In the Zeff periodicity of valence electrons, the effective nuclear charge (Zeff) increases across a period from left to right. This is because the number of protons in the nucleus increases, creating a stronger attractive force on the valence electrons.
Now, when we compare aluminum (Al) and silicon (Si), we can see that Al is located to the left of Si in the third period of the periodic table.
Aluminum has 3 valence electrons, which are located in the 3p orbital. The effective nuclear charge experienced by these electrons is relatively low because the 3s and 3p electrons shield them from the nucleus. As a result, the valence electrons in Al are relatively easy to remove, making Al a good conductor of electricity.
On the other hand, silicon has 4 valence electrons, which are also in the 3p orbital. However, the effective nuclear charge experienced by these electrons is higher than that experienced by the valence electrons in Al. This is because silicon has more protons in the nucleus, which creates a stronger attractive force on the valence electrons. This makes silicon a poorer conductor of electricity than aluminum.
In summary, the Zeff periodicity of valence electrons explains the changes in the properties of elements across a period. As you move across a period from left to right, the effective nuclear charge (Zeff) experienced by valence electrons increases, making it harder to remove them. This results in changes in the physical and chemical properties of elements such as conductivity, reactivity, and electron affinity.