The modern periodic table
For many years, an element;s atomic number was just its position on the periodic table. Then, in 1913, scientists worked out than an element's atomic number was the number of protons in its atoms. The total number of protons and neutrons in the nucleus of an atom is called its mass number. In full chemical symbols like the top number is the mass number and the bottom one is the atomic number.
This information can be used to calculate the number of protons, neutrons and electrons in an atom. For example, in , there are:
- 13 protons (because the atomic number is 13)
- 27 - 13 = 14 neutrons (mass number minus atomic number)
- 13 electrons (the same as the number of protons)
Relative atomic mass
The mass of an atom is incredibly small, so…