The reactivity of transition metals typically decreases as one moves from left to right across a period in the periodic table.

Transition metals occupy the central block of the periodic table, specifically within groups $3$ to $12$. As you progress from left to right across a period, the reactivity of these metals generally diminishes. This trend can be attributed to two primary factors: the increasing number of electrons in the outer shell and the rising nuclear charge. These changes make it more challenging for the atoms to lose electrons, thereby reducing their reactivity.

At the beginning of the transition metals series, we encounter Scandium ($\text{Sc}$) and Titanium ($\text{Ti}$). These elements exhibit relatively high reactivity due to their lower number of outer shell electrons, which facilitates electron loss and the formation of positive ions—an essential characteristic of metallic reactivity.

As we continue across the period, the number of outer shell electrons increases. For instance, Iron ($\text{Fe}$) has more outer shell electrons than Scandium, making it more difficult for Iron to lose electrons and form positive ions. Consequently, Iron displays lower reactivity compared to Scandium.

In addition to the increase in outer shell electrons, the nuclear charge—the total charge from all the protons in the nucleus—also rises across the period. This enhanced nuclear charge exerts a stronger attraction on the outer electrons, pulling them closer to the nucleus and making them more difficult to remove, further contributing to the decrease in reactivity.

It is important to note, however, that this trend is not entirely linear. There are notable exceptions due to the unique electronic configurations of certain transition metals. For example, Copper ($\text{Cu}$) and Zinc ($\text{Zn}$) exhibit lower reactivity than one might anticipate based on their position within the periodic table. This is primarily because both elements possess full or half-full $d$ sub-shells, which provide additional stability and reduce the likelihood of electron loss.

In summary, while there are exceptions to the observed trend, the general pattern is that the reactivity of transition metals decreases as we move from left to right across a period. This trend is driven by the increasing number of outer shell electrons and the escalating nuclear charge.