Atomic Structure - Chemistry - page 73

will start by learning how to read an electron configuration. Look at the
electron configuration for hydrogen, shown in Figure 3-4a.
The superscript shows us that hydrogen has only one electron. The large
(coefficient) number represents the principal quantum number for the elec-
tron. In this case, because the value for n = 1, it means that this electron is
located in the first energy level. The letter indicates the sublevel that the
electron is located in. Because
there is only one energy level
in this particular atom, it rep-
resents the valence shell, or
outer energy level. Hydro-
gen is said to have one elec-
tron in its valence shell.
Now, let's compare that
electron configuration of hydrogen to the electron configuration of he-
lium, shown in Figure 3-4b.
Now, we see that the superscript for helium is 2, which means that he-
lium contains two electrons. This shouldn't surprise you, because a quick
check of the periodic table shows that the atomic number for helium is 2,
indicating that a neutral atom of helium contains two protons and two elec-
trons. The letter “s” tells us that both of helium's electrons occupy an “s”
sublevel. The value of n (coefficient) is 1, which tells us that both of these
electrons are in the first energy level. Again, because the electrons only
occupy one energy level, the first energy level is also helium's valence shell.
It has two electrons in its valence shell.
For clarity's sake, let's look at the electron configuration of an atom
that shows electrons in more than one energy level. Following is the elec-
tron configuration for oxygen.
The Electron Configuration of Oxygen
1s 2 2s 2 2p 4
What are we looking at here? Well, first add up the superscripts and
you will see that there are a total of 8 electrons (2 + 2 + 4 = 8). This makes
sense, because oxygen, with an atomic number of 8, has a total of 8 elec-
trons. Now, you see that there are two different coefficients (1 and 2). This
tells us that oxygen's electrons occupy two different energy levels. The three
letters tell us that the electrons are spread over three different sublevels. As
you will recall from Lesson 3-3, the first energy level (n = 1) has one sub-
level, whereas the second energy level (n = 2) has two. The six electrons that

The Electron Configuration of Helium

superscript = number of electrons in sublevel

1s 2

letter ( l ) = the sublevel

coefficient (n) = the energy level

Figure 3-4b