Electron Configurations Section 5.3. Objectives Apply the Pauli Exclusion Principle, the Aufbau Principle, and Hund’s Rule to write electron configurations.

Electron Electron ConfigurationConfiguration

ssSection 5.3Section 5.3

ObjectivesObjectivesApply the Pauli Exclusion Principle, Apply the Pauli Exclusion Principle,

the Aufbau Principle, and Hund’s Rule the Aufbau Principle, and Hund’s Rule to write electron configurations using to write electron configurations using orbital diagrams and electron orbital diagrams and electron configuration notation.configuration notation.

Define valence electrons and draw Define valence electrons and draw electron-dot structures representing electron-dot structures representing an atom’s valence electrons.an atom’s valence electrons.

Electron configurationElectron configuration: : the arrangement of electrons the arrangement of electrons

in an atomin an atomElectrons tend to assume an Electrons tend to assume an

arrangement that gives the atom the arrangement that gives the atom the lowest possible energy. Why?lowest possible energy. Why?

This arrangement is the ground-state This arrangement is the ground-state electron configuration.electron configuration.

Three (3) rules define how electrons Three (3) rules define how electrons can be arranged in an atom’s orbitals.can be arranged in an atom’s orbitals.

Rule #1Rule #1The Aufbau The Aufbau

Principle states Principle states that each electron that each electron occupies the occupies the lowest energy lowest energy orbital available.orbital available.

An An aufbau diagramaufbau diagram show the sequence show the sequence of orbitals from of orbitals from lowest to highest lowest to highest energy.energy.

Features of the Aufbau Features of the Aufbau Diagram Diagram

Each box or circle represents an orbital.Each box or circle represents an orbital.All orbitals in the All orbitals in the same sublevelsame sublevel have have

equal energy values.equal energy values.Sublevels Sublevels within an energy levelwithin an energy level have have

different energies - in order of increasing different energies - in order of increasing energy are s, p, d, & f.energy are s, p, d, & f.

Orbitals of one energy level CAN overlap Orbitals of one energy level CAN overlap with orbitals of another energy level - 4s with orbitals of another energy level - 4s has a lower energy than any orbital of 3d.has a lower energy than any orbital of 3d.

Electron SpinElectron Spin

Electrons behave as though they were Electrons behave as though they were spinning on their own axis.spinning on their own axis.

The spin can be either: The spin can be either: Clockwise - represented by Clockwise - represented by Counterclockwise - represented by Counterclockwise - represented by

Rule #2Rule #2The Pauli The Pauli

Exclusion PrincipleExclusion Principle States that a States that a maximum of 2 maximum of 2 electrons may electrons may occupy a single occupy a single atomic orbital, but atomic orbital, but only if the only if the electrons have electrons have opposite spins.opposite spins.

Rule #3Rule #3Hund’s ruleHund’s rule states states

that single that single electrons with the electrons with the same spin must same spin must occupy occupy eacheach equal-energy equal-energy orbital orbital beforebefore additional additional electrons with electrons with opposite spins can opposite spins can occupy the same occupy the same orbital.orbital.

Representing Electron Representing Electron ConfigurationConfiguration

Aufbau or orbital diagrams can be used.Aufbau or orbital diagrams can be used.

Carbon

C 1s 2s 2p

Practice ProblemsPractice Problems

Use an orbital diagram to represent the Use an orbital diagram to represent the electron configurations for the following electron configurations for the following atoms:atoms:

1. Ge1. Ge

2. Mg2. Mg

3. Ti3. Ti

Representing Electron Representing Electron ConfigurationConfiguration

Electron configuration notation can be Electron configuration notation can be used.used.The energy level is written first.The energy level is written first.The sublevel is written next to the The sublevel is written next to the

energy level.energy level.A superscript is used to represent the A superscript is used to represent the

number of electrons in all the orbitals of number of electrons in all the orbitals of the sublevel.the sublevel.

For example, the electron configuration For example, the electron configuration notation for carbon is notation for carbon is 1s1s22 2s 2s22 2p 2p22..

Practice ProblemsPractice Problems

Use the orbital diagrams already done to Use the orbital diagrams already done to write the electron configuration for:write the electron configuration for:1.1. GeGe2.2. MgMg3.3. TiTi

Representing Electron Representing Electron ConfigurationConfiguration

Noble-gas notation can also be used.Noble-gas notation can also be used. A bracket around a noble gas symbol is used to A bracket around a noble gas symbol is used to

represent the inner level electrons.represent the inner level electrons. [He] represents [He] represents 1s1s22

[Ne] represents [Ne] represents 1s1s2 2 2s2s2 2 2p2p66

The remaining electrons are represented with The remaining electrons are represented with electron configuration notation.electron configuration notation.Carbon could then be written as Carbon could then be written as

[He] [He] 2s2s2 2 2p2p22

Sodium could be written as [Ne] Sodium could be written as [Ne] 3s3s11

Valence electronsValence electrons

Valence electrons are the electrons in an Valence electrons are the electrons in an atom’s outermost orbitals. In other words, atom’s outermost orbitals. In other words, they are they are the electrons in the highest principal the electrons in the highest principal energy levelenergy level..

Valence electrons Valence electrons determine the chemical determine the chemical propertiesproperties of an element. of an element.

They are easy to identify in electron They are easy to identify in electron configuration or nobleconfiguration or noble-gas notation: -gas notation: S [Ne] S [Ne] 3s3s2 2 3p3p44 or or 1s1s2 2 2s2s2 2 2p2p6 6 3s3s2 2 3p3p44

Sulfur has 6 valence electrons, identified as Sulfur has 6 valence electrons, identified as 3s3s2 2 3p3p4 4

Electron-dot (or Lewis) Electron-dot (or Lewis) StructuresStructures

Since valence electrons are involved in Since valence electrons are involved in bond formation, scientist use a visual bond formation, scientist use a visual shorthand to represent them.shorthand to represent them. The element’s symbol is written. It The element’s symbol is written. It

represents the nucleus and all inner-level represents the nucleus and all inner-level electrons of the atom.electrons of the atom.

Dots are drawn to represent the valence Dots are drawn to represent the valence electrons.electrons.

Proper placement of dots is important. They Proper placement of dots is important. They are placed 1 at a time on the 4 “sides” of the are placed 1 at a time on the 4 “sides” of the symbol and then they are paired up until all symbol and then they are paired up until all are used.are used.

Electron-dot (or Lewis) Electron-dot (or Lewis) StructuresStructures

Practice ProblemsPractice Problems

Draw the electron-dot structures for the Draw the electron-dot structures for the following:following:

1. Tin1. Tin

2. Bromine2. Bromine

3. Rubidium3. Rubidium