Metallic bonding representations

 

Featured paper

de Posada, J. E. M. (1999). The presentation of metallic bonding in high school science textbooks during three decades: Science educational reforms and substantive changes of tendencies. Science Education, 83, 423-447.

In this study the authors analysed 58 Spanish high school chemistry textbooks from 1974-1998 in relation to their treatment of metallic bonding

The authors found that every textbook explained that metals are made up of metal cations (positive ions) and free electrons, but very few explicitly acknowledged that this is a theoretical model. Unsurprisingly at the level of textbook being examined, very few mentioned the band theory of metals.

Graphical representations

The graphics used to represent the metallic bonding model varied. Some diagrams represented electrons as particles, but others showed the delocalised electrons as a cloud. A student who is operating only in terms of the surface features of the diagrams would observe lines, circles, symbols and shading. The authors suggest that the diversity of these diagrams is confusing for students.

Language

The authors noted that the language used to describe metallic bonding uses metaphors, for example a ‘sea’ of electrons and a metallic ‘lattice’. Students use these words in everyday life and may therefore bring their own existing ideas with them to the chemistry classroom. The authors suggest that textbooks should make clear which attribute of the metaphor is linked to metallic bonding.

Reflective questions

How could you support students to interpret the range of metallic bonding diagrams that they may encounter?

How could you clarify the metaphorical language used so that students understand which attributes apply to metallic bonding and which do not?

BEST Diagnostic question

Metallic structure diagrams

This question was devised to show four students that are thinking in different ways about representations of metallic structure and bonding. Exploring your own students' views may help you to understand if their thinking is similar to any of the students in the question. 

Some students have drawn diagrams to show how they think about the structure of a metal.

1. Whose thinking do you most agree with?

2. Whose thinking do you disagree with?

Explain your answers

The diagram by student A has a balance of positive and negative charge and pictures the electrons in particle form which is consistent with the model of the atom commonly taught to the 14 to 16 age group.

Student C’s thinking does not include any negative charge at all. Agreement with this way of thinking could suggest a literal interpretation of the idea that metallic structure is made up of positive metal ions surrounded by a ‘sea of electrons'.

A student who most agrees with student B’s thinking may also have been influenced by references to a ‘sea of electrons. The student may be taking the metaphor more literally than it is intended by representing the structure with a vast number of electrons.

Agreement with the thinking of student D may also indicate misunderstanding about a ‘sea of electrons’ as it could be representing the electrons as being in the sea. Alternatively, it may be indicating a continuous area of negative charge. This is inconsistent with the model of the atom commonly taught at this age which treats electrons as being particles. 

Useful resources

BEST Topic 7 Key Concept1: Metallic structure model

Diagnostic questions to check for student misconceptions about metallic bonding as part of a five-part progression (and including response activities)

University of York Science Education Group

Developing Understanding: Metallic bonding

A ramped student worksheet that aims to help students to deepen their understanding of metallic bonding and to strengthen their mental models.

Royal Society of Chemistry

Metallic bonding models

Featured paper

Cheng, M. M. W. and Oon, P.T. (2016). Understanding metallic bonding: Structure, process and interaction by Rasch analysis. International Journal of Science Education, 38(12), 1923-1944.

 

The authors categorise three ways in which students think about metallic bonding.

1.      As a structure – a lattice arrangement of cations (positive ions) and delocalised electrons

2.       As a process – metal atoms losing outer electrons to form a ‘sea of electrons’

3.       As an interaction – a lattice arrangement of cations held together by electrostatic attractions between    these positive ions and the free electrons

The study aimed to test if structure, process and interactions were at increased levels of understanding. 3006 year 10-12 students in Hong Kong took part in the research survey.

The research study supported an order of difficulty where thinking of metallic bonding as an interaction was the model that was the most difficult:

interaction > process > structure

In Year 7-9 in Hong Kong a ‘particle model’ was used. This describes metals as being made of particles with each metal being made of one kind of particle. Changes of state were explained as a change in arrangement of these particles, but forces of attraction are not considered.

From Year 10 a ‘free electron’ model was used in which metals are made of a lattice of positive metal ions with delocalised electrons moving around them. This structure is held together by an all-directional electrostatic force.

When students are taught the initial particle model in this curriculum the focus of thinking is on structure. Later, to fully understand the ‘free electron model’ students must move to thinking in terms of interactions. The authors suggest that this shift is a challenge for students. Some students may remain thinking in terms only of structure. Consequently, they may not fully gain an understanding of metallic bonding as a form of electrostatic interaction.

Reflective questions

In your curriculum, does the initial particle taught at age 11-14 include the idea of forces of attraction between particles? What are the benefits and drawbacks of including this concept from the outset?

Are your students able to select and justify their choice of the most appropriate model to explain different properties of metals such as malleability and electrical conductivity?

Useful resources

Developing Understanding: Metallic bonding  

A ramped student worksheet that aims to help students to deepen their understanding of metallic bonding and to strengthen their mental models.

Royal Society of Chemistry

BEST Topic 7 Key Concept 1: Metallic structure model

Diagnostic questions to check for student misconceptions about metallic bonding as part of a five-part progression with additional response activities.

University of York Science Education Group