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# Help Zone

### Student Question

Secondary I • 1yr.

I don't understand the logic of the periodic table. Why does it have this weird shape? Why not just a rectangle? How come the items are classified like this?

Science

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## Explanations (1)

• Explanation from Alloprof

Explanation from Alloprof

This Explanation was submitted by a member of the Alloprof team.

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Team Alloprof • 1yr.

Thanks for your question!

The periodic table of the elements brings together all the chemical elements known to humans.

Although it may seem odd, the shape of the periodic table is actually very well planned.

First, the elements are arranged in ascending atomic number order reading from left to right, then from top to bottom.

Then the items are organized into families (sometimes called “groups”) and periods.

• Families are the different columns of the periodic table. Except for the middle columns (the transition metals) and the two rows at the bottom (the actinides and lanthanides), the column number gives the number of valence electrons (electrons on the last electron shell of an atom) . Several families have names, like alkalis, for example, the first family from the left.

• The periods are the different lines of the periodic table. The properties of the elements of the same period are said to be periodic. More precisely, there are precise trends in some of their properties, like the atomic radius, for example. As we order the elements of a period, we will see an increase or decrease in some of these properties.

Two clusters of elements stand out from the others: the central cluster, offset from the others, and the two rows separated from the rest of the table below.

The central cluster is called the “transition metals”. The two separate rows in the table below are named actinides (the first row) and lanthanides (the second row) and should actually be placed between Ba (barium) and Hf (hafnium) as well as Ra (radium) ) and Rf (rutherfordium). Their separate position is only an aesthetic choice, in order to avoid having to stick them in a small space.

Do not hesitate if you have other questions!