Atomic structure

The section of the syllabus on atomic structure required students to develop a much deeper understanding of the structure of atoms and to start to think of electron occupancy, and therefore electronic structure, in terms of orbitals rather than energy levels, or shells.

They need to become familiar with the different types of orbitals and their multiplicities as far as s, p and d orbitals and will need to be able to deduce filling order. They should also become familiar with Hund's Rule (electrons occupy degenerate orbitals singly in preference to pairing when possible, for example when filling a set of p orbitals of the same energy level), and Pauli's exclusion principle (electrons in a single orbital must have opposite spins).

Students need to be able to apply these principles in order to write down the electronic configuration of atoms and ions in terms of orbital occupancy, and represent configurations either as shorthand (for example Na 1s2 2s2 2p6 3s1) or as energy level diagrams using box notation. Once this is achieved atoms are related to their position in the periodic table in terms of the s, p, and d blocks. It is a simple process to extent this logic to include the lanthanides and actinides as the f block with able and interested students.

The section then looks at evidence for energy levels and orbitals obtained by looking at graphs of ionisation energy. This is an area which often confuses students. The key is that evidence for energy levels is obtained from plots of successive ionisation energy for a single atom. Whereas evidence for orbital structure comes from graphs of first ionisation energy for different atoms as we cross a period. Crucially, students must be able to state and explain these trends in terms of distance of valence electrons from the nucleus and the nuclear charge experienced by the valence electron once screening is taken into account. Students often find explanation of the "sawtooth" pattern of the second type of graph challenging where they not only have to be able to explain the general increase in first ionisation energy as a period is traversed, but also the decreases between Be and B and then again between N and O, to use period two as an example. It is important in answering exam quaestions that students address both the distance of the valence electron from the nucleus and the attractive force of the nucleus. Often students will focus on one aspect and neglect the other.

It is a great pity that many students tend to see this more up-to-date model of the atom as replacing an incorrect model which they learned at GCSE, and there is an opportunity here to help students to understand the process of scientific modelling and to appreciate that the GCSE model (Bohr's model) is a simplification of the model that we are now developing at A level, or conversely that the modern model adds another layer of sophistication to Bohr's model.

This meaty section of the syllabus rounds off with a consideration of the rigorous definition of relative atomic mass and the application of the mass spectrometer in measuring Mr. Students often find the different methods of calculating Mr from data confusing given that information in exam questions may be in the form of absolute masses or relative masses and these require a different approach. Time of flight mass spectrometry is replacing the magnetic field instrumentation.

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Links and Resources

Unit S2: Atomic Structure

This comprehensive resource provides much detailed background material on the topics covered in this module. The resource would be useful for teachers when planning lessons for this module, but also many of the exercises, with a few dated exceptions, are still useful and pertinent to current syllabuses and can be scanned or retyped and edited to provide class or homework material. A number of questions from this text have appeared very recently on exam papers at A level in very near identical format.

This is an invaluable resource for planning, and for setting example problems to support teaching and learning.

publication year
1980 - 1989

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The Atom Detectives

This is a nice resource tracing some of the important events in the development of atomic theory. It includes a brief biography of key scientists such as Dalton, Rutherford and Bohr and their contribution, as well as summarising the consequences of their ideas. It builds upon and extends what students will bring to A level from their GCSE and could be usefully employed as an introductory activity at the start of the atomic structure module to revise and extend students' knowledge and elicit interest in the new material to be introduced subsequently. 

publication year
2000 - 2009

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History, People and FAQs

This is a more comprehensive resource on the development of atomic theory than the previous resource and may prove useful for project work on this area for those syllabuses that place greater emphasis on the history and development of atomic theory.

publication year
2000 - 2009

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Models of the Atom

This article from the April 2015 edition of Catalysist provides a short, but interesting summary of the development of ideas on atomic theory.

publication year
2010 to date

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An Analogy for the Atom

This resource, from the Royal Society of Chemistry publication on Chemical Misconceptions, explores the planetary model of the atom and gets students to compare the physical characteristics of an atom with those of the solar system.

Teachers would probably not want to follow the resource completely since the planetary model per se is not important at A level, other than within its historical perspective. However the resource does get students to think critically about the nature and magnitude of forces within the atom and as such it could be used as a starter activity to develop ideas about ionisation energy.

publication year
2000 - 2009

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7. Chemical Structure

The first part of this resource from the same volume as An analogy for the atom critiques the previous resource and is a very useful discussion of the types of misconception that students very often bring to their A level studies from GCSE.

It then goes on to consider ionisation energy and orbitals, all of which is useful material to teachers planning lessons on this topic.

The latter part of the resource extends the discussion to ideas of chemical structures and would be useful reading in preparation for later modules on bonding. The resource is written for teachers, and is designed to stimulate discussion and reflection on common conceptual difficulties.

publication year
2000 - 2009

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Ionisation Energy

This resource extends on the previous resource, An Analogy for the atom, and is a diagnostic probe to elicit students' understanding related to ionisation energy. It would be a natural continuation of the activities in the aforementioned resource, and extends those ideas more into A level territory. This would be a very useful activity to prepare students for a more in depth analysis of trends in ionisation energy as discussed in the list introduction. The previous resource on Chemical Structure would be useful prior reading for the teacher planning lessons on this topic.

publication year
2000 - 2009

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2. Atomic Structure

This resource provides useful worksheets covering the development of modern atomic theory, electronic configuration and box notation, and trends in ionisation energy. The material can usefully be used to reinforce learning either in class or as homework exercises.

publication year
2000 - 2009

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10. Analysis

This resource covers the mass spectrometer and isotopes, including calculations on relative molecular mass from spectroscopic data. The principles of magnetic sector instruments are covered, but not time of flight spectrometers. The material can usefully be used to reinforce learning either in class or as homework exercises.

More in depth resources on mass spectrometry may be found in the list on Modern Analytical Techniques.

publication year
2000 - 2009

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Spectroscopy in Chemistry

This detailed resource on spectroscopy provides useful detailed treatments of a number of relevant areas of this topic and would prove a useful reference for teachers in their planning.

Chapter two covers emission spectroscopy and its relationship to the energy levels within atoms, which is a requirement for some syllabuses. There is also a detailed treatment in this section of the use of ionisation energy data to deduce orbital structure. Patience is needed in scrolling through sections of the text containing coloured plates as they can take a little time to load.

publication year
1960 - 1969

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