DP Chemistry: A.1 Introduction to materials science

A.1 Introduction to materials science

Written specifically for students to provide help and support for the IB Diploma chemistry programme this page provides full coverage of the syllabus content of Option A - sub topic A.1. It encourages you to think critically and provides many questions with full worked answers so that you can monitor and improve your knowledge and understanding. 

Learning outcomes

After studying this topic you should be able to:

Understand:

  • The classification of materials can be based on their uses, properties, or on their bonding and structure.
  • The properties of a particular material can be deduced from its position on a bonding triangle which is based on the degree of covalent, ionic or metallic character in a compound
  • Mixtures in which materials are composed of two distinct phases, a reinforcing phase that is embedded in a matrix phase, are known as composites.

Apply your knowledge to:

  • Use bond triangle diagrams for binary compounds from electronegativity data.
  • Evaluate various ways of classifying materials.
  • Relate physical characteristics of a material (e.g. melting point, permeability, conductivity, elasticity and brittleness) to its bonding and structures (packing arrangements, electron mobility and ability of atoms to slide relative to one another).

Relationships & vocabulary

Nature of science

In the past some materials were used for different purposes than they are today. This is due to improvements in technology and the increasing scientific understanding of their properties.

History has been characterized by the materials civilisations have developed and used, e.g. Stone Age, Bronze Age and Iron Age. There are different ways of classifying materials according to desired patterns.

International-mindedness

Consider the materials used by ancient civilizations, e.g. the Aztecs, Romans, and Chinese. The materials they used were similar, even though these ancient civilizations were located in very different parts of the world.

Vocabulary

triangular bonding diagramcompositephasepermeability
van Arkel-Ketelaar diagramelasticitybrittleness

Learning slides

You can use this slide gallery for learning or for reviewing concepts and information. It covers all the key points in the syllabus for this sub-topic.

  

Something to think about

It is the first time that triangular bonding diagrams have appeared on the IB syllabus, although they have been on the Cambridge pre-U syllabus (another international examination for 16-19 year olds) for several years. I’m not convinced as to how useful they are. To me they do not really go much further than the old rule of thumb that if the electronegativity difference is greater than about 1.8 the compound is likely to be ionic. They remind me a bit of the octet rule. Both work for simple compounds but both are based on dubious assumptions and fail for more complex examples. Even if we can deduce the percentage of covalent to ionic bonding in a compound, does that really help to understand the properties? Metallic, covalent and ionic are nice labels and ways of classifying materials but their properties can only be generalised. For example metals “have high boiling points” but not all metals do – mercury is a liquid with an appreciable vapour pressure at room temperature. ‘‘Covalent substances have lower boiling points than ionic substances”– try telling that to diamond and sodium chloride!

One of the problems is electronegativity itself. It is not an absolute value and indeed there are different scales of electronegativity. The values are arrived at by considering how a bonding pair of electrons is shared between the two bonding atoms so in a sense the percentage of covalent and ionic bonding has already been taken into account. My two biggest criticisms though are:

1. In the triangular diagram given in Section 29, ‘covalent’ only applies to simple covalent molecules and does not consider macromolecules like diamond, silica or graphite. Diamond and graphite both appear in the covalent section but what new knowledge is gained from that information? It does not enable us to deduce that one conducts electricity and one does not. The tetrahedral diagram (above right), which is not on the syllabus, does attempt to address this.

2. It does not distinguish between different oxidation states. Consider the chloride of a metal M, where M has an electronegativity value of 1.8. Chlorine’s value is 3.2 so the difference is 1.4 and the average is 2.5. This puts the chloride of M in the polar covalent region so we might conclude that it will not conduct electricity when molten and will have a reasonably high boiling point although lower than that of an ionic compound. The problem is that M is lead and lead forms two chlorides, lead(II) chloride, PbCl2, and lead(IV) chloride, PbCl4. Lead(II) chloride is ionic, conducts electricity when molten and boils at 950 oC, Lead(IV) chloride is a yellow non-conducting covalent oily liquid at room temperature and boils at 50 oC. The van Arkel – Ketelaar diagram is of absolutely no use whatsoever in deducing this information.

Test your understanding of this topic

(Note that your teacher may have restricted your access to some or all of these questions and worked answers if they are going to use them as a class test or set them as an assignment.)

For ten 'quiz' questions (for quick testing of knowledge and understanding with the answers explained) see MC test: Introduction to materials science.

For short-answer questions see Materials science introduction questions together with the worked answers on a separate page Materials science introduction answers.

More resources

1. A demonstration of some of the physical properties of metals, such as tensile strength, elasticity, malleability and melting point.

  Physical properties of metals

2. A good introduction to composites and their properties by BioNetwork.

  Composites

3. An excellent video describing and explaining how van Arkel- Ketelaar triangular bonding diagrams work. The discussion then goes further to look at tetrahedral bonding diagrams. It will give you a really good understanding of how it all works.

  Triangular bonding diagrams

4. Richard Thornley gives a simple animated video of van Arkel-Ketelaar diagrams specifically for the IB.

  van Arkel-Ketelaar diagrams

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