# Why are lighter nuclei dominated by $\beta$-decay and heavier ones by $\alpha$-decay?

While I'm trying to understand the nature of radioactive decay, I found that the lighter nuclei mostly have $$\beta$$-decay while heavier nuclei have $$\alpha$$-decay.

From what I know is that the $$\beta$$-decay happens to stabelize an atom whith more neutrons than protons. This mostly happen for the lighter atoms.

For heavy nuclei, the nuclear force is related with atomic mass $$A$$ while the Coulomb force is related with the electric charge $$Z^2$$ (with $$Z$$ the numbers of protons). So the repulsive Coulomb force become much stronger than the nuclear force for heavy nuclei, even when there are far more neutrons than protons. Thereby the protons are expelled and the nuclear force cannot hold them any more, so an $$\alpha$$-particle can escape the nuclei. But why does this happen for very heavy atoms with atomic mass higher than $$200$$ and not for less heavier atoms? And why does very heavy atoms not have $$\beta$$-decay to transform protons into neutrons and stabilize the nucleus?

• protons are expelled to the surface That doesn't sound right. See en.wikipedia.org/wiki/Nuclear_shell_model May 27, 2019 at 14:29
• I know it's formulated a bit shady, but that doesn't take a away the core of my question. May 27, 2019 at 14:32
• Agreed. There's some useful info in the answers here, although that question is focused on why proton & neutron numbers are roughly equal for small stable nuclei. May 27, 2019 at 14:35
• Plenty of heavy nuclides do undergo beta decay, of one form or another. The Wikipedia alpha particle article is quite good. It explains why almost no light nuclides emit alphas. Also see en.wikipedia.org/wiki/Nuclear_drip_line May 27, 2019 at 15:14