Transition Metals and Coordination Compounds

Atomic Radius & Density of Transition Metals

The atomic radius of transition metals is influenced by their position in the periodic table and unique electronic configurations. Understanding these trends is essential for predicting chemical and physical properties of transition metals.

Atomic Radius Trends

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

• Periodic Trends:

  • Across a Period: For main group elements, atomic radius generally decreases from left to right due to increasing nuclear charge pulling electrons closer. In transition metals, the change in size is more subtle because the number of outermost electrons (d electrons) remains relatively constant across the period.

  • Down a Group: Atomic radius generally increases as you move down a group due to the addition of electron shells. However, for transition metals, the increase is less pronounced between periods 5 and 6 due to the lanthanide contraction.

• Lanthanide Contraction:

  • Occurs due to poor shielding by the 4f electrons, resulting in a higher effective nuclear charge ().

  • This increased nuclear charge pulls the electrons closer, causing a smaller than expected atomic radius for period 6 transition metals.

  • Effective Nuclear Charge (): The net positive charge experienced by valence electrons. It increases due to the filled 4f subshell.

Periodic Table of Transition Metals (Selected Atomic Radii)

The following table summarizes the atomic radii (in picometers, pm) for selected transition metals in periods 4, 5, and 6:

Additional info: Only selected atomic radii are shown for clarity. The trend demonstrates the effect of lanthanide contraction, as period 6 elements (e.g., W, Re, Os) have atomic radii similar to their period 5 counterparts.

Key Points

• Across a period: The number of outermost electrons (d electrons) is constant, so atomic radius changes only slightly.

• Down a group: The expected increase in atomic radius from period 5 to 6 is offset by lanthanide contraction, resulting in similar atomic sizes.

• Lanthanide contraction: Caused by increased effective nuclear charge due to poor shielding by 4f electrons, leading to smaller atomic radii for period 6 transition metals.

Examples

• Example 1: Predicting atomic size in pairs:

  • a) Ni & Ti: Ti is larger (Ni is further right in the period).

  • b) Tc & Ru: Tc is larger (Ru is further right).

  • c) Rh & Nb: Nb is larger (Nb is further left).

  • d) Y & Ag: Y is larger (Ag is further right).

• Example 2: Which 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

  • Answer: c) Ta and d) Re (period 6 elements) are expected to be larger, but due to lanthanide contraction, their atomic radii are similar to Tc.

Formulas and Equations

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

Additional info: For more details on effective nuclear charge and periodic trends, refer to the topic "Periodic Trend: Effective Nuclear Charge".