Beta decay

Also called Beta-minus decay, Beta-plus decay, Beta emission

Beta decay is the mode of radioactive decay in which one nucleon changes type inside the nucleus. Beta-minus turns a neutron into a proton and emits an electron and an antineutrino; beta-plus turns a proton into a neutron and emits a positron and a neutrino.

AP Physics 2 requires both modes, as separate essential-knowledge statements, and being vague about which particle goes with which is where marks go.

Beta-minus (15.8.A.2.iii). A neutron changes to a proton by emitting an electron and an antineutrino.

^{A}_{Z} X \rightarrow \, ^{A}_{Z+1} Y + \beta^{-} + \bar{\nu}

Beta-plus (15.8.A.2.iv). A proton changes to a neutron by emitting a positron and a neutrino.

^{A}_{Z} X \rightarrow \, ^{A}_{Z-1} Y + \beta^{+} + \nu

Both emitted light particles carry mass number 0, so $A$ is unchanged either way, which is the signature of beta decay. Charge is what separates them. On the first line, $Z = (Z+1) + (-1) + 0$ ; on the second, $Z = (Z-1) + (+1) + 0$ . The neutrino and antineutrino contribute nothing to either column, which is exactly why they are easy to leave off and still get the daughter right.

Write them anyway. Statement 15.8.A.2.i makes lepton number a conserved quantity alongside nucleon number and charge, and the balanced equation is the thing an examiner marks. The bar over the symbol is the whole difference between $\nu$ and $\bar{\nu}$ , so form it clearly.

Beta-plus and electron capture are not the same process, and only one is examinable. The Topic 15.8 boundary statement removes electron capture and neutron emission from the framework outright, along with types of neutrinos and any application of the weak force. If a source offers you a third beta mode, it is describing a course you are not taking.

Topic 15.8 works both.

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