Alpha decay

Also called Alpha emission

Alpha decay is the mode of radioactive decay in which a nucleus ejects an alpha particle, a helium nucleus of two protons and two neutrons. The mass number of the parent falls by 4 and the atomic number by 2, so the daughter is a different element.

Essential knowledge 15.8.A.2.ii states it in one line: alpha decay occurs when a nucleus ejects an alpha particle. Everything quantitative follows from what that particle is, which is 15.8.A.1.i: two neutrons and two protons, symbolised $\alpha$ or $\text{He}^{2+}$ , and in AP Physics 2 only He-4 is considered.

In nuclear notation, with mass number above and atomic number below:

^{A}_{Z} X \rightarrow \, ^{A-4}_{Z-2} Y + ^{4}_{2}\alpha

Check both columns rather than trusting the arrows. Mass numbers: $A = (A-4) + 4$ . Charges: $Z = (Z-2) + 2$ . A parent with $A = 210$ and $Z = 84$ therefore leaves a daughter with $A = 206$ and $Z = 82$ , and its neutron count falls from 126 to 124.

Alpha is the only one of the four modes that changes the mass number. Both betas convert one nucleon into the other kind and gamma moves no nucleons at all, so a question reporting a drop in $A$ has handed you the number of alphas: divide the drop by 4.

Two limits worth knowing before an exam. The Topic 15.8 boundary statement says AP Physics 2 does not expect you to memorise the processes by which specific isotopes decay, so a question naming a nuclide will tell you what it does. And penetrating power, ranges in air and stopping with a sheet of paper appear nowhere in any of the four current CEDs, however prominent they are in older prep material.

Topic 15.8 works the balancing.

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