"Transition metal elements" are the type of element which have atomic radii which remain relatively constant across a period.
Periods refer to the horizontal lines of the periodic table. Each period represents the sequential occupation of an atom's valence shell orbitals, also with longer periods representing the employment of an atom's d subshell orbitals.
As the atomic number rises, more electrons, as well as protons, are present, increasing attraction over time, which drives the atomic radius to shrink over time.
Therefore, "Transition metal elements" are the type of element which have atomic radii which remain relatively constant across a period.
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