Question

In largely ionic compounds (e.g. NaBr and NaI), it seems to be generally true that, the greater the differences of electronegativity between the forming substances (i.e. The compound has more ionic character or has lower covalent character), the stronger its bond would be. (e.g. melting points: NaF>NaCl>NaBr>NaI with all of them having the same crystal structure. Pradyot, Patnaik (2003). Handbook of Inorganic Chemicals. The McGraw-Hill Companies, Inc. ISBN 978-0-07-049439-8.)

What I do not understand is that, as covalent bonds are not necessarily weaker than ionic bonds (For example, diamond and silicon both has very strong covalent bonds), why is it proper to claim that more ionic bonds have greater strength for such salts?

Answer 1

More ionic character generally leads to stronger ionic bonds in salts due to higher lattice energies, but this is not an absolute rule as other factors affect bond strengths. Covalent bonds in network solids can also be extremely strong, and the difference between ionic and covalent bond strengths is not simply related to electronegativity differences.

Step-by-step explanation:

The statement that more ionic bonds have greater strength than covalent bonds in the context of salts like NaBr and NaI involves considering the nature of ionic and covalent bonds. Ionic bonds involve the electrostatic attraction between oppositely charged ions, leading to formation of a rigid crystal lattice with high melting and boiling points.

As the difference in electronegativity between bonding atoms increases, the bond becomes more ionic and the crystal lattice energy increases, which can result in a higher melting point. However, this is not always directly proportional, and factors such as lattice energy, charge density, and ionic size play important roles.

Covalent bonds, particularly in network solids like diamond or silicon, involve the sharing of electrons and can also result in very strong materials with high melting points. These bonds form between nonmetal atoms and can manifest as gases, liquids, or solids at room temperature, typically with lower melting points than ionic compounds unless they are part of a large network structure.