Metals are commonly being used in our everyday lives.
For example, silver and gold are commonly used as jewellery because of their shiny appearance. Copper is widely used in metal wires and cables. Titanium and zirconium are also commonly used for making aircraft and space shuttle parts.
Pure Metals
In general, metals have the following physical properties:
- Usually have high densities
- Malleable and ductile
- Usually have high melting points and boiling points i.e. exists as solids at room temperature
- Good conductors of electricity (and heat)
These physical properties can be easily explained by looking at the structures of metal.
Physical Properties of Metals
1) Usually have high densities
Atoms in a metal are packed tightly in layers and are held together by strong metallic bonds. As such, there is large number of atoms per unit volume i.e. high densities.
2) Malleable and ductile
Malleability refers to the ability to be beaten or hammered into shapes without breaking. Ductility refers to the ability to be stretched or drawn into wires without breaking. In pure metals, same size atoms are packed closely together in a regular pattern. When a force is applied to a metal, the layers of metal atoms can slide past each other. As such, pure metals are known to be soft, malleable and ductile.
3) Usually have high melting points and boiling points
In the metal lattice, the atoms lose their valence (outermost shell) electrons and become positively charged metal ions. The valence electrons are said to be delocalised out into the empty spaces (‘sea’). Strong electrostatic forces of attraction exists between the positively charged metal ions and the negatively charged electrons. Large amount of energy is required to overcome these forces.
4) Good conductors of electricity (and heat)
The valence (outermost shell) electrons delocalised into the empty spaces (‘sea’) and are able to move freely within the metal lattice. These ‘sea’ of delocalised electrons are mobile
(free moving) and act as charge carriers which help to conduct electricity and heat.
Pure metals have many useful properties but they are not widely used because of two main reasons:
- Pure metals are too soft and malleable
- Pure metals may react with air and water i.e. corrode easily
As such, we often use alloys instead of the pure metals.
Alloys
An alloy is defined as a mixture of a metal with at least one other element. The element(s) added can be either a metal or a non-metal.
4 Main Reasons Why Metals are used as Alloys:
- To make metals stronger and harder
- To make metals more resistant to corrosion
- To improve the appearance of metals
- To lower the melting points of metals
1) To make metals stronger and harder
The most common question that students will be asked in their Chemistry tests and examinations is how does alloying increase the strength and hardness of a pure metal.
First of all, in an alloy, the atoms of the main metal and the element(s) added have different sizes. This disrupts the regular arrangement of the atoms in the pure metal. Atoms of different sizes cannot slide past each other easily when a force is applied. As such, an alloy is stronger, harder and less malleable than the pure metal itself.
For the purpose of GCE O-Level Pure Chemistry and IP Chemistry examinations, students would need to know the following examples of alloys:
- Brass = Copper + Zinc
- Bronze = Copper + Tin
- Steel = Iron + Carbon
- Stainless Steel = Iron + Carbon + Nickel + Chromium
Let’s take a look at a potential question which you might be tested in the Chemistry examination. This is modified from a question in Paper 2 (Written Paper) of the Nov 2011 GCE O-Level Pure Chemistry Examination in Singapore.
Question:
Uranium is a chemical element with the symbol U and atomic number 92. It is a silvery-grey metal in the actinoids series of the periodic table.
(i) Describe the bonding in uranium. [2]
Answer: Strong electrostatic forces of attraction between the positively uranium ions and the ‘sea’ of delocalised electrons in the giant metallic lattice structure.
(ii) Explain what happens when an electric current passes through uranium. [1]
Answer: The ‘sea’ of delocalised valence electrons in uranium are free moving and they act as charge carriers and move throughout the metallic structure.
Source: Paper 2, Nov 2011 GCE O-Level Pure Chemistry Examination
2) To make metals more resistant to corrosion
Coins that we used in our everyday lives are made of cupronickel, which is an alloy of copper and nickel. It does not corrode easily.
3) To improve the appearance of of metals
Pewter is an alloy of tin, antimony and copper. It is commonly used to make ornaments such as key chains and decorative displays because it looks more beautiful than pure tin itself.
4) To lower the melting points of metals
The main method used in the industry to join two metal parts together is through the use of soldering. The solder material is usually an alloy of tin and lead, and it has a lower melting point than most metals.
Can you draw simple diagrams to show the difference in the arrangement of atoms between copper and bronze?
YouTube Video Tutorial on Physical Properties of Pure Metals and Alloys
You can watch the YouTube Video below to have an even clearer idea about the properties of pure metals and the reasons why metals are often used in the form of alloys .
Click on the following link for the video on O-Level Chemistry . IP Chemistry: Physical Properties of Pure Metals and Alloys
Length of Video: 12.32 minutes
Before we end this blog post, a quick food-for-thought exam-based question for you to ponder:
Q) Can you draw simple diagrams to show the difference in the arrangement of atoms between copper and bronze?
Source: Possible exam question in O-Level Pure Chemistry and IP Chemistry
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PS: In the next blog post, i will share with you on the Reactivity Series of Metals. Do look out for it. Also, under related articles below, there are several blog post discussions and questions related to Metals. You can also do a keyword search using the search box at the top right hand corner.
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