Chemistry Practice Test: Nuclear Chemistry

An alpha particle is two protons and two neutrons bound together, which is the same as a helium-4 nucleus. A high-energy electron is a beta particle, and a photon is gamma radiation.

Losing an alpha particle lowers the mass number by 4 (238 → 234) and the atomic number by 2 (92 → 90), and element 90 is thorium. Keeping the mass at 238 forgets that the alpha particle carries 4 mass units away.

In beta-minus decay a neutron turns into a proton and an electron is emitted, so the atomic number rises by 1 (6 → 7, nitrogen) and the mass number stays 14. Boron would mean the atomic number went down.

15 days is three half-lives: 80 → 40 → 20 → 10 g. Stopping after two halvings gives 20 g; a sample never drops to zero in a fixed number of half-lives.

Each half-life multiplies the amount by 1/2, so after four: (1/2)⁴ = 1/16. The answer 1/4 comes from dividing by the number of half-lives instead of halving each time.

200 → 100 → 50 → 25 mg is three half-lives, so each one takes 18 ÷ 3 = 6 hours. Dividing 18 by 2 assumes only two halvings happened.

Gamma rays are high-energy electromagnetic waves with no mass or charge, so they penetrate the most. Alpha particles are stopped by paper and beta particles by a few millimeters of aluminum.

In the Sun’s core, extreme heat and pressure force hydrogen nuclei together to form helium, releasing energy. Fission splits heavy nuclei, which is what nuclear power plants on Earth use.

Carbon-14 dating works on things that were once alive and are up to roughly 50,000 years old. After millions of years almost no carbon-14 is left, and rocks and metals were never alive.

The products of fission or fusion have slightly less mass than the starting nuclei, and that missing mass becomes energy according to E = mc². Breaking chemical bonds involves far smaller energies.