Periodicity

In a nutshell

Periodicity refers to the trends seen in the properties of elements across periods in the periodic table. These trends are explained by the electron arrangements of each element. There are several trends seen such as trends in atomic radii, melting point and boiling points. 

Trends in atomic radii

The atomic radius tells you about the size of an atom. The atom does not have a well-defined boundary which can be measured therefore atomic radius is half the distance between the centres of a pair of atoms.

The term atomic radius can be used to describe different types of bonds for example, ionic, covalent, metallic, van der Waals and more. 

Across each period, the atomic radius decreases and as you go down each group the atomic radius increases.

Why does atomic radius decrease across a period?

Looking at the electronic structures of elements explains these trends. 

The atomic radius decreases across a period because there is an increased attractive force between the outer electrons and the nucleus. This is because across a period, the number of protons in each nucleus increases, similarly the nuclear charge also increases. 

Although the nuclear charge increases, the number of shells doesn't and therefore there are no additional electron shells to provide extra shielding. This leads to the outer electrons being pulled closer, hence causing a decrease in the atomic size. 

Why does atomic radius increase down each group?

As you go down each group, the outer electron is found in an extra level of electrons, for example,  lithium has the outer electron in a $$2s$$​ shell, whereas sodium has its outer electron in a $$3s$$​ shell. This extra level of electrons means that the outer electrons are further away from the nucleus and experience less pull. This results in a larger atomic radius.

Trends in melting points and boiling points

The graph below shows the trends seen across boiling points and melting points in period $$3$$​. 

Elements with a higher boiling point and melting point are found on the left and those with a lower boiling point and melting point are found on the right of the periodic table. These trends are explained by the structures formed by the elements. Elements which have a giant structure will tend to have a higher melting point and a higher boiling point. Those elements which form molecular structures or atomic structures will tend to have lower melting points and lower boiling points. 

​In period $$3$$​ sodium, magnesium and aluminium form metallic structures. For these metals, the strength of the metallic bonding increases from left to right thus the melting point and the boiling point also increase. This is because from left to right in this period, the charge on the ion increases for each element, meaning more electrons join the delocalised electrons which hold the lattice together - creating a stronger metallic bond.

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Silicone, phosphorus, sulfur and chlorine (the non-metals) form structures with covalent bonds. The melting point for these elements depends on the size of the van der Waals forces occurring between the molecules. This is determined by the number of electrons in the molecule and how closely packed together the molecules are. The melting point and boiling point decrease going from left to right as dictated by the decrease in molecular packing and van der Waal forces. 

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