### Electron configuration

The lowest orbit level is called the ground state As the electrons absorb more quantum energy, they get more excited and can jump to a higher orbit or energy level There are different principal energy levels each with their own sublevels Typically the principal levels correspond to the row number (except the transition elements) The sublevels include s, p, d, and f.

### What do quantum numbers mean?

Principal Quantum number – tells you the primary energy shell where the electrons are found (includes all the sublevels) Sublevel – tells you where the electrons are by the orbitals that are found in the sublevel. The “s, p, d, f” Orbitals – the individual electron clouds that are found based on the probability (s, px,y,z, dxz, yz, xy, x2-y2, z2, f (has 7) Each px, py and pz are individual orbitals Sublevel organization As you see below, the sublevels (s, p, d, f) are all found in different sections. The energy level (distance away from the nucleus) increases from s < p < d < f. So the f sublevel has the highest energy or the greatest distance from the nucleus.               

Each orbital holds a maximum of 2 electrons. “s” orbital holds – 2 electrons Each px, py, pz orbital holds – 2 electrons But recall… calculating the maximum number of electrons is 2(n^2). The energy level 2 (n=2) will give you 8 electrons… how? Or from where? Which orbitals? 2 electrons in the s orbital, 6 in the p orbital.

### Electron configuration

The electron configuration of an atom assigns a number to each electron. The number includes the principal energy level of the electron, the name of the orbital and the number of electrons in that orbital. 

The electron configuration shows that

• there are 2 electrons in the s-orbital of the first energy level (1s^2)
• 2 electrons in the s-orbital of the second energy level (2s^2)
• And 4 electrons in the p-orbital of the second energy level (2p^4).

The orbitals are always written as lower case letters and the number of electrons is always written as superscripts. Figuring out Electron configuration 3 Principles must be followed –

1. Aufbau
2. Pauli Exclusion Rule
3. Hund’s Rule

Aufbau – German for “build up”, the principle was proposed by Neils Bohr. Electrons are added starting at the lowest energy level and build up to the higher levels. The “1s” orbital is filled first, and then 2s and then 2p and so forth. *Look at the periodic table above* Notice 4s orbital has a lower energy than 3d orbital so the 4s is filled before the 3d orbital.

Pauli Exclusion Principle – Only a maximum of two electrons can occupy an orbital if they have opposite spins. 

Hunds Rule – Proposed that as the electrons fill up the orbitals, they tend to maximize unpaired electrons in filling all the orbitals before completely filling one. In other words, before filling the first p-orbital with two electrons, an electron is placed into the px orbital, then another electron into the py then pz before filling the px orbital.

A great tool to use to figure out the order of the orbitals and energy levels is the diagram below. Start by creating a column of 1s orbitals, and then beside the 2s, start your column of p orbitals, and then go down to the 3rd energy level for the d orbital etc… Draw your arrow diagonally down for your order!