The periodic table shows all the elements


The atom



Electronic shells

Atomic number

Electronic subshells

Atomic orbitals

Atomic orbitals d

Rutherford's experiment





The atomic number Z characterizes an atom.

It is possible that 2 or more atoms have the same Z, so that they are the same element, but different mass number A. Such atoms are called isotopes.

Example: hydrogen is the most simple atom, as it has only 1 electron. It may have 3 different masses:

1) hydrogen with 1 proton

2) hydrogen with 1 proton and 1 neutron (called deuterium)

3) hydrogen with 1 proton and 2 neutrons (called tritium)

In all 3 cases above the number Z=1, so that we are dealing with the element hydrogen. However their masses are 1, 2 and 3 respectively. So, what is the mass number that is shown in the periodic table? (check yourself here)

It is 1.008 ! Strange?

That is because the mass number A is an average of the mass of all the isotopes of an element. The reason the number in this case is so close to 1 is because this is an weighted average, i.e., the mass number of each isotope enters the average in proportion to its abundance. Because the simple hydrogen atom (Z=1, A=1) is by far the most abundant, the average is very close to 1.



© Ricardo Esplugas. All images in this site can be bought in an enlarged version. Please contact me on