Title: Lesson 2a Physical Properties Continued

Learning Objectives:

– Identify and explain the trends in the physical properties of the first 20 elements including:

• Atomic radius• Ionic radius• First ionisation energy• Electronegativity• Melting point

– Use Microsoft Excel to produce a spreadsheet to graph the above physical data

Solutions

Melting Point

• This is the temperature (in Kelvin…i.e. Celsius + 273) at which an element melts

• Values range over:– 3935 K for Carbon– 1 K for Helium

• Values are influenced by:– Nature of bonding: giant covalent, giant ionic, metallic– Strength of bonding– Strength of intermolecular forces

Melting Points• Example of group 1 and 17 trends:

• MP decreases down group 1: Metallic structure held by attractive forces between delocalised outer electrons and positively charge ions. Attraction decreases with distance.

• MP increases down group 17: Molecular structures held together more strongly as the number of electrons increase. (More on this in chapter 4)

Graph of Periodic Pattern of Melting Points

• Rise across period to maximum at group 14, then falls to a minimum at group 18.

Periodicy and the properties of elements in period 3

Na, Mg, Al, Si, P, S, Cl, ArNon metals, either accept electrons to form ionic bonds or share the outer electron to make a covalent compound

All metals, they have giants structures. They lose their outer electrons to form ionic compounds

Four electrons in the outer shell which form 4 covalent bonds. The element has some metallic properties and is classed as semi-metal

Noble Gas, full outer shell and unreactive

Plot of the melting and boiling points

Na, Mg and Al: melting and boiling pointsThe melting and boiling points increase for the three metallic

elements from sodium to aluminium.

This is because the strength of the metallic bonds increases. More energy is needed to break the stronger metallic bonds, so melting and boiling points are higher.

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Na Mg Alelement

melting point

boiling point

Na, Mg and Al: metallic bond strengthThe increase in metallic bond strength from sodium to aluminium is due to two factors:

1. Charge density. This is the ratio of an ion’s charge to its size. Na+ ions are large with a small charge, so have a low charge density. Al3+ ions are smaller with a larger charge, and so have a higher charge density. They are therefore more strongly attracted to the delocalized electrons.

2. Number of free electrons. Sodium has one free electron per metal ion, whereas aluminium has three. This leads to more attractions that must be broken in aluminium.

SiliconSilicon has a macromolecular structure similar to that of diamond.

Each silicon atom is bonded to four neighbouring silicon atoms by strong covalent bonds. These must be broken in order for silicon to melt. This requires a lot of energy, so silicon's melting and boiling points are high.

Period 3 non-metalsThe melting and boiling points of phosphorus, sulfur and chlorine are much lower than those of silicon.

Breaking these forces of attraction requires much less energy than breaking covalent bonds.

This is because they have asimple molecular structure with weak van der Waals forces holding the molecules together. 0

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Na Mg Al Si P S Cl Arelement

Period 3 non-metals: structure

Melting points in period 3

Trends in Physical Properties• You need to produce an Excel spreadsheet to help you analyse the

physical data.– Use the blank here and follow the instructions on the instructions page

• Once you have done this you need to use this to help you identify and explain the following trends:– Atomic and ionic radius, first ionisation energy, electronegativity and

melting point• Down group I (alkali metals)• Down group VII (halogens)

– Atomic and ionic radius, first ionisation energy, electronegativity• Across period 3

– The general trend in electronegativity over the whole PT

Key Points

• Each of the following physical parameters follow trends and patterns in the PT

• These patterns are generally explained by:– Charge in the nucleus– Number of electron shells– Electron shielding