Why is there a similarity in properties across a transition series?

The similarity in properties across a transition series is due to the similar electron configurations of the elements involved.

In a transition series, the elements have similar electron configurations because they are filling the same d-orbital. This is a key characteristic of transition metals, which are defined as elements whose atoms have an incomplete d sub-shell, or which can give rise to cations with an incomplete d sub-shell. The d-orbital can hold up to 10 electrons, and as you move across a transition series from left to right, electrons are added one at a time to the same d-orbital.

This consistent addition of electrons to the same orbital results in the elements in a transition series having similar chemical and physical properties. For example, they all tend to form coloured compounds, have multiple oxidation states, and are good conductors of heat and electricity. These properties are a direct result of the similar electron configurations of the elements in a transition series.

The electron configuration also affects the reactivity of the elements. As you move across a transition series, the increase in nuclear charge is offset by the addition of electrons to the same d-orbital, which shields the outer electrons from the increased nuclear charge. This means that the reactivity of the elements does not change significantly across a transition series.

However, it's important to note that while the elements in a transition series have similar properties, they are not identical. Small differences in electron configuration can result in significant differences in chemical behaviour. For example, chromium and copper do not follow the general trend of electron configuration in a transition series, which results in them having unique chemical properties.

In summary, the similarity in properties across a transition series is a result of the elements having similar electron configurations as they are filling the same d-orbital. This leads to consistent chemical and physical properties, although there can be significant differences in behaviour due to small variations in electron configuration.