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Chapter 5

Electrons In Atoms

5.1 Revising the Atomic Model

5.2 Electron Arrangement

in Atoms

5.3 Atomic Emission Spectra and

the Quantum Mechanical Model

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Energy and stability play an important role in determining how electrons are configured in an atom.

CHEMISTRY & YOU

What makes the electron configuration of an atom stable?

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Electron Configurations

What are the three rules for writing the electron configurations of elements?

Electron Configurations

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Electron Configurations

The ways in which electrons are arranged in various orbitals around the nuclei of atoms are called electron configurations.

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Electron Configurations

Three rules—the aufbau principle, the Pauli exclusion principle, and Hund’s rule—tell you how to find the electron configurations of atoms.

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Electron Configurations

Aufbau Principle

According to the aufbau principle, electrons occupy the orbitals of lowest energy first. In the aufbau diagram, each box represents an atomic orbital.

Increasing energy

6s

5s

4s

3s

2s

1s

6p

5p

5d

4p

4d

4f

3p

3d

2p

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Electron Configurations

Aufbau Principle

Increasing energy

6s

5s

4s

3s

2s

1s

6p

5p

5d

4p

4d

4f

3p

3d

2p

The aufbau diagram shows the relative energy levels of the various atomic orbitals. Orbitals of greater energy are higher on the diagram.

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Electron Configurations

Aufbau Principle

Increasing energy

6s

5s

4s

3s

2s

1s

6p

5p

5d

4p

4d

4f

3p

3d

2p

The range of energy levels within a principal energy level can overlap the energy levels of another principal level.

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Electron Configurations

Pauli Exclusion Principle

According to the Pauli exclusion principle, an atomic orbital may describe at most two electrons.

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Electron Configurations

Pauli Exclusion Principle

According to the Pauli exclusion principle, an atomic orbital may describe at most two electrons.
To occupy the same orbital, two electrons must have opposite spins; that is, the electron spins must be paired.

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Electron Configurations

Pauli Exclusion Principle

Spin is a quantum mechanical property of electrons and may be thought of as clockwise or counterclockwise.

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Electron Configurations

Pauli Exclusion Principle

Spin is a quantum mechanical property of electrons and may be thought of as clockwise or counterclockwise.
A vertical arrow indicates an electron and its direction of spin (↑ or ↓).

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Electron Configurations

Pauli Exclusion Principle

Spin is a quantum mechanical property of electrons and may be thought of as clockwise or counterclockwise.
A vertical arrow indicates an electron and its direction of spin (↑ or ↓).
An orbital containing paired electrons is written as .

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Electron Configurations

Hund’s Rule

According to Hund’s rule, electrons occupy orbitals of the same energy in a way that makes the number of electrons with the same spin direction as large as possible.

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Electron Configurations

Hund’s Rule

Three electrons would occupy three orbitals of equal energy as follows.

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Electron Configurations

Hund’s Rule

Three electrons would occupy three orbitals of equal energy as follows.

Electrons then occupy each orbital so that their spins are paired with the first electron in the orbital.

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Electron Configurations

Look at the orbital filling diagram of the oxygen atom.

Electron Configurations of Selected Elements
Element	1s	2s	2p_x 2p_y 2p_z	3s	Electron configuration
H					1s¹
He					1s²
Li					1s²2s¹
C					1s²2s²2p²
N					1s²2s²2p³
O					1s²2s²2p⁴
F					1s²2s²2p⁵
Ne					1s²2s²2p⁶
Na					1s²2s²2p⁶3s¹

An oxygen atom contains eight electrons.

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Electron Configurations

Look at the orbital filling diagram of the oxygen atom.

Electron Configurations of Selected Elements
Element	1s	2s	2p_x 2p_y 2p_z	3s	Electron configuration
H					1s¹
He					1s²
Li					1s²2s¹
C					1s²2s²2p²
N					1s²2s²2p³
O					1s²2s²2p⁴
F					1s²2s²2p⁵
Ne					1s²2s²2p⁶
Na					1s²2s²2p⁶3s¹

The 1s orbital has two electrons of opposite spin.

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Electron Configurations

Look at the orbital filling diagram of the oxygen atom.

Electron Configurations of Selected Elements
Element	1s	2s	2p_x 2p_y 2p_z	3s	Electron configuration
H					1s¹
He					1s²
Li					1s²2s¹
C					1s²2s²2p²
N					1s²2s²2p³
O					1s²2s²2p⁴
F					1s²2s²2p⁵
Ne					1s²2s²2p⁶
Na					1s²2s²2p⁶3s¹

The 1s orbital has two electrons of opposite spin.
The 2s orbital also has two electrons of opposite spin.

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Electron Configurations

Look at the orbital filling diagram of the oxygen atom.

Each of the three 2p orbitals has one electron. The remaining electron now pairs with an electron occupying one of the 2p orbitals.

Electron Configurations of Selected Elements
Element	1s	2s	2p_x 2p_y 2p_z	3s	Electron configuration
H					1s¹
He					1s²
Li					1s²2s¹
C					1s²2s²2p²
N					1s²2s²2p³
O					1s²2s²2p⁴
F					1s²2s²2p⁵
Ne					1s²2s²2p⁶
Na					1s²2s²2p⁶3s¹

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Electron Configurations

A convenient shorthand method for showing the electron configuration of an atom involves writing the energy level and the symbol for every sublevel occupied by an electron.

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Electron Configurations

A convenient shorthand method for showing the electron configuration of an atom involves writing the energy level and the symbol for every sublevel occupied by an electron.

You indicate the number of electrons occupying that sublevel with a superscript.

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Electron Configurations

For hydrogen, with one electron in a 1s orbital, the electron configuration is written 1s¹.

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Electron Configurations

For hydrogen, with one electron in a 1s orbital, the electron configuration is written 1s¹.

For oxygen, with two electrons in a 1s orbital, two electrons in a 2s orbital, and four electrons in 2p orbitals, the electron configuration is 1s²2s²2p⁴.

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Electron Configurations

For hydrogen, with one electron in a 1s orbital, the electron configuration is written 1s¹.

For oxygen, with two electrons in a 1s orbital, two electrons in a 2s orbital, and four electrons in 2p orbitals, the electron configuration is 1s²2s²2p⁴.

Note that the sum of the superscripts equals the number of electrons in the atom.

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CHEMISTRY & YOU

Explain why the correct electron configuration of oxygen is 1s²2s²2p⁴ and not 1s²2s²2p³3s¹.

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CHEMISTRY & YOU

Explain why the correct electron configuration of oxygen is 1s²2s²2p⁴ and not 1s²2s²2p³3s¹.

The 2p orbitals are lower in energy than the 3s orbital, so they will be completely filled before any electrons will be found in the 3s orbital.

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Sample Problem 5.1

Writing Electron Configurations

The atomic number of phosphorus is 15. Write the electron configuration of a phosphorus atom.

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Sample Problem 5.1

Analyze Identify the relevant concepts.

Phosphorus has 15 electrons. There is a maximum of two electrons per orbital. Electrons do not pair up within an energy sublevel (orbitals of equal energy) until each orbital already has one electron.

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When writing electron configurations, the sublevels within the same principal energy level are written together.

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Sample Problem 5.1

Solve Apply the concepts to this problem.

Use the aufbau diagram to place electrons in the orbital with the lowest energy (1s) first.

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1s

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Sample Problem 5.1

Solve Apply the concepts to this problem.

Use the aufbau diagram to place electrons in the orbital with the lowest energy (1s) first.
Continue placing electrons in each orbital with the next higher energy level.

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1s

2s

2p

3p

3s

4s

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Sample Problem 5.1

The electron configuration of phosphorus is 1s²2s²2p⁶3s²3p³.
The superscripts add up to the number of electrons.

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Solve Apply the concepts to this problem.

Write the electron configuration.

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Electron Configurations

Exceptional Electron Configurations

You can obtain correct electron configurations for the elements up to vanadium (atomic number 23) by following the aufbau diagram for orbital filling.

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Electron Configurations

Exceptional Electron Configurations

You can obtain correct electron configurations for the elements up to vanadium (atomic number 23) by following the aufbau diagram for orbital filling.
If you were to continue in that fashion, however, you would assign chromium and copper the following incorrect configurations.

Cr 1s²2s²2p⁶3s²3p⁶3d⁴4s²

Cu 1s²2s²2p⁶3s²3p⁶3d⁹4s²

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Electron Configurations

Exceptional Electron Configurations

The correct electron configurations are as follows:

Cr 1s²2s²2p⁶3s²3p⁶3d⁵4s¹

Cu 1s²2s²2p⁶3s²3p⁶3d¹⁰4s¹

These arrangements give chromium a half-filled d sublevel and copper a filled d sublevel.

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Electron Configurations

Exceptional Electron Configurations

Some actual electron configurations differ from those assigned using the aufbau principle because although half-filled sublevels are not as stable as filled sublevels, they are more stable than other configurations.

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What is the correct electron configuration of a sulfur atom?

A. 1s²2s²2p⁴3s²3p⁶

B. 1s²2s²2p⁶3s²3p³

C. 1s²2s²2p⁶3s²3p⁴

D. 1s²2s²2p⁶3s⁶3p²

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What is the correct electron configuration of a sulfur atom?

A. 1s²2s²2p⁴3s²3p⁶

B. 1s²2s²2p⁶3s²3p³

C. 1s²2s²2p⁶3s²3p⁴

D. 1s²2s²2p⁶3s⁶3p²

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Key Concept

Three rules—the aufbau principle, the Pauli exclusion principle, and Hund’s rule—tell you how to find the electron configurations of atoms.

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Glossary Terms

electron configuration: the arrangement of electrons of an atom in its ground state into various orbitals around the nuclei of atoms
aufbau principle: the rule that electrons occupy the orbitals of lowest energy first
Pauli exclusion principle: an atomic orbital may describe at most two electrons, each with opposite spin direction

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Glossary Terms

spin: a quantum mechanical property of electrons; it may be thought of as clockwise or counterclockwise
Hund’s rule: electrons occupy orbitals of the same energy in a way that makes the number of electrons with the same spin direction as large as possible

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END OF 5.2

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