aelectron arrangements heading

Higher and foundation tiers

Electron Configurations

Imagine having to learn how all the elements in the periodic table react! That would probably be impossible. Luckily you don't have to. Once you know a little about the internal structure of atoms, and in particular how the electrons are arranged then you will have an excellent idea how each of the elements will react.

the periodic table

In the periodic table the elements are arranged in groups or columns. There are 8 groups, these are labelled in the image above as groups 1-7 with group 0 or 8, the Noble gases being the last group or column in the periodic table. In each group the elements all react in a similar way, so if you know how one element in any group reacts then you can easily predict how the others will react e.g. lithium and sodium are the two metals at the top of group 1, the alkali metals.

symbols for Lithium and Sodium

You have probably watched your teacher drop these two metals into a bowl of water and seen the violent reaction that happens. Have you ever wondered why two metals react in a similar way? The way a substance reacts depends on how the electrons are arranged inside the atoms, and in particular how many electrons are in the last electron shell.

Electron arrangements and chemical reactions

The electrons fill up the electron shells in order. The first electron shell is the lowest in energy and it can hold a maximum of 2 electrons, once it is full then any additional electrons must go into the second shell. The second shell can hold a maximum of 8 electrons. Once this shell is full then any extra electrons go into the third shell, this shell also holds a maximum of 8 electrons. You need to be able to draw diagrams similar to the one opposite for any of the first 20 elements in the periodic table. I have drawn out a few electronic structure diagrams below for a few elements but I would suggest you draw them out for the first 20 elements in the periodic table and try to spot any patterns between the number of electrons in the last shell and the group the element is in the periodic table.
Remember:

electronic structure of an atom

More electron arrangements

Lets go back to the examples above using lithium and sodium metal, both from group 1 in the periodic table.

lithium atom

You should notice that both lithium (2,1) and sodium (2,8,1) have 1 electron in their outer electron shell. It is the number of electrons in the outer shell that determines how an atom will react. When you move down a row in the periodic table from lithium (2,1) to sodium (2,8,1) one extra electron shell is added. So although lithium (2,1) and sodium (2,8,1) do not have an identical electron arrangement, they have a similar electron arrangement. This is why they react in a similar way. Potassium is the next element under sodium in group 1 of the periodic table. Potassium has an atomic number of 19, so has 19 protons as well as 19 electrons. Since potassium is in group 1 of the periodic table it will have 1 electron in its outer electron shell. Its electron arrangement will be 2,8,8,1. Again moving down one row from sodium to potassium in the periodic table simply adds one extra electron shell.

Electron arrangements and the periodic table

There is an easy way to check if you have worked out the correct electron arrangement for an element, simply check what group in the periodic table an element is in and the element should have the same number of electrons in the last shell as the group it is in. The horizontal rows in the periodic table are called periods. Hydrogen and helium are in period 1, lithium is the first element in period 2 and sodium the first element in period 3. Whatever period an element is in will tell you how many electron shells it has. e.g.

Element Group in periodic table Atomic number electron arrangement Atomic structure diagram
carbon 4 6 2,4 carbon atom
silicon 4 14 2,8,4 silicon atom
nitrogen 5 7 2,5 nitrogen atom
phosphorus 5 15 2,8,5 phosphorus atom
oxygen 6 8 2,6 oxygen atom
sulfur 6 16 2,8,6 sulfur atom

If you look at the elements in the table above you will notice that the group number in the periodic table corresponds to the number of electrons in the last shell and the number of shells corresponds to the row or period the element is in. This is the reason why chemists do not have to learn how every element will react. Since chemical reactions depend upon the number of electrons in the last shell, then if you know how one element in any one particular group reacts with other substances then you can easily predict how all the elements in that group will react.

Key Points


Practice questions

Check your understanding - Electron arrangements for the first 20 elements

Check your understanding - Electron arrangements and reactivity.

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