Textbook Question
Briefly account for each of the following observations:
(a) Atomic radii decrease in the order Sc > Ti > V.
(b) Densities increase in the order Ti > V > Cr.
Ch.21 - Transition Elements and Coordination Chemistry
Chapter 21, Problem 45
The atomic radii of zirconium (160 pm) and hafnium (159 pm) are nearly identical. Explain.
Verified step by step guidance1
Understand the concept of atomic radii, which is the measure of the size of an atom from its nucleus to the outer boundary of the surrounding cloud of electrons.
Recognize that zirconium (Zr) and hafnium (Hf) are both in Group 4 of the periodic table, indicating they have similar chemical properties and electron configurations.
Consider the concept of the lanthanide contraction, which explains why elements in the same group but in different periods might have similar atomic radii. This phenomenon is due to the poor shielding effect of the f-orbitals in the lanthanides, which precedes hafnium in the periodic table.
Note that zirconium is in the 5th period and hafnium is in the 6th period of the periodic table. Despite being in different periods, the effect of lanthanide contraction makes their atomic radii very close.
Conclude that the nearly identical atomic radii of zirconium and hafnium can be attributed to their position in the periodic table and the influence of lanthanide contraction on hafnium, which compensates for the expected increase in atomic radius as one moves down a group in the periodic table.
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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Periodic Trends
Periodic trends refer to the predictable patterns observed in the properties of elements across the periodic table. Atomic radius generally decreases across a period due to increased nuclear charge, which pulls electrons closer to the nucleus. However, trends can vary in transition metals, where d-orbitals influence electron shielding and atomic size.
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Transition Metals
Transition metals are elements found in the d-block of the periodic table, characterized by their ability to form variable oxidation states and complex ions. The atomic radii of transition metals can be similar due to the filling of d-orbitals, which leads to comparable electron shielding effects and effective nuclear charge, as seen in zirconium and hafnium.
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Lanthanide Contraction
Lanthanide contraction refers to the phenomenon where the atomic and ionic radii of the lanthanide series decrease with increasing atomic number. This occurs due to poor shielding of the nuclear charge by the f-electrons, resulting in a stronger attraction between the nucleus and the outer electrons. This effect extends to the subsequent transition metals, contributing to the similar atomic sizes of zirconium and hafnium.
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