Radioactive elements.

Alpha P. Jamison has written:

'Elements of mechanical drawing ...'

One way that elements decay is by alpha-emission. That means that the nucleus of the atom spews out alpha particles. An alpha particle is made of two protons and two neutrons, like a helium nucleus. Of course, once an alpha particle is emitted, the number of protons in the nucleus decreases, and the element changes with the number of protons.

alpha radiation

The elements emit especially alpha particles.