The Valence Shell Electron Pair Repulsion Vsepr Theory

To determine the correct decreasing order of negative electron gain enthalpy for the elements C, Ca, Al, F, and O, let's analyze the concept of electron gain enthalpy.

Electron gain enthalpy is the energy change that occurs when an electron is added to a neutral atom in the gaseous state. A more negative value indicates a higher tendency to gain an electron.

Analysis of the Elements:

• Fluorine (F): Has the highest electron gain enthalpy, as it is highly electronegative and readily accepts an electron to attain a stable electronic configuration.
• Oxygen (O): Has a high negative electron gain enthalpy but not as high as fluorine due to slightly less electronegativity.
• Carbon (C): Has less tendency to gain an electron compared to oxygen but more than aluminum and calcium due to its smaller size and relatively higher electronegativity.
• Aluminum (Al): Being a metal, has a lower electron gain enthalpy compared to nonmetals like carbon, but higher than calcium.
• Calcium (Ca): As an alkaline earth metal, it showcases the least tendency to gain an electron due to its larger size and low electronegativity.

Based on these observations, the correct decreasing order of negative electron gain enthalpy is:

The reason that Ti²⁺ is purple while Ti⁴⁺ is colorless lies in their electronic configurations and the concept of crystal field theory. Let's explore this in detail.

Electronic Configurations:
1. Ti²⁺ has a configuration of [Ar] 3d². This means that in its d-orbital, there are two electrons. Electrons in d-orbitals can absorb visible light, leading to electron transitions, which result in color.

2. Ti⁴⁺ has a configuration of [Ar] with empty 3d orbitals. Since there are no electrons in the d-orbitals, Ti⁴⁺ cannot absorb visible light, thus appearing colorless.

Crystal Field Effect:
The color in transition metal complexes is majorly due to the crystal field splitting of d-orbitals and the transitions of electrons between these split d-orbitals. In Ti²⁺, the presence of 3d electrons allows such transitions, causing the absorption of certain wavelengths of visible light (particularly green and yellow), and the reflection of other wavelengths (such as red and blue), which we perceive as purple. Conversely, for Ti⁴⁺, the absence of d-electrons means no such transitions can occur, thus no color is observable.

Therefore, the correct statement is Ti²⁺ has 3d² configuration.