Some atoms are naturally radioactive, especially atoms of uranium and plutonium. These atoms have so many protons and neutrons that even the strong nuclear force is barely strong enough to hold them together, and they sometimes lose a proton or a neutron, making them a little more stable. The number of protons and neutrons in its nucleus keeps changing, as the atom shoots out some of its protons and neutrons into space. The heavier the atom, the more protons and neutrons it has, and the more likely it is to lose some of them, kind of like a shaggy dog is more likely than a short-haired dog to shed dog hair all over your house.
A uranium atom usually loses two protons and two neutrons at the same time, so that what it shoots out is the same as the nucleus of a helium atom. We call this an alpha particle. Alpha particles aren’t dangerous to people unless you drink them or breathe them in.
But sometimes the atom may not be able to put together a whole alpha particle. In this case, one neutron may change into a proton, and that makes the atom more stable. When the neutron changes into a proton, it has some energy left over, which we call a beta particle. A beta particle has the same mass as an electron. Sometimes this change also creates neutrinos.
Another way that an unstable atom may get more stable is for a proton to change into a neutron. In this case, the atom shoots out a positron and a neutrino. The positron ends up running into an electron, and this destroys both of them, but it shoots out two gamma ray photons instead.
When an atom shoots out this energy – the neutrinos or the beta particles or the gamma ray photons – the energy can pass through people’s skin and damage their DNA inside their cells. People who get a lot of radiation die immediately, while people who get less die more slowly, because their bones can’t make new blood anymore. That takes several weeks.