The Aufbau Principle (College Board AP® Chemistry)
Study Guide
Written by: Martín
Reviewed by: Stewart Hird
The Aufbau Principle
The Aufbau Principle helps us understand how electrons are arranged in atoms
It states that electrons fill the lowest energy orbitals first before moving to higher energy ones
Electrons follow the order of increasing energy
The order of filling the subshells is determined by the arrows in Aufbau Diagram
The Aufbau Diagram
Electrons fill the lowest energy orbitals first before moving to higher energy ones
Writing Electron Configurations
Electron configuration is a way to represent the arrangement of electrons in an atom's orbitals
Electrons occupy the lowest energy orbitals available following the Aufbau Diagram
Each orbital can hold up to two electrons
Electrons are represented as exponents
A condensed electron configuration uses the closest previous noble gas to describe the electron arrangement
Worked Example
Write the full electron configuration and the condensed electron configuration for fluorine (F)
Answer:
Step 1: Identify the element
Fluorine has an atomic number of 9, which represents the number of protons. Since F is a neutral atom, the number of electrons is the same as the number of protons
Step 2: Use the Aufbau Diagram to distribute the 9 electrons the electrons
Start by filling the orbitals with electrons following the Aufbau Diagram
The first two electrons will go into the 1s subshell
1s²Next, move to the 2s subshell and add 2 electrons more
2s²Finally, proceed to the 2p subshell. Since there are three p orbitals, the max number of electrons is 6. Considering there are just 5 electrons remaining,
2p5
Step 3: Combine the individual electron configurations and interpret the information
1s² 2s² 2p5
This tells us that fluorine has 9 electrons arranged as follows:
2 electrons in the 1s subshell
2 electrons in the 2s subshell
5 electrons in the 2p subshell
This also implies, that fluorine has 2 core electrons and 7 valence electrons
Step 4: Write down the condensed electron configuration using the closest previous noble gas
The closest previous noble gas is helium (He)
The electron configuration for He is 1s²
Therefore, the condensed electron configuration for F is,
[He] 2s² 2p5
Electron configuration and the Periodic Table
Elements in the same column (group) of the periodic table have similar electron configurations in their valence shell
This similarity explains why elements in the same group exhibit similar chemical properties
The periodic table is organized in a way that allows you to predict the electron configuration of an element based on its position in the table
The Periodic Table
Electron configuration in the periodic table
Orbital diagrams
Orbital diagrams are a visual representation of electron configurations
Boxes are used to represent orbitals
Arrows are used to represent electrons with their spins indicated as up (↑) or down (↓)
Each orbital can hold a maximum of two electrons with opposite spins
Electrons will fill orbitals with the same energy one at a time before pairing up with an electron with opposite spin
g. the orbital diagram for the atom of fluorine (F) is shown below
Note that an orbital is filled first with (↑) and then paired with (↓)
Electrons occupy first the lowest energy subshells
Representing the electron configuration in a fluorine atom
Orbital diagram for the electron configuration of the fluorine (F) atom
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