Atomic structure
Read our general notes on Risk Assessment
Some exam "Dos and Don'ts": Atomic Structure
Some advice on answering questions on topics often highlighted by examiners as being poorly answered in exams. Make sure that the detail covered by the question is included in your syllabus specification.
ATOMIC STRUCTURE
• Isotopes
When defining an isotope, it is best to say 'Isotopes are - - -', and be sure to say ' - - are ATOMS of the same element which - - - -' to get full marks.
When working out relative atomic masses, remember to give your answer to at least 2 decimal places, bearing in mind the accuracy of the data given.
• H-atom line spectrum and energy levels
Make sure you know the difference between a continuous spectrum (a spread of ALL wavelengths/frequencies/energies of light), and a line spectrum (only 'lines' corresponding to specific wavelengths/
frequencies/energies of light).
In the H-atom spectrum there are several series of lines, and in each series the lines get closer together as the frequency or energy increases, or the wavelength decreases - be able to draw a diagram showing this.
Each line in the spectrum represents a specific amount of energy, given out by the excited atom when the electron jumps (a transition) from a higher to a lower energy level. The lines are NOT energy levels, as exam candidates frequently state!
• Evidence for fixed (quantised) energy levels
As only certain specific lines appear only certain fixed jumps must be possible. The energy of the H-atom must therefore be limited to a set of fixed energy levels. The fact the lines get closer together towards higher energies shows that the energy levels are getting closer together at higher energies - think of a step ladder with steps getting closer towards the top, and you jumping off to the ground!
• Questions about energy levels and distance from the nucleus
The higher the energy level, the further the electron is from the nucleus - and the more energy is released when it drops to the lowest energy level, closest to the nucleus. The energy involved in the jump from the very highest energy level (the limit of the highest energy series of lines) is the same as the energy needed to remove the electron from the atom - it is therefore equal to the ionisation energy.
• Balmer series in H-atom spectrum
You may be expected to know something about the series of visible lines in the H-atom spectrum. This is called the Balmer series, and all the lines arise from electron jumps from higher energy levels to the second lowest energy level.
• Orbitals and energy levels
We tend to use the words energy levels and orbitals or shells interchangeably when describing where electrons are in an atom. Early theories put an electron in a given energy level in a fixed orbit around the nucleus. In our modern picture of the atom, the electrons still occupy fixed energy levels, but their place in space around the nucleus, in orbitals, is less clearly defined. An orbital is the region in space around the nucleus where an electron in a given energy level is most likely to be found.
You will be expected to know the shapes of s- and p-orbitals.
• Filling orbitals
You will know the general rules and that 4s is filled before 3d. It is most important to remember that when writing the electron arrangement for metal ions (which have lost electrons), that the 4s electrons must be removed before any from the 3d subshell.
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updated: 01 June 2007
