The parent nuclide of the thorium decay series is 239 90 Th. The first four decays are as follows: alpha emission, beta emission, beta emission, and alpha emission. Can you write the nuclear equations for this series of emissions?
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You can work it out if you know that each alpha decay equates to the loss of a Helium nucleus, 4/2He, therefore reduces mass by four and atomic number by two and beta (minus) decay has no effect on mass but increases atomic number by one, overall a neutron is changed to a proton. Of course changing the atomic number changes the element so above you expect a total of four element changes.
239/90 Th -------> 235/88 Ra + 4/2 He -------> 235/89 Ac + β- ---------> 235/90 Th + β- ---------> 231/88 Ra + 4/2 Ηe
239/90 Th -------> 235/88 Ra + 4/2 He -------> 235/89 Ac + β- ---------> 235/90 Th + β- ---------> 231/88 Ra + 4/2 Ηe