Atomic structure

The section of the syllabus on atomic structure requires 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 extend 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 M_r. Students often find the different methods of calculating M_r 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.  Please note that time of flight mass spectrometry is replacing the magnetic field instrumentation, so please be concious of this when viewing older resources.

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