The electron configuration shows where electrons are supposed to be and how many are in each orbital. The first orbital can hold only two electrons and then it's full. The next few orbitals can hold up to eight electrons and then they are full.
Some orbitals can hold lots of electrons. But we aren't going that far in the periodic table. We will stop at the third orbital.
Hydrogen and helium are the first two elements on the periodic table. Hydrogen has one electron in the only orbital around the nucleus. Remember, we said it could hold up to two electrons. If the orbital isn't full, then the atom isn't stable.
Helium, on the other hand, has two electrons in the orbital. Since the orbital is full, the atom is stable. Stable means it doesn't react with anything.
Consider stable as happy. We want atoms to be stable, so they don't blow things up. Hydrogen and helium must follow the duet rule. Their outermost orbital must have two electrons to be stable. All other atoms must have eight electrons in their outermost orbital if they want to be stable.
Since they must have eight electrons, they have to follow something called the octet rule. It states that for an atom to be stable, it must have eight electrons in the outermost orbital. In the diagrams, we have three atoms and their electron configurations. Nitrogen has how many electrons in the outermost orbital?
There are five electrons. Since there are not eight, nitrogen atoms are unstable. Oxygen has six electrons in its outermost orbital. Again, since there are not eight electrons, oxygen atoms are unstable.
Finally, we have Neon. It has a total of 8 electrons in the outermost orbital and does satisfy the octet rule. Neon is considered stable.
It's easy to keep straight. Hydrogen and Helium are a duet, or a pair, and follow the duet rule. The rest follow the octet rule.