Question 34 — The Chemistry of Art (25 marks) Answer parts (a) and (b) of the question on pages 2–4 of the Section II Writing Booklet - HSC - SSCE Chemistry - Question 34 - 2015 - Paper 1

The coloration of transition metal ions in solution arises from the electronic transitions of their d electrons. Cr^2+(aq), which has the electronic configuration $[Ar] 3d^4$, allows electrons to transition between d orbitals when subjected to light, leading to absorption of certain wavelengths and therefore a characteristic color. In contrast, Zn^2+(aq) has the electronic configuration $[Ar] 3d^{10}$, where the d orbitals are fully filled. The absence of partially filled d orbitals in Zn^2+ means that there are no available energy levels for electronic transitions, resulting in a lack of color.

To observe the flame colors of metal ions, the following procedure can be taken:

1. Safety Precautions: Wear safety goggles and laboratory gloves to protect against splashes.
2. Use a clean wire loop or a metal spatula dipped in concentrated hydrochloric acid (HCl) to eliminate any residues.
3. Dip the clean loop into a sample of the metal salt, for example:
   • Lithium chloride (LiCl) for a red flame
   • Sodium chloride (NaCl) for a yellow flame
   • Potassium chloride (KCl) for a lilac flame.
4. Place the loop into the hottest part of a Bunsen burner flame and observe the color produced.
5. Repeat for each metal ion, ensuring to clean the loop between tests.

Flame tests are effective for detecting certain metals because the energy from the flame excites the electrons in the metal ions, causing them to emit visible light at specific wavelengths. However, only certain types of metal ions can provide distinct flame colors due to their electron configurations. Most notably, aluminum ions (Al^3+) cannot be identified using flame tests because they do not provide a characteristic flame color due to their higher ionization energy and the inability to effectively excite their electrons within common flame conditions.

The first ionisation energies of the elements in the third row of the Periodic Table show a trend that supports the existence of sub-shells. For instance, elements such as aluminum (Al) exhibit a lower ionisation energy compared to magnesium (Mg), demonstrating that the outer electrons of Al are in a higher energy p sub-shell. The $3p$ sub-shell, which contains the valence electrons in aluminum, requires less energy to remove an electron than the fully filled $3s$ sub-shell of magnesium. This observation indicates that the electron shielding and energy levels differ between s and p sub-shells, thus providing evidence for their existence.

The graph illustrating electronegativity trends across Periods 1, 2, and 3 demonstrates that electronegativity generally increases with increasing atomic number across a period. This can be attributed to the effective nuclear charge acting on the electrons; as the number of protons increases, the attraction between the nucleus and valence electrons also intensifies. Consequently, elements like fluorine exhibit high electronegativity due to having a small atomic radius and high effective nuclear charge, which pulls the bonding electrons closer. Conversely, elements with larger atomic radii have lower electronegativity as their electrons are further removed from the nucleus, reducing the effective pull.

The Bohr model of the atom provided significant insights into atomic structure by introducing the concept of quantized energy levels for electrons. According to Bohr, electrons orbit the nucleus at fixed distances, and when they absorb or emit energy, they transition between these levels. This concept of quantization explains the distinct lines seen in emission spectra when electrons return to lower energy levels, releasing specific wavelengths of light. The model laid the groundwork for the understanding that electrons exist in defined orbits, which correlate to the emission spectrum of an element. Although incomplete, the Bohr model was pivotal in bridging classical and quantum theories in atomic physics.