How do transition metals' radii change across the periodic table?

Transition metals' radii generally decrease across the periodic table due to increasing nuclear charge.

As you move across the periodic table from left to right, the atomic radii of transition metals typically decrease. This is primarily due to the increase in nuclear charge, which pulls the electrons closer to the nucleus, reducing the atomic radius. This trend is not as straightforward as it is in the main group elements, but it is still noticeable.

The transition metals are found in groups 3-12 of the periodic table. These elements have partially filled d orbitals. As you move from left to right across a period, electrons are added one at a time to the d orbital. Simultaneously, protons are being added to the nucleus. This increase in nuclear charge attracts the electrons more strongly, pulling them closer to the nucleus and causing the atomic radius to decrease.

However, the decrease in atomic radius is not as pronounced in transition metals as it is in the main group elements. This is because the electrons being added to the d orbitals experience a shielding effect. The inner electrons shield the outer electrons from the full effect of the nuclear charge, which somewhat counteracts the decrease in atomic radius.

In addition, the d orbitals are more diffuse than the s and p orbitals, meaning they extend further from the nucleus. This can also contribute to a less significant decrease in atomic radius across the transition metals.

It's also worth noting that there are exceptions to this general trend. For example, the atomic radii of the elements in the second and third transition series do not always decrease across the period. This is due to the lanthanide contraction, a phenomenon where the 4f electrons in the lanthanides do not shield the increasing nuclear charge as effectively as expected, resulting in smaller atomic radii.

In summary, while the atomic radii of transition metals generally decrease across the periodic table due to increasing nuclear charge, the trend is not as straightforward as in the main group elements due to factors such as the shielding effect and the diffuse nature of the d orbitals.