Question

The lanthanide contraction explains which of the following periodic trends? (a) The atomic radii of the transition metals first decrease and then increase when moving horizontally across each period. (b) When forming ions the transition metals lose their valence s orbitals before their valence $d$ orbitals. (c) The radii of the period 5 transition metals (Y-Cd) are very similar to the radii of the period 6 transition metals (Lu-Hg).

Short answer

The correct choice is (c), as it accurately describes the periodic trend observed due to the lanthanide contraction. The radii of period 5 transition metals (Y-Cd) are very similar to the radii of period 6 transition metals (Lu-Hg) because of the effect of the lanthanide contraction on the atomic radii.

Step-by-step solution

Option (a) analysis

The atomic radii of the transition metals do exhibit some variation while moving across each period, with a general decrease followed by an increase. However, this trend is not directly attributed to the lanthanide contraction, but rather it's caused by factors like increasing nuclear charge and the addition of electrons. The lanthanide contraction specifically refers to atomic radii variations in the lanthanide series.

Option (b) analysis

The fact that transition metals lose their valence s orbitals before their valence d orbitals is indeed related to the energetic ordering of the orbitals. However, this is not a trend directly explained by the lanthanide contraction, which is more related to atomic radii variations.

Option (c) analysis

The statement claims that the radii of period 5 transition metals (Y-Cd) are very similar to the radii of period 6 transition metals (Lu-Hg), discussing a trend regarding atomic radii. In fact, this is the trend directly correlated to the lanthanide contraction. Due to the poor shielding by the 4f electrons, the effective nuclear charge experienced by the outer electrons increases, leading to the observed decrease in atomic radii. This causes the elements of the d-block in period 5 to have atomic radii comparable to their period 6 counterparts, despite being one period apart.

Answer

Based on our analysis of the given options, the correct choice is (c), as it accurately describes the periodic trend observed due to the lanthanide contraction. The radii of period 5 transition metals (Y-Cd) are very similar to the radii of period 6 transition metals (Lu-Hg) because of the effect of the lanthanide contraction on the atomic radii.

Key concepts

Atomic Radii

Atomic radii refer to the size of an atom from its nucleus to the boundary of the surrounding cloud of electrons. It's an important measure as it is related directly to the volume of the atom.
Several factors can influence the atomic radii of elements, such as the nuclear charge, electron-electron repulsions, and electron shielding. The lanthanide contraction is a phenomenon mainly affecting the atomic radii. It occurs in elements with atomic numbers from 57 (La) to 71 (Lu), which are known as the lanthanides.
In these elements, as protons are added to the nucleus, electrons are added to the increasingly diffuse 4f subshell. Despite more electrons being added, their poor shield capacity results in a stronger pull of the electrons towards the nucleus.
This tightening manifests in a steady decrease in atomic size from La to Lu.
Interestingly, this contraction impacts the subsequent transition metals in period 6 because the elements end up with smaller radii than expected.

Periodic Trends

Periodic trends are patterns observed among the elements in the periodic table, such as variations in atomic radii, electronegativity, ionization energy, and more.
These trends stem from the regular increase of atomic numbers and the arrangement of electrons in each shell. In the context of atomic radii, a common periodic trend is that radii decrease across a period. This is attributed to the increasing effective nuclear charge, drawing electrons closer to the nucleus. However, the lanthanide contraction can influence this trend.
Because of the lanthanide contraction, the radii of the transition metals in period 5 (Y-Cd) and period 6 (Lu-Hg) can appear unexpectedly similar, despite being separated by an entire additional shell of electrons, as seen in option (c) of the exercise.

Transition Metals

Transition metals are found in the center of the periodic table spanning across groups 3 to 12.
They are known for their unique ability to form various oxidation states and colored compounds.
Transition metals play a crucial role in several biological processes and industrial applications. Their atomic structure, characterized by the filling of the d subshell, endows them with specific properties like high melting points, electrical conductivity, and malleability.
An interesting aspect of transition metals, highlighted in the exercise, is their atomic radii pattern.
As the transition metals span a period, there is generally a decrease in atomic radii due to the increase in nuclear charge. However, this trend can be disrupted by the lanthanide contraction between periods 5 and 6, resulting in similarly sized atoms.
Understanding this relationship between lanthanide contraction and transition metals is crucial for explaining why elements like yttrium (Y) and lutetium (Lu) have comparable atomic radii.