As atomic mass increases the stability of the nucleus increases but after a while they become too heavy and the stability decreases.

The graph above shows how the energy changes with atomic number. Note that the energy axis goes the other way to the way you would expect. I have shown the graph this way since it is the way it is usually presented.

We can see that Hydrogen nucleus has the highest energy and with the exception of Helium (He) the energy decreases to Iron (Fe) which is the most stable. Then the energy increases to Uranium (U). After Uranium the atoms are so unstable that they do not exists to any real extent in nature.

Some of the elements heavier than Iron do have a way of decreasing their atomic number and lowering their energy. They do this by a helium nucleus (two protons and two neutrons) breaking off. This is called an α particle and the process is called α decay.

This helium nucleus has an atomic mass of 4 and a double positive charge. This mass and charge means that it interacts quite strongly with matter. Therefore it can be stopped by a thin piece of tissue paper or the layer of dead skin that we have covering our body.

Since the atomic number has now changed (decreased by two) the atom is now a different element and its mass has also decrease by four.

For example Uranium – 238 decays by alpha decay to give the element Thorium

Wikipedia