Charge Density and Melting Point

What is Charge Density?

The density of charge around an ion is referred to as its charge density. Silberberg defines it as follows:

“The ratio of an ion’s charge to its volume.”

Charge density is equal to charge/volume.

Table of Contents

• Factors Affecting Charge Density
• The trend of Charge Density in the Periodic Table
• What is Melting Point?
• Relationship between Charge Density and Melting Point
• Frequently Asked Questions – FAQs

Factors Affecting Charge Density

The factors influencing the strength of an ion’s charge density are the ion’s charge (e.g., 2+ for Mg, 1+ for Na) and the effective volume over which that charge acts – this is where the ionic radius comes in.

Since the magnesium ion and sodium ion have the same electronic configuration, namely that of neon (because Mg has lost two electrons and Na has lost one, and they now both have ten electrons), the charge of the electrons surrounding the ions is the same.

Magnesium, on the other hand, has one more proton in its nucleus. As a result, magnesium has a smaller radius. Mg²⁺ has an ionic radius of 72 pm, while Na⁺ has an ionic radius of 102 pm.

And since magnesium has a lower volume and a higher charge, its charge density is higher than that of sodium.

The trend of Charge Density in the Periodic Table

The charge of the ion formed from a particular element remains constant on moving down a group, but the outer electron moves further away from the nucleus due to more shielding from more full inner electron shells, and the increase in shielding is greater than the increase in proton number. This means that the atomic radius/size of the electron cloud is larger, and the outermost electron is at a higher energy level, which means it has more intrinsic energy and is easier to remove because of the electrostatic forces of attraction between the outer electron and the nucleus are weaker.

As a result, as the charge remains constant while atomic radius increases, charge density decreases from top to bottom in a group.

As the period progresses, the proton number increases while the shielding remains constant because the outermost electron is shielded to the same extent by the same number of full inner electron shells. This means that as the outermost electron approaches the nucleus, it is at a lower energy level (has lower intrinsic energy) and thus requires more energy to remove due to stronger electrostatic forces of attraction between the outer electron and the nucleus. As a result, the atomic radius/size of the electron cloud of an atom of a specific element decreases over time.

As a result, as the number of protons increases and the atomic radius decreases over time, the charge density increases from left to right.

What is Melting Point?

The melting point is typically defined as the temperature at which a material transition from a solid to a liquid.

The melting point of a liquid is the temperature at which solid changes from solid to liquid at atmospheric pressure. This is the point where both the liquid and solid phases are in equilibrium. The substance’s melting point varies with pressure and is specified at standard pressure.

Relationship between Charge Density and Melting Point

The melting points of giant ionic structures are extremely high. The strength of inter-ionic attraction is determined by the ion’s charge and the size of the ions. Structures containing double-charged ions have significantly higher melting points than structures containing single-charged ions.

The melting point is affected by the ion’s size and charge density.

The smaller the ion, the greater the charge density and the stronger the forces between the ions, which results in a higher melting point. As a result, KCl has a lower m.p. than NaCl.

It is important to note that lithium salts do not follow this pattern. According to electrostatic attraction force theory, lithium chloride should have a higher melting point than sodium chloride. The high charge density of lithium ions explains why it does not (due to their small size). These polarise the electron shell of the much larger negative ion, resulting in a degree of covalent character in the lattice that lowers the melting point.

For the same reasons, beryllium chloride is a simple covalent molecule. The high charge density of the beryllium 2+ ion (which is even smaller than lithium and has a double charge) would repolarise the chloride ions and produce covalent bonds.

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