1 Quantum Numbers and Electronic Configuration. F Scullion: www.justchemy.com

Each electron has a set of four numbers, called quantum numbers, that specify it completely; no two electrons in the same atom can have the same four. That is essentially the Pauli Exclusion Principle.

1. The Principal Quantum Number. Given the symbol “n”.

This denotes the energy level (shell).

This has integer values 1, 2, 3.

2. The Angular Momentum Quantum Number. Given the symbol “l ”

It denotes the number of sub-levels (orbitals) in each energy level and the shape of these orbitals.

The number of orbitals in any level = the number of the energy level. The number of electrons in any level is found by the formula:

No. of electrons = 2n2 where n is the principle quantum number (energy level)

Energy Maximum d Number of s p F level Number of orbital (Value of n) orbital orbital orbital Subshells Electrons Clover- Spherical Dumbell Complex = 2n2 leaf 1 1 2 √ 2 2 8 √ √ 3 3 18 √ √ √ 4 4 32 √ √ √ √

2 3. The Magnetic Quantum Number. Given the symbol “m”.

This defines the number of atomic orbitals making up a sublevel.

Sublevel Number of Atomic Orbitals s 1 ↑↓ p 3 ↑↓ ↑↓ ↑↓ d 5 ↑↓ ↑↓ ↑↓ ↑↓ ↑↓ f 7 ↑↓ ↑↓ ↑↓ ↑↓ ↑↓ ↑↓ ↑↓

It also describe their orientation in space.

s orbital

P orbital

px py pz

Each atomic orbital can hold 0, 1 or 2 electrons.

4. The Spin Quantum Number. Given the symbol “s”.

This describes the spin of the electron in the atomic orbital. There are just 2 possible spins: clockwise and counterclockwise.

↑ ↑ ↑ ↑ ↑ ↑

Sample p orbitals Parallel spins

↑↓ ↑ ↑ ↑↓ ↑↓ ↑ ↑↓ ↑↓ ↑↓

Hund's rule: every orbital in a subshell is singly occupied with one electron before any one orbital is doubly occupied, and all electrons in singly occupied orbitals have the same spin. 3 The Aufbau (building up) Principle for Electronic Configuration.

In the so-called “ground state” of an atom the electronic configuration generally follows this principle. According to this principle, electrons enter into states in order of the states increasing energy. Electrons are reluctant to pair up with another in the same atomic orbital (due to repulsions) and to go to a higher energy level than is necessary. When faced with the need to do one or other, they generally settle for pairing up with opposite spins, rather than go into the next highest energy level.

Complete for Oxygen (8) Complete for Sodium (11)

Exceptions to this rule include Chromium (4s1 3d5) and Copper (4s1 3d10) These result because it requires less energy to maintain an electron in a half-filled or filled d sublevel than paired in the s sublevel below. s p d and f block elements Assigned according to the oribtal getting the final electron. Colour in the chart below to show each of these blocks. 4 Complete the diagrams to show the normal electronic configurations of the elements indicated. Hydrogen (1) Helium (2) Lithium (3)

Beryllium (4) Boron (5) Carbon (6)

Nitrogen (7) Oxygen (8) Fluorine (9)

Neon (10) Sodium (11) Magnesium (12)

5 Aluminium (13) Silicon (14) Phosphorus (15)

Sulphur (16) Chlorine (17) Argon (18)

Potassium (19) Calcium (20) Scandium (21)

Chromium (24) Copper (29) Bromine (35)

6 A brief look at examples of hybridization. (This is beyond the syllabus, but helpful to know.)

Phosphorus (15)

Ground State Hybrid State Move one of the paired 3s electrons up to one of the empty 3d orbitals

There are 3 unpaired electrons giving a There are 5 unpaired electrons giving a valency of 3, which would explain the valency of 5, which would explain the existence PF3 for example. existence PF5 for example. The phosphorus is surrounded by 8 The phosphorus is surrounded by 10 electrons in this molecule. electrons in this molecule. This is known as a “completed octet”. This is known as an “expanded octet”.

Shape of PF5: Trigonal Bipyramid Bonding

7 Sulphur (16)

Ground State Hybrid State Move one of the paired 3s electrons up to one of the empty 3d orbitals. Do the same with one of the pair in

the 3px atomic orbital.

There are 2 unpaired electrons giving a There are 6 unpaired electrons giving a valency of 2, which would explain the valency of 6, which would explain the existence H2S for example. existence SF6 for example. The sulphur is surrounded by 8 electrons The phosphorus is surrounded by 12 in this molecule. electrons in this molecule. This is known as a “completed octet”. This is known as an “expanded octet”.

Shape of SF6: Octahedral Bonding

8 Now you are ready to move on to the next topic:

The Origin of Atomic Spectra - Atomic Absorption and Emission.

When aquainted with the circumstances in which electrons can jump up and down between energy levels in an atom, you should be able to understand the origin of Atomic Spectra.

An atomic line spectrum.

The electrons jumps that we will learn about – The “Balmer Series” in particular

F Scullion. www.justchemy.com