Radioactive decay

A nucleus changes into a new element by emitting alpha or beta particles. These changes are described using nuclear equations.

Alpha decay (two protons and two neutrons) changes the mass number of the element, decreasing by four and decreasing the atomic number by two. An alpha particle is the same as a helium-4 nucleus.

Example

_{86}^{219}\textrm{Rn} \rightarrow _{84}^{215}\textrm{Po} + _{2}^{4}\textrm{He}

Beta decay changes the atomic number, increasing by one (the nucleus gains a proton) but the mass number remains unchanged (it gains a proton but loses a neutron by ejecting an electron, so a beta particle is an electron).

Example

_{6}^{14}\textrm{C} \rightarrow _{7}^{14}\textrm{N} + _{-1}^{0}\textrm{e}

Gamma is pure energy and will not change the structure of the nucleus in any way.

Question

Uranium 238, _{92}^{238}\textrm{U} , emits an alpha particle to become what nucleus?

Alpha decay (two protons and two neutrons) decreases the mass number of the element by four and decreases the atomic number by two, so the remaining nucleus will be _{90}^{234}\textrm{}.

A periodic table shows that element number 90 is thorium, _{90}^{234}\textrm{Th} .