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TOPIC 1.5: ATOMIC STRUCTURE & ELECTRON CONFIGURATION
Enduring Understanding
SAP-1	Atoms and molecules can be identified by their electron distribution and energy.
Learning Objective
SAP-1A	Represent the electron configuration of an element or ions of an element using the Aufbau principle.

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Atoms

Atoms are made up from protons (positive), neutrons (neutral) and electrons (negative). The nucleus contains the protons and neutrons, while the electrons move around the nucleus.

The majority of the mass of the atom comes from the protons and neutrons, while most of the volume of an atom comes from the electrons.

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Electron Configuration

Electron Configurations are a way of describing the arrangement of electrons within an atom.

The inner electrons are called core electrons.

The outer electrons are called valence electrons.

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Energy Levels

The first shell is small and only has room for the 1s orbital.

The second shell is larger and there is room for a 2s orbital and a set of 3 2p orbitals which are dumb-bell shape.

The third shell is even larger and there is room for a 3s orbital, a set of 3p orbitals and a set of 5 3d orbitals which are double dumb-bell shape.

Energy Level	Possible Shapes (orbitals)	Number of Electrons
1	s	2
2	s p	8
3	s p d	18

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RULES FOR ELECTRON CONFIGURATIONS:

1. Aufbau principle: electrons are added to the lowest subshells first and build up.

2. Hund’s Rule: each subshell should have one electron before any are doubled up.

3. Pauli Exclusion Principle: no two electrons can have the same set of 4 quantum numbers.

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Writing Electron Configuration

(Shell#)(orbital name)^{# of e-}

Fluorine Example: 1s² 2s² 2p⁵

Writing Electron Configurations is more convenient than filling diagrams.

Follows Aufbau Principle

Hund’s Rule and Pauli Exclusion are not represented.

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Writing Electron Configurations

Determine how many electrons are in the atom. Iron has 26 electrons.

Arrange the energy sublevels according to increasing energy:

1s 2s 2p 3s 3p 4s 3d … (explained on next slide)

Fill each sublevel with electrons until you have used all the electrons in the atom:

Fe: 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d⁶

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Sublevel Increasing Energy

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Noble Gas Abbreviation/Shorthand Electron Configuration

An abbreviated form of electron configurations was developed using the final column of the periodic table, the noble gases.

Example: Scandium

Instead of Sc: 1s²2s²2p⁶3s²3p⁶4s²3d¹

We can abbreviate as [Ar]4s²3d¹

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Electromagnetic Forces

Chemistry is governed by the forces that exist between charged particles.

The strength of these electromagnetic forces are determined by two factors:

the amount of charge (q1 and q2)

F ∝ q1 x q2

the distance between charges (r)

F ∝ 1 / r²

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★ Coulomb’s Law ★

The force between charged particles is proportional to the product of the two charges and the force is inversely proportional to the squared radius between them.

The force will decrease the further away the particles are.

Higher charges and smaller distances between the charges result in a greater force of attraction. This explains why it takes more energy to remove electrons that are closest to the nucleus.

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Shielding

In addition to the distance, the electrons that are on the valence shell experience less of the nuclear pull because the electrons that are in the core of the atom shield the attraction of the nucleus from the valence electrons.

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Example 1

Write the ground state electron configuration for Arsenic.

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Example 2

Write the electron configuration for Calcium ion, Ca²⁺.

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TOPIC 1.6: PHOTOELECTRON SPECTROSCOPY
Enduring Understanding
SAP-1	Atoms and molecules can be identified by their electron distribution and energy.
Learning Objective
SAP-1B	Explain the relationship between the photoelectron spectrum of an atom or ion and: The electron configuration of the species. The interactions between the electrons and the nucleus.

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PES

Photoelectron spectroscopy (PES) is an experimental technique that measures the relative energies of electrons in atoms or molecules.

It works by ejecting electrons from the materials using high energy electromagnetic radiation (like UV or x-rays) and then measuring the kinetic energy of those electrons. This process can be described as photoionization.

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PES Graphs

PES graphs show the relative number of electrons and their corresponding binding energy.

The binding energy can be described as the amount of energy needed to remove an electron from an atom.

The electrons with the highest binding energy are the ones that have the greatest coulombic attraction to the nucleus because they are the closest to the nucleus.

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PES Graphs

A PES graph directly correspond to the electron configuration.

The graphs are often (but not always) set up so that the x axis gives the largest values first. The graphs are scaled so that they can show many orders of magnitude.

The highest value for the ionization energy (binding energy) will be the innermost electrons.

ALWAYS read the axis!

On this graph they are the peak on the left. There are 2 electrons in the 1s orbital so we can use the height of that peak to estimate the others. Often the graph is not labeled with the number of electrons in each peak.

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