The atomic radii of Y (180 pm) and La (187 pm) are significantly different, but the radii of Zr (160 pm) and Hf (159 pm) are essentially identical. Explain.

Verified step by step guidance

Step 1: Understand the concept of atomic radii, which is the distance from the nucleus of an atom to the outermost electron shell. It is influenced by the number of electron shells and the effective nuclear charge.

Step 2: Recognize that Yttrium (Y) and Lanthanum (La) are elements in the same group of the periodic table but in different periods. Y is in the 5th period, and La is in the 6th period, which means La has an additional electron shell compared to Y, leading to a larger atomic radius.

Step 3: Consider the concept of the lanthanide contraction, which is the phenomenon where the atomic radii of the lanthanide series elements decrease with increasing atomic number. This is due to the poor shielding effect of the f-electrons, which results in a greater effective nuclear charge and a smaller atomic radius.

Step 4: Apply the concept of lanthanide contraction to Zirconium (Zr) and Hafnium (Hf). Although Hf is in the 6th period and Zr is in the 5th period, the lanthanide contraction causes Hf to have a similar atomic radius to Zr despite having more electron shells.

Step 5: Conclude that the significant difference in atomic radii between Y and La is due to the additional electron shell in La, while the similar radii of Zr and Hf are a result of the lanthanide contraction, which offsets the expected increase in size due to the additional electron shell in Hf.

Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Atomic Radius

Atomic radius is a measure of the size of an atom, typically defined as the distance from the nucleus to the outermost electron shell. It can vary significantly across different elements due to factors such as electron shielding and effective nuclear charge. Generally, atomic radii increase down a group in the periodic table and decrease across a period.

Lanthanide Contraction

Lanthanide contraction refers to the phenomenon where the atomic and ionic radii of the lanthanide series elements decrease with increasing atomic number. This occurs due to the poor shielding effect of the f-electrons, leading to a greater effective nuclear charge felt by the outer electrons. As a result, elements like Y and La exhibit larger differences in atomic radii compared to transition metals like Zr and Hf.

Transition Metals and Similar Radii

Transition metals often exhibit similar atomic radii due to their comparable electron configurations and the presence of d-electrons. In the case of Zr and Hf, both elements belong to the same group and have similar effective nuclear charges, resulting in nearly identical atomic sizes. This trend is common among transition metals, where the addition of electrons occurs in the inner d-orbitals, leading to minimal changes in atomic radius.

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