SECTION 3: ELECTRON CONFIGURATIONS CHAPTER 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 orbital 2 electrons

2s orbital 2 electrons

2p orbital 6 electrons

3s orbital 2 electrons

3p orbital 6 electrons

4s orbital 2 electrons

3d orbital 10 electrons

4p orbital 6 electrons

5s orbital 2 electrons

4d orbital 10 electrons

5p orbital 6 electrons

6s orbital 2 electrons

4f orbital 14 electrons

5d orbital 10 electrons

6p orbital 6 electrons

7s orbital 2 electrons

5f orbital 14 electrons

6d orbital 10 electrons

7p orbital 6 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 Examples c. Al d. V

Orbital Notation Examples e. Li + f. O 2-

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 Configuration c. W d. S 2-

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 Notation a. W b. Eu

Noble Gas Notation c. 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]4s 1 3d 5 Copper: [Ar]4s 1 3d 10 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 4s 2 3d x (1e–) for 4s 1 3d 5 or 4s 1 3d 10