http://www.ck12.org Chapter 6. The Periodic Table
FIGURE 6.4
A block diagram of the periodic table shows which sublevels are being filled at any point.Valence Electrons
Because they are held more loosely to the nucleus than the inner electrons, it is the outermost electrons that dictate
the chemical behavior of a given element. Specifically, much can be predicted about the chemical reactivity of a
given element based solely on the number of electrons in its highest occupied principal energy level (n). These
electrons are referred to asvalence electrons. The remaining electrons, which are closer to the nucleus and less
available for interaction with other atoms, are referred to ascore electrons.
Consider the element magnesium, which has 12 electrons in a configuration of 1s^2 2s^2 2p^6 3s^2. The highest occupied
principal energy level is 3, so all electrons with a quantum number of n = 3 are valence electrons. Thus, magnesium
has two valence electrons. The other 10 electrons (in the n = 1 and n = 2 levels) are its core electrons.
Representative Elements
We will now examine each block of the periodic table in more detail. The s and p sublevels for a given principal
energy level are filled during the correspondingly numbered period. For example, the 2s and 2p sublevels fill during