Transition Metals Study Notes

Introduction to Transition Metals

• Focus for the semester on transition metals, studying them by group.

• Application of previously discussed concepts to learn interesting properties of the elements.

Transition Metals Overview

• Definition: Transition metals refer to elements characterized often by their metallic properties, particularly those that exhibit some common characteristics through groups 3 to 12 of the periodic table.

• Clarification of Terms:

  • Atoms vs Ions: Distinction between discussing atoms of metals, cations, and complexes is crucial.

  • Transition metals are typically referred to as a chunk of metal.

Characteristics of Transition Metals

• Metallic Bonding:

  • Strong metallic bonding is a defining characteristic.

  • Described by orbital band theory: numerous electrons in the orbital bands result in strong attraction between electrons and protons in the nucleus.

  • Tightly bonded atoms result in high melting points.

• Melting Points:

  • Good indicator of atomic attraction.

  • 10 transition metals have melting points above $2000 ext{°C}$.

  • 3 transition metals exceed melting points of $3000 ext{°C}$.

  • For context, a Bunsen burner flame is roughly between $1500 ext{°C}$ and $1600 ext{°C}$, which is marginally sufficient to melt iron (melting point: $1550 ext{°C}$).

• Density:

  • Transition metals have high densities due to strong atomic attraction, resulting in close packing of atoms.

  • Iridium and Osmium (elements 76 and 77) have the highest densities (~$23 ext{ g/cm}^3$), significantly more than water which has a density of 1.

  • Example: A football-sized piece of iridium weighs approximately $500 ext{ kg}$.

  • Controversy exists among metrologists regarding density comparisons between iridium and osmium.

Reactivity of Transition Metals

• Transition metals are generally characterized as fairly unreactive due to:

  • Passivation: A crucial process where the surface atoms react with moisture and/or oxygen, creating a protective layer of metal oxide that prevents further oxidation of the underlying metal.

  • Example in practice: Chromium forms a layer of chromium(III) oxide ($Cr_2O_3$), typically only a few nanometers thick, preventing the bulk material from oxidizing effectively.

  • However, iron is an exception, known for rusting completely due to the different structure of its oxidation product, iron(III) oxide ($Fe_2O_3$), which does not adhere well to the underlying metal.

Understanding Passivation

• Importance of Passivation:

  • Enables certain transition metals like chromium, titanium, and copper to resist corrosion.

  • Iron's oxidation creates flaking rust, continuously exposing new metal surface, leading to deterioration.

  • Only iron does not develop this protective oxide layer effectively; it continuously reacts with oxygen and moisture.

Trends in Transition Metals

• Size of Ions:

  • Discussed trends concerning the size of ions; cations generally grow larger as we go down the group.

  • Notably, vanadium(III), niobium(III), and tantalum(III) exhibit similar sizes due to the same charge density and other physical properties impacting size.

• Oxidation States:

  • General trend: First-row transition metals tend to have lower oxidation states compared to the second and third-row transition metals.

  • Smaller atomic size = higher attraction = more energy required to remove electrons.

  • Trends reveal that the left half of transition metals shows greater tendency towards higher oxidation numbers than the right half.

Crystal Field Stabilization Energy (CFSE)

• CFSE explains how the d-orbitals split when in a complex:

  • For instance, Cobalt in a strong field ligand complex shows significant CFSE indicating greater stability compared to iridium.

  • As d-orbitals increase in size, CFSE increases due to the increased repulsion experienced between electrons.