


The electronic structures of atoms

Relating orbital filling to the Periodic Table

UK syllabuses for 16 - 18 year olds tend to stop at krypton when it comes to writing electronic structures, but it is possible that you could be asked for structures for elements up as far as barium. After barium you have to worry about f orbitals as well as s, p and d orbitals - and that's a problem for chemistry at a higher level. It is important that you look through past exam papers as well as your syllabus so that you can judge how hard the questions are likely to get.



The first period

Hydrogen has its only electron in the 1s orbital - 1s1, and at helium the first level is completely full - 1s2.

The second period




Note:  The orbitals where something new is happening are shown in bold type. You wouldn't normally write them any differently from the other orbitals.








Shortcut 2: You can lump all the inner electrons together using, for example, the symbol [Ne]. In this context, [Ne] means the electronic structure of neon - in other words: 1s22s22px22py22pz2 You wouldn't do this with helium because it takes longer to write [He] than it does 1s2.


The third period


For example:

short version


The beginning of the fourth period

At this point the 3-level orbitals aren't all full - the 3d levels haven't been used yet. But if you refer back to the energies of the orbitals, you will see that the next lowest energy orbital is the 4s - so that fills next.


There is strong evidence for this in the similarities in the chemistry of elements like sodium (1s22s22p63s1) and potassium (1s22s22p63s23p64s1)


s- and p-block elements

The elements in Group 1 of the Periodic Table all have an outer electronic structure of ns1 (where n is a number between 2 and 7). All Group 2 elements have an outer electronic structure of ns2. Elements in Groups 1 and 2 are described as s-block elements.


Note:  If you use the current IUPAC (international Union of Pure and Applied Chemistry) system for group numbering, you will probably know what I call Group 3 as Group 13. My reasons for not using the IUPAC system are discussed on this page in the Questions and Comments section.

d-block elements






If you are interested:  A transition element is defined as one which has partially filled d orbitals either in the element or any of its compounds. Zinc (at the right-hand end of the d-block) always has a completely full 3d level (3d10) and so doesn't count as a transition element.


d electrons are almost always described as, for example, d5 or d8 - and not written as separate orbitals. Remember that there are five d orbitals, and that the electrons will inhabit them singly as far as possible. Up to 5 electrons will occupy orbitals on their own. After that they will have to pair up.

d5 means

d8 means

Notice in what follows that all the 3-level orbitals are written together - with the 4s electrons written at the end of the electronic structure.



Mn1s22s22p63s23p63d54s2(back to being tidy again)
Cu1s22s22p63s23p63d104s1(another awkward one!)


Filling the rest of period 4


Useful exercise:  Work out the electronic structures of all the elements from gallium to krypton. You can check your answers by comparing them with the elements directly above them in the Periodic Table. For example, gallium will have the same sort of arrangement of its outer level electrons as boron or aluminium - except that gallium's outer electrons will be in the 4-level.


Writing the electronic structure of an element from hydrogen to krypton

  • Use the Periodic Table to find the atomic number, and hence number of electrons.

  • Fill up orbitals in the order 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p - until you run out of electrons. The 3d is the awkward one - remember that specially. Fill p and d orbitals singly as far as possible before pairing electrons up.

  • Remember that chromium and copper have electronic structures which break the pattern in the first row of the d-block.






What about the inner electrons if you need to work them out as well? The 1, 2 and 3 levels will all be full, and so will the 4s, 4p and 4d. The 4f levels don't fill until after anything you will be asked about at A'level. Just forget about them! That gives the full structure: 1s22s22p63s23p63d104s24p64d105s25px25py25pz1.

When you've finished, count all the electrons to make sure that they come to the same as the atomic number. Don't forget to make this check - it's easy to miss an orbital out when it gets this complicated.


Including all the inner levels: 1s22s22p63s23p63d104s24p64d105s25p66s2.

It would be easy to include 5d10 as well by mistake, but the d level always fills after the next s level - so 5d fills after 6s just as 3d fills after 4s. As long as you counted the number of electrons you could easily spot this mistake because you would have 10 too many.


Questions to test your understanding


questions on electronic structures of atoms


Where would you like to go now?

To working out electronic structures for ions . . .

To the atomic properties menu . . .

To the atomic structure and bonding menu . . .

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© Jim Clark 2000 (last modified October 2012)