WJEC Chemistry for AS Level Student Book: 2nd Edition (Draft)

 1.2 Basic ideas about atoms

There are three different p orbitals (dumbell shaped lobes) known as the p x , p y and p z orbitals. They are at right angles to each other. These are represented as

z

z

z

y

y

y

x

x

x

p x

p y

p z

▲ Representation of p orbitals

There are five different d orbitals and seven different f orbitals. Therefore:

▪ an s subshell can hold 2 electrons ▪ a p subshell can hold 6 electrons ▪ a d subshell can hold 10 electrons ▪ an f subshell can hold 14 electrons.

Filling shells and orbitals with electrons The way in which an atom’s electrons

are arranged in its atomic orbitals is called electronic structure or configuration. The electronic structure can be worked out using three basic rules: 1. Electrons fill atomic orbitals in order of increasing energy (Aufbau principle). 2. A maximum of two electrons can occupy any orbital each with opposite spins (Pauli exclusion principle). 3. The orbitals will first fill with one

6 s

5 p

5 s

4 p

4 s

4 d

3 p

3 s

3 d

Energy

2 p

2 s

electron each with parallel spins, before a second electron is added with the paired spin (Hund’s rule). The order of filling is shown in the diagram on the right. An expected order is followed up to the 3p subshell, but then there is a variation, as the 4s subshell is filled before the 3d. The most common way of representing the electronic configuration of an atom is to write the occupied subshells in order of increasing energy with the number of electrons following as a superscript, e.g. nitrogen has two electrons in the 1s orbital, two electrons in the 2s orbital and three electrons in the 2p subshell so it is shown as 1s 2 2s 2 2p 3 . ▲ The order of electron filling 23 DRAFT 1 s Key term Electronic configuration is the arrangement of electrons in an atom. Stretch & challenge The reason for the 4s orbitals filling before the 3d orbitals is due to increasingly complex influences of nuclear attractions and electron repulsions upon individual electrons.