Question: why can't xrays past through lead

Serena Corr answered on 19 Jun 2013:

Zulaikanu – this is a great question!

The reason lead is so good at not letting x-rays pass through it is because it is so heavy. If you look at the periodic table of elements and find lead (the symbol for lead is Pb – that’s because the latin name for lead is plumbum) you can see that it has an atomic number of 82 and atomic mass of 207.21g mol-1. From this, we can see that lead has a high atomic mass. The atomic mass tells us how many protons are in a lead atom, so that means there are 82 protons in a lead nucleus. To make a neutral atom, we need an equal number of electrons as we have protons, so that means that lead has 82 electrons. Lead also has a small atomic radius, so we can pack a lot of lead atoms together really well and it has a high density (11.34 g cm-3). It is because of these properties (lots of electrons, high density) that x-rays do not penetrate lead very well. That is why lead is used for shielding in x-ray machines. I hope this answers your question!