## Main Energy Levels

The number of main energy levels for electrons is another very important consideration. As the atomic number increases, so does the number of electrons. But that does not necessarily increase the number of energy levels.

As you go across a period  all of the new electrons fit into the energy levels that are already being used. For example, looking at carbon, nitrogen, and oxygen, the number of protons increases from 6 to 7 to 8; so does the number of electrons. However, when we look at their electron arrangements, notice that all of the electrons are in the first two energy levels. The number of energy levels being used does not change even though the number of electrons does.
 C N O electrons 6 7 8 configuration 1s22s22p2 1s22s22p3 1s22s22p4 levels 2 2 2
(This is also shown in example 5a in your workbook. The format there shows the symbol for the element, the number of protons in the nucleus, the electron configuration, and also the total number of electrons in each energy level or shell.)

It is only when you go from one period to the next that you have to increase the number of energy levels. (Also shown in a different way in part b of example 5 in your workbook.) As we go from fluorine to neon to sodium, the number of protons increases from 9 to 10 to 11 and thus the number of electrons increases from 9 to 10 to 11. Notice what happens to the number of energy levels that must be used. For both fluorine and neon, two energy levels accommodate all of the electrons. But once there are 10 electrons in those two energy levels (2 in the first and 8 in the second as with neon), any additional electrons have to go into the next energy level.
 F Ne Na electrons 9 10 11 configuration 1s22s22p5 1s22s22p6 1s22s22p63s1 levels 2 2 3

Next, let’s consider what happens within a group. (Also shown in example 5c in your workbook.) As you go from carbon to silicon to germanium, the number of protons increases in large jumps. The number of electrons also increase and the number of energy levels used also increases. Notice that carbon has two energy levels, silicon has three, and germanium (Ge) has four levels being used.
 electrons configuration levels C 6 1s22s22p2 2 Si 14 1s22s22p63s23p2 3 Ge 32 1s22s22p63s23p64s23d104p2 4
The number of energy levels used to accommodate the electrons in the atoms of a particular element is going to be the same as the number of the period. As you go down the periodic table, you will increase the number of energy levels being used.

Like the atomic number, the number of energy levels is not really a periodic or repeating feature of atoms. It stays the same throughout a period and then increases by one when a new period starts.

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