The heisenberg uncertainty principle:

Super smart guy, Heisenberg, laid the foundation for modern quantum theory by saying that electrons don't live in these perfect little circles around the nucleus like Bohr had drawn

Instead he said that not only do electrons not live in perfect little circles, but it is IMPOSSIBLE to know both the position and velocity of an electron. Which is a pain, but hey, it's a fundamental principle in our current understanding of light and matter, so it's ok.

So...we don't know where electrons are...what do we know? DO WE KNOW ANYTHING?

Yes. We know regions where electrons are likely to be found. We call these regions orbitals. Let's break it down a little bit.

1. Energy Levels

Electrons have energy levels (the numbers we wrote beside the rows on the periodic table) The higher the energy level, the further away the electron is from the nucleus.

2. Sublevels

Sublevels are s, p, d, and f.

3. Orbitals

Those sublevels are each different shapes, and those shapes can be oriented different ways. The number of ways it can be oriented, is how many orbitals that sublevel has.

The s- sublevel is a sphere, which always looks the same, no matter how you turn it, so there is only 1 s orbital. The p -sublevel can be oriented 3 ways, so there are 3 p orbitals. d has five orbitals, f has seven.

4. Pairings

Each orbital holds 2 electrons. Those electrons have opposite spins.

Watch this video if you feel like you need help writing out electron configurations:

Different Ways to Write Electron Configurations:

Let's write it multiple ways for Bromine (Br)

1) Write it all out, which is super long and takes forever, but you need to know how to do it

2) Noble Gas Notation. Put the Noble Gas that comes before this element in brackets, and then write out the remaining electron configuration. Much shorter.

3) Orbital Diagram. I would write out the whole electron configuration first, and then transfer it into the orbital diagram. This is where you have little blanks representing each orbital (with the energy level and sublevel written underneath) and then arrows representing the electrons.

Remember Hund's rule! Every orbital gets 1 electron in it first before you start doubling up!

Also remember: for these, the arrows represent the electrons -- NOT THE SUPERSCRIPTS! You don't need the superscripts (exponents) anymore!!!!

exceptions:

These go for any element that end in d4 or d9 -- they are so close to being either half full or completely full, that they are going to steal from the s orbital.