Answer (Randall Munroe): There are people who collect elements. There collectors try to gather samples of as many of the elements as possible into periodic-table-shaped display cases.

Of the 118 elements, 30 of them – like helium, carbon, aluminum, iron, and ammonia — can be bought in pure form in local retail stores. Another few dozen can be scavenged by taking things apart (you can find tiny americium samples in smoke detectors). Others can be ordered over the Internet.

All in all, its’ possible to get samples of about 80 of the elements — 90, if you are wiling to take some risks with your health, safety, and arrest record. The rest are too radioactive of short-lived to collect more than a few atoms of them at once.

But what if you did?

The periodic table of elements has seven rows. You could stack the top two rows without much trouble. The third row would burn you with fire. The fourth row would kill you with toxic smoke. The fifth row would do all that stuff PLUS give you a mild does of radiation. The sixth row would explode violently, destroying the building in a cloud of radioactive, poisonous fire and dust. DO NOT build the seventh row.

Here is a link to the original resource with a lot more detail (randall-munroe-periodic-wall-of-elements).