Electron Configuration

The periodic table

How are electrons distributed in the atom?

A designation of the most probable distribution is called electron configuration. It refers to the distribution of electrons in the different sublevels or orbitals as well as in the main energy levels. There are 3 principles that you have to consider.

1. As the electrons get farther from the nucleus, their energy becomes greater.2. The same principle holds true for

the sub-levels or orbitals. Therefore, s<p<d<f.

3. The maximum number of electrons per orientation is 2. Therefore, only 2 electrons can occupy the s orbital, 6 electrons for p orbitals, 10 electrons for d orbitals, and 14 electrons for f orbital.

We also have to consider three other principles that will help us distribute the electrons properly:

1.Pauli’s Exclusion Principle

2.Aufbau Principle3.Hund’s Rule

Pauli’s Exclusion Principle

1. States that no two electrons in an atom should have the same four quantum numbers.

2. Electrons will go into available orbitals singly before beginning to pair up.

3. Two electrons can only be in the same orbital if they have opposite spins.

Aufbau PrincipleWhen placing electrons

into the orbitals of polyelectronic atoms, we use the Aufbau Principle.

AUFBAU (German: BUILD UP)

• Electrons will successively occupy the available orbitals in order of increasing energy.

• States that an electron tends to occupy the level with a lower energy first.

• This principle states that in addition to adding protons and neutrons to the nucleus, one simply adds electrons to the hydrogen-like atomic orbitals.

The aufbau Principle

Hund’s Rule

• States that the most stable arrangement of electrons is that which allows the maximum number of unpaired electrons.

• Applies to electrons that need to occupy the same sub-level.

• This arrangement minimizes electron-electron repulsions.

• Electrons occupy an orbital having equal energy one by one first before pairing up.

Summary• Electrons go into hydrogen-like orbitals toconstruct polyelectronic atoms.• Elements in the same column have the

samenumber of valence electrons.• Elements in the same column will havesimilar chemical properties.• Remember the “trick” for the order of theenergies of the orbitals.