Homework # 8 Electron Configuration

AIM # 8: How are electrons arranged in an atom?

Electron configuration Where the electrons are in the atom.

1. Three rules are used to build the electron configuration: A. Aufbau principle B. Pauli Exclusion Principle C. Hund’s Rule

A. Aufbau Principle Electrons occupy orbitals of lower energy first. ___ ___ ___ ___ ___ 1s 2s 2 p

Violates Aufbau Principle electrons fill lowest available energy levels ? ___ ___ ___ ___ ___ 1s 2s 2 p

Aufbau Diagram

B. Pauli Exclusion Principle An orbital can hold only two electrons and they must have opposite spin. No two electrons in an atom have the same four quantum numbers ___ ___ ___ ___ ___ 1s 2s 2 p

Violates Pauli Principle No more than 2 electrons /orbit + different spin ___ ___ ___ ___ ___ 1s 2s 2 p ?

C. Hund’s Rule Every orbital in a subshell is singly occupied with one electron before any one orbital is doubly occupied, and all electrons in singly occupied orbitals have the same spin. ___ ___ ___ ___ ___ 1s 2s 2 p

Violates Hund’s Rule Must occupy all orbitals of equal energy, before pairing up. ___ ___ ___ ___ ___ 1s 2s 2 p ?

2. Diagonal Rule 1s 2s 2p 3s 3p 3d 4s 4p 4d 4f 5s 5p 5d 5f 6s 6p 6d Write the energy levels top to bottom. Write the orbitals in s, p, d, f order. Write the same number of orbitals as the energy level. Draw diagonal lines from the top right to the bottom left. To get the correct order, follow the arrows! 2s 2p 3s 3p 3d 4s 4p 4d 4f 5s 5p 5d 5f 6s 6p 6d 7s 7p

Aufbau Diagram for Hydrogen

Aufbau Diagram for Helium

Aufbau Diagram for Lithium

Aufbau Diagram for Beryllium

Aufbau Diagram for Boron

Aufbau Diagram for Carbon

Aufbau Diagram for Nitrogen

3. Notations of Electron Configurations A. Standard B. Shorthand

Aufbau Diagram for Fluorine 1s2 2s2 2p5

A. Standard Notation of Fluorine Number of electrons in the sublevel 2 + 2 + 5= 9 1s2 2s2 2p5 PEL: 1, 2 Sublevels

B. Shorthand Notation 1s2 2s2 2p5 Fluorine: [He] 2s2 2p5 Use the last noble gas that is located in the periodic table right before the element. Write the symbol of the noble gas in brackets. Write the remaining configuration after the brackets.

Exceptions to the Aufbau Rule Write the electron configuration of : Cr and Cu Cr [Ar] 3d44s2 Cu [Ar] 3d94s2

Exceptions to the Aufbau Rule Cr [Ar] 3 d44s2 Cu [Ar] 3 d94s2 However, the actual electron configuration, determined experimentally, are: Cr [Ar] 3d54s1 Cu [Ar] 3d104s1

Notice that for chromium, an electron is Cr [Ar] 3d54s1 Cr [Ar] ___ ___ ___ ___ ___ ___ Notice that for chromium, an electron is borrowed from the 4s subshell to give a 3d subshell that is exactly half filled.

For Copper the 4s electron is borrowed to Cu [Ar] 3d104s1 Cu [Ar] ___ ___ ___ ___ ___ ___ For Copper the 4s electron is borrowed to give a completely filled 3d subshell.

Apparently , half filled and filled subshells have some special stability that makes such borrowing energetically favorable. This subtle but nevertheless important phenomenon affects not only the ground state configurations of atoms but also the relative stabilities of some of the ions formed by the transition elements.

Exceptions to the Aufbau Rule Ag [Kr] 4d105s1 Au [Xe] 4f145d106s2

Blocks in the Periodic Table

Test your understanding

Violates Hund’s rule? ____ Violates the Aufbau principle? ____ c) Violates the Pauli exclusion principle? ___ d) Represents C in the ground state? ____ e) represents C in the excited state? ____

a) violates Hund’s rule? If equal energy orbitals are available, don’t pair e’s B

A b) violates the Aufbau principle? “Build up” by adding e’s from bottom to top A

c) violates the Pauli exclusion principle? 2e’s max per orbital; must have different spins C

d) represents C in the ground state? Hund’s, Pauli’s, and the Aufbau are followed D

e) represents C in the excited state? An electron has been promoted to a higher E orbital A

End of the show

a) violates Hund’s rule a) violates Hund’s rule? __B___ If equal energy orbitals are available, don’t pair e’s b) violates the Aufbau principle? __A___ “Build up” by adding e’s from bottom to top c) violates the Pauli exclusion principle? __C__ 2e’s max per orbital; must have different spins d) represents C in the ground state? __D__ Hund’s, Pauli’s, and the Aufbau are followed e) represents C in the excited state? __A__ An electron has been promoted to a higher E orbital

Diagonal Rule Must be able to write it for the test! This will be question #1 ! Without it, you will not get correct answers ! The diagonal rule is a memory device that helps you remember the order of the filling of the orbitals from lowest energy to highest energy _____________________ states that electrons fill from the lowest possible energy to the highest energy