Hardness Parameters in Chemistry

Study Notes

Hardness parameters are used to describe the ability of a central metal ion to polarize or deform nearby molecules or ions.

Hardness (η): A measure of the resistance of an atom or group to deformation or polarizability.
Hard acid: A metal ion that is hard, tends to form bonds with hard bases, and is characterized by high oxidation states, small size, and high electronegativity.
Softness (σ): A measure of the ease of deformation or polarizability of an atom or group.

Fajans' Rule is used to predict the covalent character of an ionic bond.

States that the smaller the cation and the larger the anion, the more covalent the bond.
Factors that contribute to covalent character:
- Polarization of anion by cation: The cation's ability to distort the electron cloud of the anion.
- Polarizability of anion: The ease with which the anion's electron cloud can be distorted.
- Charge density of cation: The higher the charge density, the greater the polarization of the anion.
Fajans' Rule is often used to rationalize the behavior of ionic compounds, such as their solubility and reactivity.

Hard acids tend to form bonds with hard bases, while soft acids tend to form bonds with soft bases.
The hardness of a metal ion determines its ability to form ionic or covalent bonds.
Fajans' Rule provides a framework for understanding the covalent character of ionic bonds and the behavior of ionic compounds.

Hardness (η) measures the resistance of an atom or group to deformation or polarizability.
Hard acid: a metal ion that is hard, tends to form bonds with hard bases, and is characterized by high oxidation states, small size, and high electronegativity.
Softness (σ) measures the ease of deformation or polarizability of an atom or group.

Predicts the covalent character of an ionic bond based on the sizes of the cation and anion.
The smaller the cation and the larger the anion, the more covalent the bond.
Factors contributing to covalent character include:
Polarization of anion by cation: the cation's ability to distort the electron cloud of the anion.
Polarizability of anion: the ease with which the anion's electron cloud can be distorted.
Charge density of cation: the higher the charge density, the greater the polarization of the anion.

Hard acids tend to form bonds with hard bases, while soft acids tend to form bonds with soft bases.
The hardness of a metal ion determines its ability to form ionic or covalent bonds.
Fajans' Rule helps understand the covalent character of ionic bonds and the behavior of ionic compounds, including their solubility and reactivity.