Re: How do you do electron configuration just using the periodic table?

Date: Thu Sep 16 14:34:17 1999
Posted By: Nigel Barker, Head of Science, International School of Lusaka
Area of science: Chemistry
ID: 937355721.Ch

Message:

I agree with your teacher - I don't even know the diagonal rule myself, although I have seen some of my students using it.

Here's what you do:

You remember that the number of sub-levels is the same as the main energy level (principal quantum number, if you call it that).
So the first level has one sub-level, the second has two, and so on.

The names of the sub-levels is another thing you have to remember:

main level	sub-levels present
1	s
2	s,p
3	s,p,d
4	s,p,d,f

etc (sub-levels beyond f are always empty in the ground state)

The number of the main level corresponds to the period number in the periodic table. First period, first main energy level; period 2, main level 2, etc.
Its also worth remembering the labelling of the 'blocks' of the periodic table:

Thats it for things to remember (almost). From now on you can use the periodic table to work out the rest.

Look at period 1. There are two elements. You remember that there is only one sublevel, and you also remember that it is called 's'. How many electrons can go in the s sub-level?
Two - one for H and two for He
The electron configurations are 1s¹ for H and 1s² for He.
Each element has the electron configuration of the previous element +1 electron.

Now period 2. There are 8 elements. The first two are in the s-block, so their configurations are
Li: 1s² 2s¹
Be: 1s² 2s²
The next bunch are in the p-block, and there are 6 of them. That means the p sub-level can hold up to 6 electrons
B: 1s² 2s²2p¹
C: 1s² 2s²2p²
....
Ne 1s² 2s²2p⁶

Now comes the final thing to remember. Period 3 is level 3, which has three sub-levels (s,p,d). However, if you look at period 3, there are no d-block elements there. The 3 d sub-level is in the row below (period 4). So the thing to remember is that the d-block has been shoved down a row. 3d is in period 4, 4d is in period 5, 5d is in period 6, etc.

Some examples from period 3:
Mg: 1s² 2s²2p⁶3s²
Al: 1s² 2s²2p⁶3s²3p¹
Ar: 1s² 2s²2p⁶3s²3p⁶

Now period 4:
K: 1s² 2s²2p⁶3s²3p⁶4s¹
Sc: 1s² 2s²2p⁶3s²3p⁶4s²3d1
Zn: 1s² 2s²2p⁶3s²3p⁶4s²3d10
Br: 1s² 2s²2p⁶3s²3p⁶4s²3d104p⁵
notice that the d-block has 10 columns - the d sub-level takes up to 10 electrons.

when you get down to period 6, you see that there is a gap where the f-block is supposed to be. A quick glance at the table above shows that the first level with an f sub-level is 4, so its 4f that is being filled up from La to Yb, and 5 f for the actinides.

To double check that you've got it right, count the total number of electrons in your configuration. It has to be the same as the atomic number of the element.

Thats it!

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