orbital. Second, hybrid orbitals, which we will talk about in the next chapter,
also violate the Aufbau principle by mixing orbitals of different energy levels.
The Stable Octet
Look at element 10, neon (Ne). Its electron configuration is 1s^22 s^22 p^6 . Neon’s
configuration has one 1 subshell and two 2 subshells. It has no 3 subshells or 4
subshells, so the two 2 subshells (indicating the 2nd subshell) constitute its
outermost shell.
Now take a look at neon’s outermost shell: 1s^22 s^22 p^6 . Count the electrons in this
shell: 2 + 6 = 8. The fact that neon has 8 electrons in its outermost shell means
that it has a stable octet: 8 electrons.
Examine element number 18, argon (Ar), and look especially at its outermost
shell, which is the 3rd shell: 1s^22 s^22 p^63 s^23 p^6 . Argon, too, has a stable octet. That
is, it has 8 electrons in its outermost shell. The same is true for the following:
All of the elements with stable octets are called noble gases or inert gases.
They’re very stable. They don’t like to react with anything or change themselves
in any way. They’re very happy the way they are. Why? Because atoms are
happiest with 8 electrons in their outermost shell. Helium (He, atomic number 2)
is also very stable. It, too, is an inert gas although it has only 2 electrons in its
outermost shell.
The electrons in an atom’s outermost shell are called valence electrons. So
another way of saying “stable octet” is to say “8 valence electrons.” All of the