In This Topic You Will Cover the Following Learning Outcomes

Foundation Chemistry – Inorganic/ Periodicity

Periodicity

In this topic you will cover the following learning outcomes:

By the end of this topic you will be able to:-

• Know and understand the atomic properties of elements in periods 2 and 3 (including trends in first ionisation energy, atomic radius and electronegativity).
• Understand the trends in melting and boiling points across periods 2 and 3.
• Understand the trend in electrical conductivity across periods 2 and 3.

Review of the atomic trends of elements in period 3

You are provided with a fact sheet on each of these three topics:

1)Trends in electronegativity

2)Trends in atomic radius

3)Trends in first ionisation energy

In your group choose one of the topics and produce a poster explaining the main points and key trends – try to keep it as simple as possible! You will then need to explain your poster and what you have learnt to the rest of the class.

Trends in melting and boiling points of Period 3 elements

The trends in melting points and boiling points going across Period 3 are not straightforward!

• Melting points generally increasegoing from sodium to silicon, then decrease going to argon (with a “bump” at sulphur).

• Boiling points generally increase going from sodium to aluminium, then decrease to argon (again with a “bump” at sulphur).

Table of physical data

Graph of physical data

Why are the values on the graph plotted in Kelvin (K) rather than oC?

Explanation of these trends

Melting

When a substance melts, some of the attractive forces holding the particles together are broken or loosened so that the particles can move freely around each other but are still close together. The stronger these forces are, the more energy is needed to overcome them and the higher the melting temperature.

Boiling

When a substance boils, most of the remaining attractive forces are broken so the particles can move freely and far apart. The stronger the attractive forces are, the more energy is needed to overcome them and the higher the boiling temperature.

Sodium, magnesium and aluminium

Sodium, magnesium and aluminium are all metals. They have metallic bonding, in which positive metal ions are attracted to delocalised electrons. Going from sodium to aluminium:



Silicon

Silicon is a ______(an element with some of the properties of metals and some of the properties of non-metals). Silicon has ______. It has a giant lattice structure similar to that of diamond, in which each silicon atom is covalently-bonded to four other silicon atoms in a tetrahedral arrangement. This extends in three dimensions to form a giant molecule or macromolecule.

Silicon has a very melting point and boiling point because:

• ______

• ______

Phosphorus, sulphur, chlorine and argon

Phosphorus, sulphur, chlorine and argon are simple molecular substances with only ______attractions between the molecules. Their melting or boiling points will be ______than those of the first four members of the period which have ______structures.

The sizes of the melting and boiling points are governed entirely by the ______of the molecules. Remember the structures of the molecules:

Phosphorus

Phosphorus contains P4 molecules. To melt phosphorus you don't have to break any covalent bonds - just the much weaker van der Waals forces between the molecules.

Sulphur

Sulphur consists of S8 rings of atoms. The molecules are bigger than phosphorus molecules, and so the van der Waals attractions will be stronger, leading to a higher melting and boiling point.

Chlorine

Chlorine, Cl2, is a much smaller molecule with comparatively weak van der Waals attractions, and so chlorine will have a lower melting and boiling point than sulphur or phosphorus.

Argon

Argon molecules are just single argon atoms, Ar. The scope for van der Waals attractions between these is very limited and so the melting and boiling points of argon are lower again.

• ______

______

• ______

______

Trends in electrical conductivity of Period 3 elements

Electrical conductivity increases going across Period 3 from sodium to aluminium, then decreases to silicon. The remaining elements have negligible conductivity.

Table of physical data

Explanation of these trends

For an element to conduct electricity, it must contain electrons that are free to move. In general, metals are good conductors of electricity and non-metals are poor conductors of electricity.

Sodium, magnesium and aluminium

Sodium, magnesium and aluminium are all metals. They have metallic bonding, in which positive metal ions are attracted to delocalised electrons. The delocalised electrons are free to move and carry charge.

Going from sodium to aluminium:

• ______

• ______

• ______

Silicon

Silicon is a metalloid (an element with some of the properties of metals and some of the properties of non-metals). Silicon has giant covalent bonding. It has a giant lattice structure similar to that of diamond, in which each silicon atom is covalently-bonded to four other silicon atoms in a tetrahedral arrangement. This extends in three dimensions to form a giant molecule or macromolecule.

Silicon is called a semiconductor because it is able to conduct electricity at high temperatures but not at room temperatures as its electrons are promoted to higher energy levels and are free to move around.

Non-metals

The remaining elements in Period 3 do not conduct electricity:

• In phosphorus, sulphur and chlorine______

______

• In argon (which exists as single atoms) ______

______

Foundation Chemistry/Periodicity/JN (2008)