Question

Explain why the transition metals in periods 5 anges have \(C\) nearly identical radii in each group.

Short answer

The transition metals in periods 5 and 6 have nearly identical radii in each group due to the lanthanide contraction. The lanthanide contraction, a result of poor shielding of 4f electrons, leads to a higher effective nuclear charge and stronger electron-proton attraction. This effect diminishes the expected increase in atomic radius as one moves down the periodic table, causing similar atomic radii for transition metals in periods 5 and 6.

Step-by-step solution

Understanding the lanthanide contraction

The lanthanide contraction refers to the steady decrease in atomic radii and ionic radii of elements along the lanthanide series (elements with atomic numbers 57 to 71). This occurs due to poor shielding of the 4f electrons, which results in an increased effective nuclear charge and increased attraction between the electrons and protons in the nucleus. Consequently, the atomic radii decrease despite the increase in the principal quantum number as one moves across the lanthanide series.

Relating the lanthanide contraction to transition metals

The lanthanide contraction has an effect on the transition metals located in periods 5 and 6 of the periodic table as well. The elements in period 6 are located after the lanthanides, and because the lanthanides show poor shielding of 4f electrons, their contraction is also experienced by the elements in period 6. This means that the increase in atomic radius that would typically be expected as one moves down the periodic table (due to an increase in the principal quantum number) is offset by the lanthanide contraction in the case of period 6 transition metals.

Comparing atomic radii of transition metals in periods 5 and 6

Due to the influence of the lanthanide contraction, the atomic radii of the transition metals in period 6 are not significantly larger than those in period 5, as one might expect based on their position in the periodic table. Instead, their radii are nearly identical within each group because the increase in atomic size resulting from an additional electron shell is counteracted by the higher effective nuclear charge and strong electron-proton attraction caused by poor shielding of the 4f electrons in the lanthanides. In conclusion, the transition metals in periods 5 and 6 have nearly identical radii in each group due to the lanthanide contraction, which diminishes the expected increase in atomic radius as one moves down the periodic table. The poor shielding of 4f electrons in the lanthanides leads to a higher effective nuclear charge and stronger electron-proton attraction, ultimately causing similar atomic radii for transition metals in periods 5 and 6.

Key concepts

Transition Metals

Transition metals are those elements found in the central block of the periodic table, specifically in groups 3 to 12. They include elements such as iron, copper, nickel, and platinum.
These metals are characterized by their ability to form variable oxidation states, and they often display properties like high melting points and electrical conductivity. Transition metals have incomplete d-orbitals, which allows them to form colorful compounds and exhibit paramagnetic properties.
Their chemistry is largely defined by the involvement of the d-electrons, which participate in bonding and can remove or accept electrons from chemical reactions. It is this flexibility in their electron configurations that makes them so versatile in forming various compounds. In periods 5 and 6, transition metals are influenced by the lanthanide contraction, which affects their atomic radii and other properties.

Atomic Radii

Atomic radii refer to the size of an atom, usually measured from the nucleus to the outermost electron orbital. As you move across a period in the periodic table, atomic radii generally decrease due to the increase in effective nuclear charge.
This compresses the electron cloud closer to the nucleus, despite an increase in the number of electrons.
However, moving down a group, you would typically expect atomic radii to increase because of the addition of electron shells. In the case of transition metals in periods 5 and 6, this pattern doesn't apply straightforwardly due to the lanthanide contraction.
This contraction leads to a similar atomic radii for transition metals in these periods because of the poor shielding effect of the 4f electrons, which enhances the effective nuclear charge.

Effective Nuclear Charge

Effective nuclear charge is the net positive charge experienced by an electron in an atom. It takes into account not just the positive charge of protons in the nucleus (nuclear charge) but also the shielding effect caused by electron-electron repulsions.
In simpler terms, as more electrons are added to an atom, especially those in inner shells, they provide a shielding effect that reduces the perceived positive charge reaching the outer electrons.
However, in the lanthanide series, this shielding is not perfect, particularly by the 4f orbitals. This poor shielding increases the effective nuclear charge experienced by the outer electrons.
Consequently, despite an increase in atomic number, the atomic radii of elements do not increase as expected, leading to a noticeable effect on transition metals in periods 5 and 6, where their atomic sizes become nearly identical.