Cranial Contents

## Question:

My friend calls me an airhead. I think that peoples skulls are filled with brains. Am I right?

## Answer:

Actually it is worse than your friend claims. Your head...and and your friend's and mine is almost entirely filled with empty space - perfect vacuum, no air, nothing.

Let's do the math. Consider the inside of a person's head. The matter filling the cranial cavity is made up of atoms, mostly of hydrogen, oxygen, carbon and a few other elements. Each of the atoms enclosed by the skull has more than 1999/2000 of its mass in the atomic nucleus. The rest of it is in the electrons circulating around the outside of the atom. Between the nucleus and the electrons is nothing.

A typical nucleus is only about 1/100,000 the radius of its atom, which means if the nucleus was the size of a beach ball, located in Melbourne Beach, the electron shells would extend from Cape Canaveral to Sebastian Inlet. Since the volume of a thing is proportional to the cube of its dimensions, the volume of an atom's nucleus is about 1/1,000,000,000,000,000 the volume of the atom. Making allowance for the space between the outside electronic shells of the atoms, the volume filled with matter is less than 1/1,000,000,000,000,000 of the total volume enclosed by the skull. Clearly "air head" is excessively generous.