Energy Levels and Electron Shells

In an atom, electrons aren't just flying around randomly like bees in a hive. Instead, they live in specific areas called electron shells or energy levels. You can think of these like the floors of a hotel or the rows of seats in a stadium. Each shell is a fixed distance from the nucleus. The shells closest to the nucleus have the lowest energy, while the shells further away have higher energy. Because electrons are attracted to the positive nucleus, they always try to find the 'cheapest' or lowest-energy seat available.

Just like a small car can only fit a few people, each electron shell has a strict limit on how many electrons it can hold. For the first 20 elements on the Periodic Table, we follow a simple pattern known as the 2-8-8 rule.

1. The first shell ($n=1$) is the smallest and can only hold $2$ electrons. 2. The second shell ($n=2$) is larger and can hold up to $8$ electrons. 3. The third shell ($n=3$) can also hold $8$ electrons before the next shell starts to fill.

If a shell is full, any extra electrons must move to the next 'floor' up.

Electrons follow a very specific 'inside-out' filling order. They are lazy! They will always occupy the lowest energy level (the one closest to the nucleus) before moving to a higher one. A higher shell will remain completely empty until the shell below it is 100% full. This is why we always fill the $n=1$ shell before touching $n=2$.