What are Valence Electrons?
They are electrons that are in the outermost shell of the electron cloud
Why are they Important?
These are the electrons that is used in chemical bonding. They are the ones that are being gained, lost, or shared.
How do you figure out how many there are?
You have to count which column the element is on (excluding the transition metals) and that is going to be the number of valence electrons there is going to be. This only applies to the natural elements.
What are the radius of the elements?
In order to find out the radius of an element, you would have to look at the atomic number. The larger the atomic number is, then the more electrons the element is going to have. Therefore, the larger the element (and the radius) is going to be.