Electron Configurations

The Bohr Model Electrons are in fixed orbits. Orbits are associated with very specific energies, and are called energy levels.

The Quantum Mechanical Model Like Bohr Model, QMM restricts energies of electrons to certain values. Unlike Bohr Model, QMM does not define an exact path electron takes around the nucleus. According the QMM, the exact location of an electron cannot be exactly determined at a given time. Estimates probability of finding an electron in a certain position. Regions in which electrons are going to be found are called Orbitals

Electron Configurations—The Basics Chemists describe the placement of electrons based on principal energy levels n=1,2,3,4, etc. Within these energy levels, there are sublevels labeled s, p, d, and f. Within these sublevels are orbitals Every orbital can hold 2 electrons.

Principal energy levels (shells) Principal energy levels are assigned values in order of increasing energy n=1, 2, 3, 4, … What do the numbers tells us? Distance from the nucleus increases with increasing n. Energy increases with an increasing n.

Sublevels (subshells) Within each principal energy level, electrons occupy energy sublevels Sublevels denoted by the letters s, p, d, f In terms of energy, s < p < d < f What do the letters tell us? Shape of orbitals s = sphere, p= dumbbell, d = clover leaf

Orbitals Each sublevel contains a certain number of orbitals Orbital = region in which electrons are likely to be found What else do the letters tell us? s = 1 orbital p = 3 orbitals d = 5 orbitals f = 7 orbitals

Electrons Each orbital can only contain 2 electrons In order for electrons to occupy the same orbital, they must have opposite spin

Orbitals S sublevel 1 orbital

Orbitals S sublevel 1 orbital 2 e-

Orbitals P sublevel 3 orbitals

Orbitals P sublevel 3 orbitals 6 total e- 2 in each orbital

Orbitals d sublevel 5 orbitals

Orbitals d sublevel 5 orbitals 10 total e- 2 in each orbital

Summary Energy Levels Sublevel # of Orbitals # of Electrons 1 1s 2 2s 2p 3 6 3s 3p 3d 5 10 4 4s 4p 4d 4f 7 14

Summary 8 32 Energy Levels Sublevel # of Orbitals # of Electrons 1 1s 2p 3 6 8 3s 3p 3d 5 10 18 4 4s 4p 4d 4f 7 14 32

Summary What is the relationship between energy level and number of sublevels?

Summary Energy Level # = # of sublevels What is the relationship between energy level and number of sublevels? Energy Level # = # of sublevels

Let’s put it together!

Let’s put it together! It was mentioned earlier that increasing n = increasing energy Not always the case. Notice 3d is higher energy than 4s. This is not the only time when that happens!

Writing electron configurations in orbital notation RULES Aufbau Principle: Electrons fill the lowest possible energy levels first. Pauli Exclusion Principle: Two electrons in the same orbital have opposite spins. Hund’s Rule: Electrons fill in separate orbitals of the same subshell (sublevel) with parallel spins before pairing.

Writing electron configurations in orbital notation Examples: Carbon Argon: Vanadium:

Writing electron configurations in superscript notation: See Periodic Table Use Aufbau Principle: Fill lowest energy levels first

Writing electron configurations in superscript notation: Ex: Carbon Argon Vanadium

Writing electron configurations in superscript notation: Ex: Carbon: 6e- 1s22s22p2 Argon: 18e- 1s22s22p6 3s23p6 Vanadium: 23e- 1s22s22p6 3s23p64s23d3

Kernel configurations: To shorten, place previous noble gas (8A) in brackets and continue writing configuration. Ex: Vanadium: 23 e- [Ar] 4s23d3 Bromine: Al:

Kernel configurations: To shorten, place previous noble gas (8A) in brackets and continue writing configuration. Ex: Vanadium: 23 e- [Ar] 4s23d3 Bromine: 35 e- [Ar] 4s23d104p5 Al: 13 e- [Ne] 3s23p1

Valence shell/valence electrons The outermost principal energy level of an atom that includes at least one electron is called the valence shell. The electrons in the valence shell are called valence electrons. Valence electrons determine properties and reactivity or elements. Very important!

Valence shell/valence electrons How many valence electrons in examples above? Carbon: Argon:   Vanadium:

Valence shell/valence electrons How many valence electrons in examples above? Carbon: 1s22s22p2 = 4 v.e. Argon: 1s22s22p63s23p6 = 8 v.e.   Vanadium: 1s22s22p63s23p64s23d3 = 2 v.e.

Electron Configurations: Special Cases Writing electron configurations for ions For cations (+ charge) subtract electrons For anions (- charge) add electrons Ex:   Na+ 11 – 1 = 10 e- 1s22s22p6 N3- 7 + 3 = 10 e- 1s22s22p6 *Same as Neon. These are isoelectronic = Same e- configuration*

Exceptional configurations: For d orbitals only The atom will be at lowest energy (most stable) when the d sublevel is half full or full. Chromium and Copper Cr, Cu

Exceptional configurations: Chromium and Copper Cr, Cu Chromium expected configuration: 1s22s22p63s23p64s23d4 Actual Cr Configuration: 1s22s22p63s23p64s13d5 ← HALF FULL Copper expected configuration: 1s22s22p63s23p64s23d9 Actual Cu Configuration: 1s22s22p63s23p64s13d10 ← FULL