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

WJEC Chemistry for AS Level

For convenience sake we represent 1s 2 2s 2 2p 6 3s 2 3p 6 as [Ar] rather than write it out each time, e.g. the electronic configuration of manganese, atomic number 25, can be written as [Ar] 3d 5 4s 2 . A convenient way of representing electronic configuration is using ‘electrons in boxes’. Each orbital is represented as a box and the electrons are shown as arrows with their clockwise or anticlockwise spins as ↑ or ↓ . Here are the ‘electrons in boxes’ notation and shorter form of electronic structure for the first ten elements.

Knowledge check

7

(a) Use electrons in boxes to write the electronic configuration of: (i) an atom of phosphorus, P. (ii) a magnesium ion, Mg 2+ . (b) Write the electronic configuration in terms of subshells for a chromium atom.

Element

Electronic configuration

Electrons in boxes

1s � �

2s

2p

8 Knowledge check State the number of different orbitals in the third quantum shell.

H 1s 1

� � � �

He

1s 2

Li

1s 2

2s 1

2p 1 � � � 2p 2 � � � � 2p 3 � � � � � 2p 4 � � � � � 2p 5 � � � � � 2p 6 � � � � �

Be

1s 2

2s 2

B 1s 2

2s 2

C

1s 2

2s 2

N 1s 2

2s 2

O 1s 2

2s 2

F

1s 2

2s 2

Link

Ne

1s 2

2s 2

Structure of periodic table page 63

▲ Table of electronic configuration

The electronic configuration of ions is presented in the same way as that of atoms. Positive ions form by the loss of electrons from the highest energy orbitals so these ions have fewer electrons than the parent atom. Negative ions form by adding electrons to the highest energy orbitals so these ions have more electrons than the parent atom, e.g. Na 1s 2 2s 2 2p 6 3s 1 Na + 1s 2 2s 2 2p 6 Cl 1s 2 2s 2 2p 6 3s 2 3p 5 Cl − 1s 2 2s 2 2p 6 3s 2 3p 6 Ionisation energies The process of removing electrons from an atom is called ionisation. The energy needed to remove each successive electron from an atom is called the first, second, third, etc., ionisation energy. The process for the first ionisation energy (IE) of an element is summarised in the equation: X(g) →  X + (g) + e − DRAFT Key term The first ionisation energy of an element is the energy required to remove one electron from each atom in one mole of its gaseous atoms. 24