A heavy nucleus spits out a bundle of 2 protons + 2 neutrons. No quark changes type. It is a tug-of-war between the strong force and electric repulsion, and quantum tunneling decides when it happens.
Click through the six steps. Watch the alpha form inside, rattle against the wall, tunnel out, and slow down into helium.
Step through an alpha decay
Step 1A uranium-238 nucleus: 92 protons and 146 neutrons, all constantly moving around. Nothing outside is touching it.
Step 2Inside, 2 protons and 2 neutrons briefly clump into a tight bundle: an alpha particle. That bundle (a helium-4 nucleus) is unusually tightly bound, so forming it frees up energy. Throwing out a single proton or neutron would cost energy instead, which is why the nucleus ejects a 4-pack.
Step 3The alpha rattles around and hits the edge roughly 10²¹ times per second. Past the edge is a wall of electric repulsion it doesn't have enough energy to climb (see the energy diagram below). Each hit has about a 1 in 10³⁸ chance of getting through anyway.
Step 4Eventually (after about 4.5 billion years on average for U-238) the alpha turns up outside the wall. It didn't climb over. It tunneled through, which is possible because quantum particles are spread-out waves rather than hard little balls.
Step 5Out here the strong force can't reach it, and its 2 protons are repelled by the 90 protons left behind. That push launches it at about 14,000 km/s (≈5% of light speed) with 4.2 MeV of energy. The thorium nucleus recoils the other way, the way a gun kicks back when it fires.
Step 6The alpha is a bare helium nucleus with charge +2. Moving through matter, it rips electrons off the atoms it passes (ionization), which is what makes radiation harmful, and it slows down fast: it stops within a few cm of air or a sheet of paper. Then it grabs 2 electrons and becomes an ordinary helium atom. Almost all the helium on Earth was made this way, underground.
The strong force is ~100x stronger than electric repulsion, but it only reaches the nucleons touching each one. Electric repulsion reaches everywhere. Making a nucleus bigger does not add glue per nucleon, but repulsion keeps piling up. Past lead (82 protons), pushing out a chunk becomes worthwhile.
This graph shows how much energy the alpha would need at each distance from the nucleus’s center. Inside, the strong force makes a deep pit. Right at the edge the strong force cuts off and electric repulsion takes over, making a tall hill that slopes down farther out.
The alpha only has 4.27MeV, but the hill is about 28MeV tall. A ball cannot roll over a taller hill, ever. But the alpha is a wave, and a wave hitting a wall fades away inside it — if the wall is thin enough, a tiny bit leaks through. That leak is the probability of finding the alpha outside. Once out, repulsion pushes it down the hill, and that is where its speed comes from.
This explains the enormous spread in half-lives. Polonium-212 emits 8.95MeV alphas, so its wall is thinner: it decays in 0.3 microseconds. Uranium-238’s 4.27MeV alphas face a thicker wall: 4.5 billion years. Twice the energy means roughly 1023 times faster decay.
Each proton is 2 up + 1 down. Each neutron is 1 up + 2 down. Count them: U-238 has 330 up + 384 down. Afterward Th-234 (324 + 378) plus the alpha (6 + 6) gives the same totals. Alpha decay is the only decay here where no particle changes identity and the weak force is not involved.