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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
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