Answer
Their outermost electrons are $4s^2$ electrons, so all 10 of the transition elements have similar chemical properties.
Work Step by Step
The elements with Z=21 to Z=30, as we can see from table 41.3, occur while the smaller 3d subshell is filling up with its 10 electrons. The already-filled $4s^2$ subshell, with a greater radius than the 3d subshell, makes the transition elements appear the same to the outside world.
Therefore, all 10 of the transition elements have similar chemical properties.
