Alpha particle

AP Physics 2 defines the particle at 15.8.A.1.i, and the wording is worth having exactly. An alpha particle, or helium nucleus, consists of two neutrons and two protons and is symbolised by $\alpha$ or $\text{He}^{2+}$, with the bracketed restriction that in Physics 2 only He-4 nuclei will be considered.

Read the second symbol carefully. $\text{He}^{2+}$ is not a helium atom. The two electrons a neutral helium atom would carry are absent, so the charge is $+2e$, or $3.20 \times 10^{-19}$ C with the sheet's $e = 1.60 \times 10^{-19}$ C. Its mass number is 4 and its atomic number is 2, which is all the bookkeeping an alpha decay equation needs.

Because the sheet prints the proton and neutron masses as the same rounded $1.67 \times 10^{-27}$ kg, four nucleons come to about $6.7 \times 10^{-27}$ kg. That is a working figure, not a nuclear mass: the exam prints no table of nuclide masses, so any binding-energy question hands you the data.

The useful comparison is charge to mass. An alpha has twice a proton's charge and about four times its mass, so its charge-to-mass ratio is half a proton's. Put both through the same potential difference or the same magnetic field and they do not behave alike. AP Physics 2 has an optional Topic 10.1 activity built on exactly that comparison, which is the only place outside Unit 15 the particle appears.

The one restriction to respect is the He-4 clause: no heavier helium isotope is in scope.