The atomic number, Z, of an element is numerically equal to the number of protons within its nucleus, and to the elements' identity. The atomic number of the lightest element, hydrogen, is one, whilst the atomic number of uranium is 92.
The absorption of X-rays is proportional to the fifth power of the atomic number, thus lead, atomic number 82, absorbs X-rays about half a million times more effectively than does carbon, atomic number 6.
