[constant thinker]

What makes lead so good at blocking radiation? Does it have to do with the structure it takes in its solid form?

Also are there any other elements good at blocking radiation?

[mike]

Depends on what type of radiation.

Lead has a high density of electrons

[Mitch]

A piece of plastic works fine if you're talking about blocking alpha rays.

[constant thinker]

Alpha rays are stopped by the outer most layer of our skin also. Beta Rays penetrate our skin. Gamma Rays have the ability to blast right through us.

I'm changing my question to..
How can you tell if something will be a good radiation blocker/absorber? Does it have a certain structure or density?

[Mitch]

Any high Z(proton) material will do. Lead just happens to be cheap.

[mike]

And the thickness of the shield as well, less dense but more thickness will work for some shields

[Grejak]

How can you tell if something will be a good radiation blocker/absorber? Does it have a certain structure or density?

It depends on what you want to stop.  Heavy charged particles such as alphas are emitted at a lower velocity and interact primarily through coulomb repulsion.  So, as soon as the particle leaves vacuum, it interacts with many electrons simultaneously, causing excitation or ionization of the electrons and leading to a quick energy/velocity loss of the alpha.  It turns out that up to 1/500 of the energy per nucleon is lost with each interaction with an electron.  So it will quickly stop in anything.

Beta particles are a step up in complexity, as you can have electron-electron or electron-nuclear interactions that can affect both the energy and the direction of the beta particle.  The primary means of energy loss though is through ion pair production (I think), and you lose ~35 keV for every ion pair produced.  The one thing you have to watch out for is bremsstrahlung radiation, which occurs when a beta particle is accelerated in the electric field of a nucleus.  So, you want to watch out for your higher Z elements and stick to something with lots of carbon or aluminum to minimize the bremsstrahlung radiation.  A good rule of thumb is that the ratio of energy loss by bremsstrahlung to the energy loss by ionization is EZ/800.  Where E is the energy of the beta and Z is the atomic number of the shielding.  By the time you get to lead, you lose most through bremsstrahlung, so your shielding is not very useful.  That is why you always block the beta particles and then the gamma rays if you have a mixed source.

Gammas are even more complex as they can lose energy by several different methods, the most common being photoelectric absorption, Compton scattering and pair production.  All of these processes increase by the atomic number of the shielding nucleus.  So here you want the heaviest and most dense material that you can find.  Lead is normally chosen as it has a relatively high Z, is quite dense and is also quite cheap.

If you want to stop neutrons, then you have another problem as they only indirectly lose energy with interactions with electrons, and most energy loss is through nuclear interactions.  Then you have the added problem of needed different methods for different energies.

I think I have most of that right, but no promises  

[Elgon]

There are two different strategies to shield neutrons. You can either use a low Z material, like paraffin or polyethylene. Any organic polymer with lots of hydrogen atoms will do. Here the neutron will collide with numerous atoms that have a similar mass and loss energy every time it collides and changes direction. In the end it will not have enough energy to move anymore.
The other option is to pick a material that has a high neutron capture cross section, such as boron, cadmium or gadolinium. If you use this material for shielding, the neutron will not be slowed down; instead it will be absorbed by the shielding material.

The best choice of shielding will always depend on which kind of radiation you want to stop and what the energy of the radiation is.

[constant thinker]

I'm going to have to keep all of this information in mind. One day when I get a job I'll start my element collection. Anything with a half life of well over a year is collectible by my standards.

[Grejak]

I'm going to have to keep all of this information in mind. One day when I get a job I'll start my element collection. Anything with a half life of well over a few thousand years is collectible by my standards.

There, fixed I your sentence for you.

Perhaps you disagree, but I think that if you are going to have an element collection, then you might as well have enough of the element to see, not just measure with a counter.  In that case, you will need to up your collectible half-life considerably.

[Mitch]

Assuming you need at least 1000th of a mole of an element to see that element. That is 6.022 x 10 ^20 atoms at a half-life of 1 year. So the activity would be 1.32 * 10^13 Bacquerels or 357 Curies!!! I'm sorry, you would be dead.

[constant thinker]

Whoooaaaaa. Ok I missed some of the math there, but I do know that 357 Curies is fairly high. Maybe I'll just stick with Hydrogen-Bismuth.

Wait maybe I'll skip over all the gases.