How do you use electron-dot notation to illustrate the number of valence electrons present in one atom?
Each dot placed around the symbol for an element represents a valence electron.
The number of valence electrons can be easily determined by locating which column the element belongs to. Elements in the left-most column (alkali metals) have one valence electron. As you move to right on the table, each column adds one more valence electron. The video below shows a number of examples of electron dot (Lewis structures) notation.
To take this a bit further, the number of valence electrons can be determined by writing an electron configuration for the element. The number of electrons in the highest energy level are the valence electrons.
Determining the number of valence electrons for elements in the d-block is a bit more complicated. To figure out these you will need to know the electron configuration of each element.
Hope this helps!