S1.3.5: Orbital Configuration and Rules

Electrons occupy atomic orbitals in patterns that mimimize

energy and make the atom more stable and they are described with an electron configuration. 

Aufbau Principle:

fill lowest energy orbitals first 

Hund’s Rule:

Spread out in equal- energy orbitals before pairing 

Pauli Exclusion

Principle: max 2 electrons per orbital, with opposite spins. 

 

 

Atomic Orbital =

a region of space around the nucleus where there is a high probability of finding an electron. 

 

Spin =

electron property shown by an arrow. Two electrons in one orbital must have opposite spins. 

 

Degenerate Orbitals –

 

Degenerate Orbitals – orbitals with the same energy 

 

Orbitals are grouped into sublevels. A single orbital can hold up to

2 electrons as long as they have opposite spins. So, orbitals have a predictable total capacity. 

The Aufbau principle means electrons fill orbitals in

 

order of increasing energy.

Aufbau idea helps

predict electron configurations by assigning electrons to the lowest energy available orbitals first. 

Hund’s rule + Pauli in orbital diagrams 

Orbital diagrams use: 

• A box for each orbital 

• • Arrows for electrons 

Rule while filling 

1. Aufbau: lower energy first 

2. Hund: for equal-energy orbitals (like the 3 p boxes), place one electron in each before paring 

3. Pauli: If two electrons share a box they must have opposite spins 

 

Unpaired

 --> Alone --> use rules 

Full Electron Configuration lists every

filled sublevel in order. 

Condensed configuration replaces the inner electrons with the

previous noble gases in brackets, then writes only the outer (valence) part. 

Electron configurations of ions

(including transition metals) 

 

To form ions, electrons are

gained or lost relative to the neutral atom 

 

Cations:

lose electrons from the highest principal quantum number n first. 

Anions:

gain electrons into the next available orbital by Aufbau 

Transition metals:

electrons are typically removed from the s sublevel before the d sublevel when forming cations.

This means you may need to look at which subshell corresponds to the highest n and remove from the s in that shell before adjusting the d count. 

 

4s are lost before 3d –

exception for Fe 

d half fill or full, if can add s

Two famous exceptions to the Aufbau predicted electron configurations occur because

half-filled and fully filled d sublevels are unusually stable.

As a result, an atom may adopt a configuration that slightly violates the naive filling order to gain extra stability in the d sublevel. 

CHROMIUM AND COPPER