In Chapter 1, we described the meaning of the atomic symbol for an element. In the atomic symbol, the atomic number (the number of protons in the nucleus) appears as a subscript preceding the symbol for the element. The mass number appears as a superscript, also preceding the symbol. Thus for uranium (atomic number 92) with a mass of 238, the symbol is 92 238 U {\displaystyle {}_{92}^{238}U} . To show radioactive decay in a ptcouncil.netical equation, you need to use atomic symbols. Thus, for the loss of an alpha particle from 92 238 U {\displaystyle {}_{92}^{238}U} , you need to show uranium on the “reactant” side of the equation and thorium and the alpha particle on the “product” side. Just like any other ptcouncil.netical equation, a nuclear equation must balance. The sum of the mass numbers on the reactant side must equal the sum of the mass numbers on the product side. Because we started with uranium-238 and lost four mass units in the alpha particle, the product (or products) of the decay must have a total mass of (238 – 4) = 234. We have also removed two protons from the uranium nucleus, dropping the atomic number by two. The newly formed element is therefore thorium-234.

\<_{92}^{238}\textrm{U}\rightarrow _{2}^{4}\textrm{He}+_{90}^{234}\textrm{Th}\>

In this equation, we have shown the alpha particle using the atomic symbol for helium ( 2 4 H e {\displaystyle {}_{2}^{4}He}

), but this is often shown using the symbol 2 4 α {\displaystyle {}_{2}^{4}\alpha }
. Compounds that emit alpha particles are very toxic, in spite of the poor penetrating ability of the particle. This is especially true if the emitting element is inhaled or ingested. The toxic dose of the alpha-emitter 210 P o {\displaystyle {}^{210}Po}
in a 175-pound person has been estimated to be about one microgram (1 × 10-6 g).

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