Just because an atom satisfies its octet doesn’t mean that it is stable. We also have to consider valence electrons. Sounds familiar? Let’s look into what those are.
What is a valence electron?
Every dot = 1 valence electron, and every stick = 1 valence electron. (Octet is different, see above.)
What is the difference between valence and octet electrons?
All of these carbons satisfy their octet, but only one of them is actually stable. The others suck. How can we tell which one will be stable?
Understanding the hydrogen isotopes.
We can predict how many valence electrons each atom wants to have by looking up its Group Number on the periodic table.
In our example above, carbon is in Group 4A, meaning that it wants to possess 4 valence electrons, making the first structure the most stable.
Now you don't have to guess what an atom looks like in its neutral state! Simply use the periodic table to look up this information.