Periods and groups in the periodic table of elements

There are 118 different types of atoms that we know of: different elements. We usually organise them in a special way in a table: the periodic table. There is a reason why the elements are placed the way they are. Here, take a look: In the top left corner, we find the element hydrogen. A hydrogen atom has only one electron.

The next element, helium, has two electrons. To accommodate a third electron, as does the element lithium, we need a second electron shell. This is shown by a new row in the periodic table. Lithium is the first element of the second row. For each step we go to the right, the atoms get one more electron.

Until we get over here. When there are eight electrons in the outer shell, there’s not room for any more. If we want to add another electron we need to start another new shell, the third shell. And that means we also start another new row: the third row. There is a pattern here: As we go to the right, the number of electrons increases until there are eight electrons in the outermost shell.

That’s when we start a new electron shell, on a new row. This means that all the elements with a “full outer shell” will end up under each other, in the same column. Elements with a full outer shell don’t want to react with other substances. They are the noble gases. All these, except for helium, have eight electrons in their outermost shell - eight valence electrons.

And all the elements at the start of a row, furthest to the left, have only one valence electron. All these elements, apart from hydrogen at the top, are soft and light metals - the alkali metals. Alkali metals readily react with other substances. It’s the same in these columns: the elements in each column have the same number of valence electrons, and therefore similar properties. Therefore, the columns are called groups.

In each row, the number of valence electrons increases, until we get to eight. Then it starts over again with one valence electron. This means the properties are repeated on each row: they appear periodically. That is why the rows in the periodic table are called periods. The atoms in the first period have one electron shell.

All the atoms in the second period have two shells, and so on. The atom with the most electrons that we know of, 118, is the last one on the seventh row. What does this tell us? That it has seven electron shells, since it stands in period seven. And that the outermost shell has eight electrons, since the element is in the group furthest to the right.

So: The columns in the periodic table are called groups. Elements in the same group, often have similar properties. The rows are called periods, since the properties are repeated periodically. The periods also show how many electron shells the atoms have, from one shell at the top, to seven shells at the bottom.