The electronic structures of atoms

Relating orbital filling to the Periodic Table

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

Now we need to start filling the second level, and hence start the second period. Lithium's electron goes into the 2s orbital because that has a lower energy than the 2p orbitals. Lithium has an electronic structure of 1s22s1. Beryllium adds a second electron to this same level - 1s22s2.

Now the 2p levels start to fill. These levels all have the same energy, and so the electrons go in singly at first.
B                1s22s22px1
C                1s22s22px12py1
N                1s22s22px12py12pz

The next electrons to go in will have to pair up with those already there.
O                1s22s22px22py12pz1
F                1s22s22px22py22pz1
Ne                1s22s22px22py22pz2

You can see that it is going to get progressively tedious to write the full electronic structures of atoms as the number of electrons increases. There are two ways around this, and you must be familiar with both.

Shortcut 1: All the various p electrons can be lumped together. For example, fluorine could be written as 1s22s22p5, and neon as 1s22s22p6.

This is what is normally done if the electrons are in an inner layer. If the electrons are in the bonding level (those on the outside of the atom), they are sometimes written in shorthand, sometimes in full. Don't worry about this. Be prepared to meet either version, but if you are asked for the electronic structure of something in an exam, write it out in full showing all the px, py and pz orbitals in the outer level separately.

For example, although we haven't yet met the electronic structure of chlorine, you could write it as 1s22s22p63s23px23py23pz1.

Notice that the 2p electrons are all lumped together whereas the 3p ones are shown in full. The logic is that the 3p electrons will be involved in bonding because they are on the outside of the atom, whereas the 2p electrons are buried deep in the atom and aren't really of any interest.

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.

On this basis the structure of chlorine would be written [Ne]3s23px23py23pz1.

The third period

At neon, all the second level orbitals are full, and so after this we have to start the third period with sodium. The pattern of filling is now exactly the same as in the previous period, except that everything is now happening at the 3-level.

For example:
                                                                                             short version
Mg                1s22s22p63s2                                            [Ne]3s2
S                1s22s22p63s23px23py13pz1                [Ne]3s23px23py13pz1
Ar                1s22s22p63s23px23py23pz2                [Ne]3s23px23py23pz2

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.
K                1s22s22p63s23p64s1
Ca                1s22s22p63s23p64s2

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

The outer electron governs their properties and that electron is in the same sort of orbital in both of the elements. That wouldn't be true if the outer electron in potassium was 3d1.

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.

Elements from group 3 across to the noble gases all have their outer electrons in p orbitals. These are then described as p-block elements.

d-block elements

Remember that the 4s orbital has a lower energy than the 3d orbitals and so fills first. Once the 3d orbitals have filled up, the next electrons go into the 4p orbitals as you would expect.

d-block elements are elements in which the last electron to be added to the atom is in a d orbital. The first series of these contains the elements from scandium to zinc, which at GCSE you probably called transition elements or transition metals. The terms "transition element" and "d-block element" don't quite have the same meaning, but it doesn't matter in the present context.
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, even though the 3d electrons are added to the atom after the 4s.
Sc                1s22s22p63s23p63d14s2
Ti                1s22s22p63s23p63d24s2
V                1s22s22p63s23p63d34s2
Cr                1s22s22p63s23p63d54s1

Whoops! Chromium breaks the sequence. In chromium, the electrons in the 3d and 4s orbitals rearrange so that there is one electron in each orbital. It would be convenient if the sequence was tidy - but it's not!
Mn                1s22s22p63s23p63d54s2                (back to being tidy again)
Fe                1s22s22p63s23p63d64s2               
Co                1s22s22p63s23p63d74s2               
Ni                1s22s22p63s23p63d84s2               
Cu                1s22s22p63s23p63d104s1                (another awkward one!)
Zn                1s22s22p63s23p63d104s2       

And at zinc the process of filling the d orbitals is complete.

Filling the rest of period 4

The next orbitals to be used are the 4p, and these fill in exactly the same way as the 2p or 3p. We are back now with the p-block elements from gallium to krypton. Bromine, for example, is 1s22s22p63s23p63d104s24px24py24pz1.

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.

Writing the electronic structure of big s- or p-block elements
First work out the number of outer electrons. This is quite likely all you will be asked to do anyway.

The number of outer electrons is the same as the group number. (The noble gases are a bit of a problem here, because they are normally called group 0 rather then group 8. Helium has 2 outer electrons; the rest have 8.) All elements in group 3, for example, have 3 electrons in their outer level. Fit these electrons into s and p orbitals as necessary. Which level orbitals? Count the periods in the Periodic Table (not forgetting the one with H and He in it).

Iodine is in group 7 and so has 7 outer electrons. It is in the fifth period and so its electrons will be in 5s and 5p orbitals. Iodine has the outer structure 5s25px25py25pz1.

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.

Barium is in group 2 and so has 2 outer electrons. It is in the sixth period. Barium has the outer structure 6s2.

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.