SECTION 3: ELECTRON CONFIGURATIONSCHAPTER 9: ELECTRONS IN ATOMS AND THE PERIODIC TABLE

Learning Goals•Apply the Pauli exclusion principle, the aufbau principle, and Hund's rule to write electron configurations using orbital diagrams and electron configuration notation.

Electron Configuration• An electron configuration

shows how the electrons occupy the orbitals for a particular atom.

• The aufbau principle states that each electron occupies the lowest energy orbital available.

1s orbital2 electrons

2s orbital2 electrons

2p orbital6 electrons

3s orbital2 electrons

3p orbital6 electrons

4s orbital2 electrons

3d orbital10 electrons

4p orbital6 electrons

5s orbital2 electrons

4d orbital10 electrons

5p orbital6 electrons

6s orbital2 electrons

4f orbital14 electrons

5d orbital10 electrons

6p orbital6 electrons

7s orbital2 electrons

5f orbital14 electrons

6d orbital10 electrons

7p orbital6 electrons

Electron Configuration• Hund’s rule states that single

electrons must occupy each equal-energy orbital before additional electrons with can occupy the same energy level orbitals.

• All electrons in singly occupied orbitals must have the same spin.

• School bus rule

Electron Configuration• The Pauli exclusion principle

states that a maximum of two electrons can occupy a single orbital, but only if the electrons have opposite spins.

• We symbolize this as two arrows pointing in opposite directions.

Orbital Notation Examples• Write the orbital notation electron configuration for

the following atoms or ions. State how many unpaired electrons are in each.

a. N

b. Cl

Orbital Notation Examplesc. Al

d. V

Orbital Notation Examplese. Li+

f. O2-

Electron Configuration• Electron Configurations Using

the Periodic Table:

• Read the periodic table from left to right to determine the electron configuration.

Electron Configuration• Write the ground state electron

configuration for the following:

a. C

b. Fe

Electron Configurationc. W

d. S2-

Electron Configuration• Noble gas notation uses noble

gas symbols in brackets to shorten inner electron configurations of other elements.

• The noble gas must have a lower atomic number than the atom or ion that the electron configuration is being written for.

Noble Gas Notationa. W

b. Eu

Noble Gas Notationc. As

d. O

Electron Configuration• The aufbau diagram can be used to

write correct ground-state electron configurations for all elements up to and including Vanadium, atomic number 23.

• The electron configurations for certain transition metals, like chromium and copper, do not follow the aufbau diagram due to increased stability of half-filled and filled sets of s and d orbitals.

Electron Configuration• Chromium: [Ar]4s13d5

• Copper: [Ar]4s13d10

• These exceptions occur because a half-filled d subshell and a completely filled d subshell are particularly stable

Electron Configuration•The number of outer-shell electrons in a transition series does not change as you move across a period.

• The transition series represents the filling of core orbitals and the number of outershell electrons is mostly constant—either 2 or 1.

Electron Configuration

(2e–) for 4s23dx

(1e–) for 4s13d5 or 4s13d10