Why do elements in the same group have similar properties?

Elements in the same group have similar properties because they have the same number of electrons in their outer shell.

In the Periodic Table, elements are arranged in groups and periods. The groups run vertically, and the elements within each group share similar chemical properties. This is due to the fact that they have the same number of electrons in their outermost shell, also known as valence electrons. These valence electrons are the ones involved in chemical reactions, and therefore, determine the chemical behaviour of an element.

For instance, Group 1 elements, known as alkali metals, all have one electron in their outer shell. This makes them highly reactive and eager to lose that one electron in order to achieve a stable electron configuration. As a result, they all react vigorously with water and halogens, and form similar compounds.

Similarly, Group 7 elements, the halogens, all have seven electrons in their outer shell. They are very reactive as well, but unlike alkali metals, they are eager to gain one electron to achieve a full outer shell. This makes them all react in a similar way, for example, by forming salts when reacting with metals.

In contrast, Group 0 elements, the noble gases, all have full outer shells. This makes them very stable and unreactive, as they do not need to lose or gain electrons to achieve a stable electron configuration.

In summary, the number of valence electrons, which is the same for elements in the same group, determines the chemical properties of an element. This is why elements in the same group of the Periodic Table exhibit similar chemical behaviour.