How can you tell how many valence electrons are contained in one atom of a particular element found in the periodic table of elements?

See explanation

Explanation:

The valence electrons in an atom are the electrons in the the outermost shell of the atom.

From the periodic table we can see the atomic number of elements. Remember that the atomic number is not only the number of protons in the nucleus, but also the number of electrons in the neutral atom of the element.

Now let us use sodium for example. The atomic number of sodium is 11. The first shell must contain a maximum of two electrons while subsequent shells contain a maximum of eight electrons.

The 11 electrons in sodium will be distributed in shells as follows; 2,8,1. This means that there is 1 electron in the outermost shell of sodium. Hence sodium has 1 valence electron.