What is polonium, which was used to kill Russian dissident Alexander Litvinenko?
Polonium is a chemical element discovered by Marie and Pierre Curie in 1898 and named for Marie Curie's native Poland.
It has 84 electrons orbiting a nucleus of 84 protons and a variable number of neutrons, with each different number of neutrons corresponding to a different isotope of polonium. The best-known one has a total number of neutrons and protons of 210 and is called polonium-210. It is radioactive and has a half-life of about 138 days, which means however much you have today, you have half as much in 138 days.
Polonium decays almost always by emitting an alpha particle -- a clump of two neutrons and two protons -- turning into lead. Alpha particles deposit a lot of energy in a very short distance, which means it is very easy to shield oneself against them, and it is very hard to detect polonium by its alpha emissions, since just a bit of paper will block the radiation.
What makes it very nasty is that if you for any reason eat it, it will distribute itself through your body, and wherever a polonium atom decays, it will damage cells nearby. The damage continues as the polonium decays. In the long term, it can cause cancer. If you have eaten too much, as Litvinenko did, it can cause radiation sickness and death. Basically, what happens is that so many cells all over your body keep dying that you yourself die.
A lethal dose is a few tens of a billionth of a gram, which is incredibly tiny.
Polonium is very hard to get in nature but can be made, at great cost, in reactors from a similar element called bismuth. It is a very exotic and expensive poison and is really effective only if you want to target one specific person into whose food you can put it.
Just getting it on your skin is no problem since the dead skin cells on the surface of your skin would protect you, so it shouldn't be much of a worry for most people and would make a pretty poor terrorist weapon or weapon of mass destruction.
Dr. Knowledge is written by physicists Stephen Reucroft and John Swain, both of Northeastern University. E-mail questions to drknowledge@globe.com or write Dr. Knowledge, c/o The Boston Globe, PO Box 55819, Boston, MA 02205-5819. Include your initials and hometown. 
