Classifying Elements Using Conductivity and Oxides

Explained

Telling metals from non metals by what they do

You can classify an element without knowing its name, using two pieces of evidence: whether it conducts electricity, and whether its oxide behaves as an acid or a base. Both tests come back to structure, which is why they agree with each other.

Conductivity, and the reason behind it

Metals conduct electricity as solids because they contain delocalised electrons that are free to move through the structure. A current is moving charge, and in a metal those electrons are the moving charge.

Most non metals do not have mobile charged particles when solid, so they do not conduct. Saying that a metal conducts "because it is a metal" gets nothing. The mark is for the delocalised electrons.

The exception you are expected to know

Graphite is a non metal that conducts. It is a giant covalent form of carbon made of layers, and each carbon atom bonds to three others rather than four. That leaves one electron per atom delocalised, free to move along the layers.

Graphite is worth learning properly because it is the standard way for an examiner to test whether you understand the reason for conductivity or have only memorised the rule.

Oxides: the second line of evidence

Metal oxides are basic. If they dissolve, they form alkaline solutions with a pH above 7. Non metal oxides are acidic, forming solutions with a pH below 7. Carbon dioxide and sulfur dioxide dissolving in rainwater is why unpolluted rain is already slightly acidic.

So an unknown element that conducts electricity and forms an oxide giving a pH of 10 is a metal, on two separate pieces of evidence. Questions often give you both and expect you to use both.

Answering a classification question

State the evidence, then state what it shows, then give the conclusion. The element conducts electricity, which indicates delocalised electrons, so it is a metal. Its oxide gives an alkaline solution, which indicates a basic oxide, which agrees.

Where the two pieces of evidence disagree, the question is usually pointing at graphite or at a metalloid, and saying so is the answer rather than a problem.