Activated charcoal is carbon that has been treated with oxygen to open up hundreds of thousands of pores inside of it. Opening up these tiny pores between atoms expands the surface area of the carbon enormously. In fact, in terms of raw surface area, one gram of activated charcoal would be able to cover several football fields if it were rolled flat. The surface are of activated charcoal is only one of the reasons why it is so useful. Carbon is a naturally absorbent element, and it is able to absorb a wide variety of harmful chemicals. The large surface area of activated charcoal allows it to absorb and trap millions of chemicals at its countless bonding sites. There are certain elements that it can't trap, however. These include sodium and nitrates. So although it is very absorbent, activated charcoal cannot absorb every element out there.