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The octet rule is one of the more poorly named rules in chemistry,
since it turns out to be violated almost as much as it is followed. It comes
from the realization that since atoms want to have noble gas structure and some
noble gases have eight electrons in their valence shell, atoms should have eight
electrons around them.
The atoms that commonly follow the octet rule are
- Carbon: C
- Nitogen: N
- Oxygen: O
- The halogens, F, Cl, Br, I
Each of these atoms will probably have
eight electrons around it.
However, there are almost as many exceptions to the rule as there are atoms
that follow it. Ones you will possibly see:
- Hydrogen: H will have 2 electrons to gain a noble gas shell.
- Phosphorus: P often has 10 electrons
- Sulfur: S often has 12 electrons
- Atoms in groups 1, 2, and 3 want 2, 4, and 6 electrons respectively. I.e.,
Na (group 1) wants 2, Be (Group 2) wants 4 and Al (Group 3) wants 6.
- Any molecule with an odd number of electrons: at least one atom must
therefore have an odd number of electrons (The molecule is known as a free
radical.)
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