Revise General physical and chemical properties of the first row of transition elements, titanium to copper for Cambridge 9701 (28.1)

Scandium (Sc) is in the d-block but is NOT a transition element. Its only stable ion is Sc³⁺, with the configuration [Ar] 3d⁰. The d-subshell is empty.

Zinc (Zn) is also in the d-block but is NOT a transition element. Its only stable ion is Zn²⁺, with the configuration [Ar] 3d¹⁰. The d-subshell is full.

Elements from Titanium (Ti) to Copper (Cu) all form at least one stable ion with a partially filled d-subshell (e.g., Ti³⁺ is 3d¹, V²⁺ is 3d³, Cr³⁺ is 3d³, Mn²⁺ is 3d⁵, Fe²⁺ is 3d⁶, Co²⁺ is 3d⁷, Ni²⁺ is 3d⁸, Cu²⁺ is 3d⁹).

Filling Order: Electrons fill the 4s subshell before the 3d subshell.

Ionisation Rule: Electrons are removed from the 4s subshell before the 3d subshell.

Exceptions: To achieve greater stability, Chromium and Copper are exceptions to the filling order. Chromium is [Ar] 3d⁵ 4s¹ (stable half-filled d-subshell) and Copper is [Ar] 3d¹⁰ 4s¹ (stable full d-subshell).

Variable Oxidation States: Because the 4s and 3d electrons are so close in energy, they can be lost with similar energy input. This allows elements like Manganese to exhibit a wide range of oxidation states, from +2 (in Mn²⁺) to +7 (in MnO₄⁻). The most common oxidation state is often +2, corresponding to the loss of the two 4s electrons.

Formation of Coloured Ions: When ligands bond to a central transition metal ion to form a complex, the d-orbitals are split into two different energy levels. If the d-subshell is partially filled, an electron can absorb a photon of visible light and be promoted to the higher energy level. This process is called a d-d transition. The light that is not absorbed is transmitted or reflected, which we perceive as the colour of the complex. For example, aqueous [Cu(H₂O)₆]²⁺ absorbs orange light and appears blue.

Catalytic Activity: Transition metals and their compounds are exceptional catalysts. This is due to two main features: 1) Their ability to exist in variable oxidation states allows them to act as electron carriers in redox reactions (homogeneous catalysis). 2) They can use their vacant d-orbitals to adsorb reactant molecules onto their surface, weakening bonds and lowering the activation energy (heterogeneous catalysis). A classic example is the use of an Iron catalyst in the Haber process.

Atomic and Ionic Radii: There is a slight decrease in atomic radius from Ti to Cu, but the change is much less dramatic than across period 3 s- and p-block elements. As we move across the period, a proton is added to the nucleus and an electron is added to the 3d subshell. The 3d electrons are poor at shielding, so the effective nuclear charge increases, pulling the electron shells closer. However, the increased electron-electron repulsion in the 3d subshell partially counteracts this, leading to a relatively constant radius.

Density: Density is calculated as mass/volume. Across the period, the atomic mass increases significantly. However, as we've seen, the atomic radius (and thus volume) remains relatively constant. The combination of increasing mass over a similar volume leads to a steady and significant increase in density from Titanium (4.5 g cm^{-3}) to Copper (8.9 g cm^{-3}).