The (2.8.8) version of Electronic Structure from GCSE won't get you far at AS.
It is important that:
- that you can write the full electronic structure for any element in the first 4 periods.
- that you can write short-form electronic structures for elements 20 and 21. For example Potassium (K19 = [Ar] 4s1. Unless you are studying A2 you will not have to write electronic structures for d-block elements.
- that you can do the same for ions of elements given their charge and Atomic Number.
Rules for filling shellsEdit
The aufbau principle states that the lowest energy orbitals must be completely filled before the orbitals at higher energy begin to be filled. (Transition elements provide some exceptions).
The Pauli Exclusion principle states that orbitals at the same energy will all be half-filled before any electrons are forced to "spin-pair".
Note: You're not really required to know the names of these two principles, nor are you required to know what "spin-pairing" actually means.That is Physics and therefore of no interest to anyone.
- You are most likely to be asked to write electronic structures. You can do this by simply remembering which periods have which orbitals and how many electrons each orbital can take, or you can simply look at the Periodic Table and count how many elements are in the s and p (and d) blocks up to your given element.
- What is the electronic structure of Potassium?
K (1s2 2s2 2p6 3s2 3p6 4s1
or, in the shortened form [Ar] 4s1 - to signify that the structure is the same as Argon up to that point as this is true for all elements. This notation is mostly restricted for A2.
- You are also very likely to be asked to do the same for an ion. Which you do by writing the structure for the element and adding (or subtracting) the number of electrons needed to produce the charge.
- What is the electronic structure of K+?
K (1s2 2s2 2p6 3s2 3p6 4s1)
so K+ must be (1s2 2s2 2p6 3s2 3p6) - because that is one fewer electron.
F (1s2 2s2 2p5)
so F- must be (1s2 2s2 2p6) - because that is one electron more.
- You are also very likely to be asked to identify an element from its full electronic structure - the easiest way is to count the total number of electrons and this will be the Atomic Number of the element.
- Which element is (1s2 2s2 2p6 3s2 3p3?
Answer: Total number of electrons = 15. Atomic Number 15 = Phosphorus
- You are also very likely to be asked to identify an element from the full electronic structure and the charge of one of its ions - again, count the total number of electrons and simply add the number of positive charges (or subtract the number of negative charges). The original total is the atomic number.
- Which element forms 2- ions with the electronic structure (1s2 2s2 2p6)?
Total number of electrons = 10.
Two of these electrons were added to make a 2- charge so the original total would have been 8.
Atomic number 8 = Oxygen.
- Which element forms 2+ ions with the electronic structure (1s2 2s2 2p6)?
Total number of electrons = 10.
Two electrons were removed to make a 2+ charge so the original total would have been 12.
Atomic number 12 = Magnesium.
- You are equally likely to be asked to do the same by filling in an orbital diagram?
This is no more complicated than the questions above, just remember to show opposite spins when you are pairing electrons. (See diagram below)
- For questions (and answers) involving Electronic structure press -  and select the appropriate heading.
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