9781422279502

47

Silver

and reorganizations since it was first developed in 1869, until it be- came the modern version of the table used today. The periodic table is arranged into rows and columns by increasing atomic number. Each element has a unique atomic number. It is the number of protons in the nucleus of the atom. For example, helium has an atomic number of 2—there are two protons in the nucleus of an atom of helium. (All samples of an element have the same number of protons, but they may have a different number of neutrons in the nucleus. Atoms with

The diagram on the left shows the two protons of helium. On the right, the number at the bottom is the atomic weight of an atom of the element.

8