Transition Metals and Coordination Compounds

Atomic Radius & Density of Transition Metals

The atomic radius of transition metals exhibits unique trends across periods and down groups in the periodic table. Understanding these trends is essential for predicting the physical properties and chemical behavior of transition metals.

Atomic Radius Trends Across a Period

• Definition: The atomic radius is the distance from the nucleus to the outermost electron shell of an atom.

• Main Group Elements: For main group elements, atomic radius decreases from left to right across a period due to increasing effective nuclear charge, which pulls electrons closer to the nucleus.

• Transition Metals: Transition metals follow a similar trend, but the change in size is less pronounced. This is because the number of outermost electrons (usually in the d subshell) remains relatively constant across the period, leading to smaller changes in atomic radius.

• Key Point: Across a period, the number of outermost electrons is constant, so the atomic radius decreases only slightly.

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• Example: Predict which element from each pair has the largest atomic size:

  • a) Ni & Ti

  • b) Tc & Ru

  • c) Rh & Nb

  • d) Y & Ag

Atomic Radius Trends Down a Group

When moving down a group in the transition metals, the expected increase in atomic radius is less than anticipated due to a phenomenon known as the lanthanide contraction.

• Lanthanide Contraction: This refers to the unexpected decrease in atomic radius observed in period 6 transition metals compared to period 5. It is caused by an increase in effective nuclear charge () due to the filling of the 4f subshell in the lanthanide series.

• Mechanism: The poor shielding effect of the 4f electrons means that the outer electrons experience a stronger pull from the nucleus, resulting in a smaller atomic radius than expected.

• Recall: More electrons in the same shell increase the attractive force, pulling electrons closer to the nucleus.

• Result: The atomic size of period 6 transition metals is nearly the same as that of period 5, despite the increase in principal quantum number.

• Example: Which of the following transition metals would you expect to be larger but are actually the same or nearly the same size as Tc (technetium)?

  • a) Zn

  • b) Mo

  • c) Ta

  • d) Re

Summary Table: Atomic Radii of Selected Transition Metals

Additional info: The atomic radii values for period 5 and 6 transition metals are nearly identical due to lanthanide contraction, which is not observed in main group elements.

Key Equations

• Effective Nuclear Charge: where is the atomic number and is the shielding constant.

Applications

• Understanding atomic radius trends helps explain the chemical reactivity, bonding, and physical properties of transition metals.

• Lanthanide contraction affects the separation and purification of rare earth elements and transition metals in industrial processes.