond the atomic mass of 83 are unstable radioactive elements.
Alpha decay is located at the top of the plotted line, because the alpha decay decreases the mass number of the element in order to keep the isotope stable. This is done by using the element helium (He). An unstable isotope's protons are decreased by 2 and its neutrons are decreased by 4, and because the isotope was originally unstable before it went through alpha decay, the elements are still considered unstable.
(Beta decay accepts protons so it changes the amount of protons and neutrons. the number of protons increase while neutrons decrease. To make things easier to understand think of the ratio of the isotope: there are too many neutrons compared to the number of protons therefore it is above the band of stability.
Positron emission and electron capture is when the isotope gains more neutrons. Positron emission and electron capture are below the band of stability because the ratio of the isotope has more protons than neutrons, think of it as there are too few protons for the amount of neutrons and that is why it is below the band of stability.