Shielding Electrons

Shielding electrons are the electrons in the energy levels between the nucleus and the valence electrons. They are called "shielding" electrons because they "shield" the valence electrons from the force of attraction exerted by the positive charge in the nucleus.

In fluorine there are 9 protons in the nucleus and there are 2 shielding electrons in the first level between the nucleus and the outer shell. They shield some of the charge of the nucleus from the electrons that are in the outermost energy level. (Also look at example 5b in your workbook.)
 nuclear charge shielding electrons valence electrons F +9 1s2 2s22p5 2 7
Next, neon also has 2 shielding electrons along with 8 valence electrons.
 nuclear charge shielding electrons valence electrons Ne +10 1s2 2s22p6 2 8
With sodium, we have 3 energy levels. There is one valence electron in the third level and all the electrons between that one valence electron and the nucleus are shielding electrons. In this case there are 2 in the first energy level and 8 in the second for a total of 10 shielding electrons.
 nuclear charge shielding electrons valence electrons Na +11 1s22s22p6 3s1 10 1
So notice that the number of shielding electrons increases when you reach the end of the periodic table and go on to the next period.

Now look at carbon, nitrogen, and oxygen to see that within a period there is no change in the number of shielding electrons. Even though the valence electrons increase in number from 4 to 5 to 6, the number of shielding electrons stays the same--two shielding electrons for each of those elements.
 C N O shielding electrons 2 2 2 valence electrons 4 5 6

When you deal with the changes within a group, notice what happens. (Also see part c of example 5.) Going from carbon to silicon to germanium, the number of protons in the nucleus increases from 6 to 14 to 32, the number of energy levels increases from 2 to 3 to 4, the number of shielding electrons also increases. In carbon there are 4 valence electrons and 2 shielding electrons. Silicon also has 4 valence electrons, but it has 10 shielding electrons. Germanium (Ge) also has 4 valence electrons, and it has 3 shells or energy levels of electrons that are shielding electrons. There are 2 in the first, 8 in the second, and 18 in the third for a total of 28 shielding electrons along with the 4 valence electrons.
 shielding electrons valence electrons C 2 4 Si 10 4 Ge 28 4

Notice that the shielding electrons follow a pattern somewhat like the number of energy levels. They stay the same within a period (except for increasing gradually and erratically across the transition metals). They increase in steps as you start a new period or go down a group

Top of Page

Back to Course Homepage

 E-mail instructor: Science Department 19600 South Molalla Avenue Oregon City, OR 97045 (503) 594-3352 TDD (503) 650-6649

Distance Learning questions

Clackamas Community College
©1998, 2002 Clackamas Community College, Hal Bender