Chapter 1: Where are the Electrons ?

Atomic Orbitals

The electron is the subatomic particle that is fundamental to chemical bonding. For atoms, the electrons are associated with atomic orbitals and it is convienient to think of finding the electrons in the atomic orbitals. But what about electrons in molecules ? Before we can talk about that, we should make sure we understand where they are in atoms. As we will see later, the important principles that apply to atomic orbitals also apply to molecular orbitals.

Electrons were discovered in 1897 by J.J. Thompson. They are often thought of as particles. But in 1924, De Broglie suggested that electrons also have wave like properties. This is the concept of "wave-particle duality".

QUESTION : Can you think of examples of particle type and wave type properties ? ANSWER

In 1926, Schrodinger was able to show that electrons in a hydrogen atom could be described by a mathematical wave function, Y.
Electrons are often described as being in orbitals around an atom that are mathematical "constructs" based on the wave function, Y, that describes the motion of an electron.

An orbital is, more correctly, a mathematical function, 4pr²Y², that describes the region of high probability in 3D space, around a nucleus, where an electron may be found. Orbitals are commonly represented by the "boundary" surfaces that encloses the region where there is a 90-95 % probability of finding the electron.

In organic chemistry which is focussed on carbon, one needs to be most familiar with the s- and p-type orbitals.  These properties of these orbitals can be described by using quantum numbers.  Quantum numbers arise from the wave functions and quantum mechanics.
The orbitals for the electrons in carbon, and the necessary quantum numbers are given below.  The nucleus of the carbon atom would reside at the centre of the x,y,z coordinate.
 

You should be familiar with the meaning of quantum numbers, and how to describe an orbital using its quantum numbers or what orbital a set of quantum numbers defines.
Try some questions.  If you are struggling you'll need to review the meaning of quantum numbers.

The electron configuration of an atom describes how the electrons of an atom are arranged.

e.g.  electron configuration of carbon
 

simple	1s2 2s2 2p2
short form	[He] 2s2 2p2
orbital energy diagram

You should be familiar with how to determine an electron configuration for an atom and identify the valence electrons.  You should be able to identify both ground and excited state electron configurations.

You should now  try some questions.  If you are still having trouble, you should review how to determine electron configurations.

© Dr. Ian Hunt, Department of Chemistry