When an atom is at its lowest energy, we say it is in the "ground state".  But this lowest energy does not correspond to an atom with all the electrons in the n =1 (lowest) energy level.  The Pauli exclusion principle states that no two electrons can have completely identical properties.  Some of the properties to consider are position, momentum, angular momentum, and energy.  Electrons in the same atom are roughly at the same position, so if they have the same energy they have to have different values of some other quantity like angular momentum.  But these other quantities are quantized too, so only certain combinations of properties can exist inside atoms.  To fully understand how all these restrictions arise requires quantum mechanics outside the scope of this material, so we will just state the conclusions.

The n = 1 energy level of atoms has exactly two possible states for electrons.  The n = 2 energy level has exactly eight states for electrons.  (You might notice that the first row of the periodic table has two elements and the second row has eight elements.  This is not a coincidence - the periodic table can be understood in terms of the energy states of electrons in atoms.).  The n = 3 energy level can actually hold 18 electrons, but due to some interesting science that we will gloss over, the ninth electron will go into the n = 4 energy level.  The remaining ten n = 3 states are filled later.  So for our purposes, the n = 3 energy level holds eight electrons.  Often, these energy "levels" are called "shells" of the atom.  You may have heard this terminology in another science course, and we will use often use it as well.

Consider the element silicon (atomic symbol Si).  Silicon is composed of 14 electrons, 14 protons, and (in most cases) 14 neutrons.  In its ground state, silicon has two electrons in the n = 1 energy level, eight in the n = 2 energy level, and four in the n = 3 energy level, as shown on the energy diagram to the left.  The chart also illustrates a convention (unique to this author as far as I know) followed in the rest of this unit:  blue electrons occupy filled energy levels, and green electrons occupy partially filled energy levels.  The outermost energy level that (in the ground state of the atom) contains electrons is called the "valence shell" of the atom.  These valence electrons are responsible for all chemical bonds between atoms.  For silicon, the valence shell corresponds to a principle quantum number n of 3.

The reader should note that as electrons cannot be seen with any currently available technology, they certainly cannot be seen using visible light and thus do not have color.  The colors are merely a convenience to help us identify which electrons are packed in completely filled shells and which occupy partially filled shells.
 
 

What happens to these atomic energy states when atoms bond to form solids? Go to the next page to find out!