Polonium 210

Polonium 210, an Alpha emitter, the physical chemistry of death:
One gram of Po-210 is 1/210 = 4.762 * 10-3 mole. Multiplying this by Avogadro's number, the number of atoms in a mole, we obtain
4.762 * 10-3 * 6.022 * 1023 = 2.87 * 1021 atoms of Po-210. If we multiply this many atoms by the alpha energy of 5.407 MeV, we obtain a total energy release of
2.87 * 1021 * 5.407 MeV = 1.55 * 1028 eV.
One joule equals 6.24 * 1018 eV, so this is equal to 2.48 * 109 joules, or 2.48 gigajoules.
The average adult human male has a mass of 70 kg. If we assume the Po-210 is uniformly distributed throughout the body, then the whole-body radiation dosage from 1 gram of Po-210 in one week would be
0.0345 * 2.48 gigajoules / 70 kg * 2.4 sievert/gray = 2.9 * 106 sievert. This is a truly gargantuan dose, but 1 gram is a huge amount of Po-210. If we only need 10 sieverts/week to kill our victim, then we only need
10 sievert / (2.9 * 106 sievert/gram) = 3.4 * 10-6 g.
That's 3.4 micrograms.