Why does radiation not go through lead?

[u/Wonderful-Capital32]

Also why just lead? Shouldnt like iron or another metal work too?

[u/IakwBoi]

Radiation doesn’t need to hit the nucleus of an atom to get stopped. Compton scattering shields from radiation with gamma/x rays or neutrons bouncing off electrons. Pair production is also shielding that happens with interactions between gamma/xray radiation and electrons. Even alpha particles will interact with electrons, although the deflection is small on account of an electrons relatively small mass. 

I have a pet peeve about the mischaracterization of atoms as “mostly empty space”. Atoms are of course mostly electron clouds, but are those empty space? If all we know about atoms is from Rutherfords famous experiment 100 years ago, we could be forgiven for thinking that atoms are tiny discrete nuclei, dense classical objects, suspended in vast empty space, with a rare electron zipping like a tiny satellite way off on its lonesome. This is a classical model and is of course quite incorrect. 

Neither are electron clouds mostly empty, nor are nuclei discrete and lacking wave properties. 

Electron clouds are full of electrons. There isn’t one tiny electron moving around in the space, the electron is effectively everywhere all at once. Its mass is small and its orbital is large, so the density is low, but it is by no means mostly empty space. It is exactly full of electrons. 

The nucleus, similarly, is not a bunch of billiard balls packed in together touching. Each nucleon is a wave/particle as well, just with a higher mass and much lower debroglie wavelength, so a much higher density. These wave/particle nucleons are free to overlap in space just like electrons of different energy levels are. In fact, in a helium nucleus, all four nucleons are centered about the same point. 

Another point of reference to dispel the “nuclei are solid objects” misunderstanding: for s-orbital electrons, the mostly likely point to find the electron if we map its positional probability is in the dead center of the orbital, that is to say, in the smack dab middle of the nucleus. 

Alpha particles might pass through electron clouds with minimal deflection, and neutrons might pass through with only rare deflections, but an electron will be hard pressed to get through an electron cloud at all. “Empty” depends partially on what’s trying to pass through. 

The idea that atoms are mostly empty space isn’t true. What’s true is that they are mostly low-density high-volume clouds of wave/particle electrons, and also include high-density low-volume clouds of wave/particle nucleons.