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 within atoms; then you will have an excellent idea how each of the elements will react.

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.

You have probably watched your teacher drop these two metals into a trough of water and seen the violent reaction that happens. Have you ever wondered why these 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 the first electron shell 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:

• The first electron shell holds a maximum of 2 electrons.
• The second electron shell holds a maximum of 8 electrons.
• The third electron shell holds a maximum of 8 electrons.

More electron arrangements

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

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 chemically with other substances. 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 for each row you go down.

So although lithium (2,1) and sodium (2,8,1) do not have identical electron arrangements, they both have 1 electron in their outer shell and 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 it will have 19 protons in its nucleus and 19 electrons in the electron shells. Since potassium is in group 1 of the periodic table it will also have 1 electron in its outer electron shell. Its electron arrangement will be 2,8,8,1. Again moving down one row or period 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 that particular 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. study the information in table below and you should notice that the number of electron shells an element has is the same as the period in which the element is found in the periodic table.

Element name Group in periodic table Atomic number electron arrangement Atomic structure diagram period where element is found in the periodic table
carbon 4 6 2,4 period 2
silicon 4 14 2,8,4 period 3
nitrogen 5 7 2,5 period 2
phosphorus 5 15 2,8,5 period 3
oxygen 6 8 2,6 period 2
sulfur 6 16 2,8,6 period 3

If you look at the elements in the table above you will notice that the group number where the element is found 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 found in. This is the reason why chemists do not have to learn how every element reacts. 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 element in that group will react.

Key Points

• Electrons are arranged in energy levels or shells around the nucleus.
• The 1st electron shell can hold a maximum of 2 electrons; the other shells all hold up to 8 electrons.
• The group number where an element is found in the periodic table corresponds to the number of electrons in its last or outer electron shell.
• The number of electron shells corresponds to period in the periodic table where the element is to be found.