It is important to find the electron arrangement of an atom in order to predict its reactivity.
Remember the electrons in an atom are arranged in energy levels or what used to be called shells. The most stable energy level is closest to the nucleus of the atom but it can only hold a total of two electrons. The second energy level can accomodate up to 8 electrons. If we look at Na, we see that it has 11 electrons (atomic #) so it will end up with one lone electron in the outermost energy level. These outer electrons are called valence electrons. This one lone electron in the Na atom will be very unstable and tend to react easily with other elements.
Reactivity is a chemical property. The Periodic Table of Elements arranges elements according to similar properties, e.g. group 1 elements share similar high reactivity.