Alphetta

Meet Alphetta, a heavy nucleus waving goodbye to two protons and two neutrons at once.

Some heavy nuclei hold more energy than they need. One way to shed it is alpha decay: the nucleus throws out a tight bundle of two protons and two neutrons, which is a helium-4 nucleus. The leftover daughter nucleus has an atomic number Z two lower and a mass number A four lower, so it is a different element. Ernest Rutherford named alpha rays, and in 1909 Rutherford and Thomas Royds showed that alpha particles are helium nuclei.

A worked example: uranium-238 has Z = 92 and A = 238. After alpha decay, Z = 92 − 2 = 90 and A = 238 − 4 = 234, which is thorium-234. Both totals balance, because the alpha particle carries away exactly 2 and 4.

Ordinary smoke detectors use a tiny alpha source, americium-241, to ionise the air in a small chamber; smoke disturbs that current and sets off the alarm.

Alpha particles are heavy and slow by nuclear standards, so a sheet of paper or the outer layer of skin stops them. That makes them fairly harmless outside the body but dangerous if an alpha emitter is swallowed or breathed in.