Answer:
The next to outermost electron shell begins to fill up.
Explanation:
Transition elements are a group of elements that are present in the center of the periodic table. These are essentially metals that occupy groups 3-12.
They are also known as the d-block elements as it is the inner d-orbital that gets progressively filled with electrons on moving from left to right across a period while the outer s-orbital remains filled. The general electron configuration is: (n-1)d¹⁻¹⁰ns²
Thus the common characteristic of the transitions elements is that the next to outermost electron shell begins to fill up i.e. the d orbitals fill up relative to the outer s-orbital
