Electronic Structure & Electron Configuration

Electronic Structure of Matter

Introduction to Electron Configuration

• Recall of the periodic table of elements and its blocks:
• s-block
• p-block
• d-block
• f-block

Electron Configuration

• Describes the arrangement of electrons in an atom

Bohr Model vs Cloud Model

• The Bohr Model treats electron energy levels as clearly defined orbital paths around the nucleus, similar to planets orbiting the sun.
• This model is considered inaccurate according to modern atomic theory.
• The Cloud Model treats electron energy levels as probability clouds.
• These are regions in which electrons are likely to be found.

Energy Level

• Also called a "shell".
• Represents the 3-dimensional space surrounding the nucleus where electrons are most likely to be.
• The first energy level (n=1) is closest to the nucleus and has the lowest energy.
• Each period on the periodic table represents a new electron energy level.
• Period 1 has 1 energy level.
• Period 2 has 2 energy levels.
• Period 3 has 3 energy levels.
• Each energy level can only occupy a limited number of electrons.

Sublevels

• Includes s, p, d, and f orbitals.
• Each orbital can contain different total numbers of electrons.
• s-orbital (sharp)
• p-orbital (principal)
• d-orbital (diffuse)
• f-orbital (fundamental)
• Each orbital can contain different total numbers of electrons:
• f can contain 14 electrons
• d can contain 10 electrons
• p can contain 6 electrons
• s can contain 2 electrons
• Each box (orbital) can occupy two electrons.
• Numbers before the orbitals indicate their energy level.
• The '2p' orbital is p-shaped, located in the 2nd energy level, and can hold up to six electrons.

Observing Electron Configurations

• Hydrogen (1) has an electron configuration of 1s¹.
• Helium (2) has an electron configuration of 1s².
• Lithium (3) has an electron configuration of 1s² 2s¹.
• Superscripts in each electron configuration represent the number of electrons in an atom

Concept Check

• Given the electron configuration 1s² 2s² 2p⁶ 3s² 3p⁵:
• The numbers written in red represent the numbers of electrons.
• The "3" has the highest energy.
• The letters written in purple represents orbitals
• The name of the element with that electron configuration Fluorine

Writing Electron Configurations

Notation

• The notation includes:
• Energy level
• Type of orbital
• Number of electrons in the orbital

Rules for Filling Orbitals

• Aufbau Principle: dictates that electrons will occupy the orbitals having lower energies before occupying higher energy orbitals.
• Pauli Exclusion Principle: a maximum of two electrons, each having opposite spins (spin up and spin down), can fit in an orbital.
• Hund's Rule: every orbital in a given subshell is singly occupied by electrons before a second electron occupies it.

Noble Gas Notation

• Valence Electrons: electrons in the outermost shell orbitals.
• Core electrons: electrons occupying the inner shells.
• Sodium (Na) which has an electron configuration of 1s²2s²2p⁶3s¹ with an abbreviation of [Ne]3s¹.
• Neon (Ne) electron configuration is 1s²2s²2p⁶.
• Aluminum has the electron configuration 1s²2s²2p⁶3s²3p¹ and becomes [Ne]3s²3p¹.
• Argon electron configuration is 1s²2s²2p⁶3s²3p⁶.
• Calcium electron configuration is 1s²2s²2p⁶3s²3p⁶4s² and becomes [Ar]4s².

Example

• To set up an orbital diagram for Oxygen and write its complete electron configuration and noble gas notation you would get: O 1s² 2s² 2p⁴ [He] 2s² 2p⁴

Consept Check

• To set up the orbital diagram for Titanium and write its complete electron configuration and noble gas notation:
• The Atomic number is 22 with 22 electrons, also in 3d-block Location: 3d-block, the resulting configuration:
• configuration: 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d²
• The Noble Gas Notation is: [Ar] 4s² 3d²

Rules in Filling of Orbitals

• Recap of the Aufbau Principle: dictates that electrons will occupy the orbitals having lower energies before occupying higher energy orbitals.

Remember

Key things to remember are:

• Proper notation of the standard electron configuration
• Orbital diagrams (following Aufbau Principle, Pauli Exclusion Principle, and Hund's Rule)
• Noble gas notation ([noble gas] rest of the e.c.)
• The order of the orbital subshells
• spdf blocks in the periodic table
• Elements that do not follow Aufbau Principle (Cr, Mo, Cu, Ag, Au)

Quantum Numbers

• A set of 4 numbers used to describe the electrons in terms of their:
• Distance from the nucleus
• Shape of the orbitals
• Orientation in space
• Direction of electron spin

1st Quantum Number (n)

• It is the main energy level occupied by an electron.
• values are all positive whole # integers (1,2,3,4...).
• As it increases, the electron:
• Has more energy
• Is farther from nucleus

2nd Quantum Number (l)

• The shape of the orbital (sublevel)
• The number of possible shapes = the energy level
• The 4th energy level has 4 possible shapes
• possible values: 0 to n-1
• s: 0
• p: 1
• d: 2
• f: 3

3rd Quantum Number (ml)

• Indicates the position of an orbital around the nucleus (x, y, or z direction).
• It has values from +l to -l.
• Each orbital hold a maximum of 2 electrons.
• For certain orbitals each hold Quantum Numbers:
• s with 0
• p with -1, 0- +1
• d with -2, -1, 0, +1, +2
• f subshell with -3, -2, -1, 0, +1, +2, +3

4th Quantum Number (ms)

• Indicates the spin state of the electron
• There are only 2 possible directions
• And only 2 possible values: +1/2 and -1/2
• Paired electrons must have opposite spins

Explanation of electron configuration, Bohr model, cloud model and energy levels. Each period on the periodic table represents a new electron energy level. The first energy level is closest to the nucleus and has the lowest energy.