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Understanding Charge Density and Melting Point - Testbook
IMPORTANT LINKS
The Solid State
Defining Charge Density
Charge density can be simply understood as the measure of electric charge per unit volume around an ion. In other words:
It is the quotient of the charge of an ion and its volume.
Charge density can be calculated using the formula - charge/volume.
Elements Impacting Charge Density
The charge density of an ion is influenced by the ion’s charge (for instance, Mg has a 2+ charge, while Na has a 1+ charge) and the effective volume over which that charge is spread. This is where the concept of ionic radius comes into play.
For instance, consider magnesium and sodium ions, both having the same electronic configuration, that of neon. This is because Mg loses two electrons and Na loses one, resulting in both having ten electrons. However, Magnesium has an additional proton in its nucleus, which makes its radius smaller. Mg 2+ has an ionic radius of 72 pm, while Na + has an ionic radius of 102 pm.
Since magnesium has a lower volume and a higher charge, its charge density is higher than that of sodium.
Charge Density Trends in the Periodic Table
As we move down a group in the periodic table, the charge of the ion remains constant, but the outer electron moves further from the nucleus due to increased shielding from more full inner electron shells. This leads to an increase in the atomic radius or size of the electron cloud, which in turn makes the outermost electron easier to remove. Therefore, the charge density decreases from top to bottom in a group.
Hence, with a constant charge and an increasing atomic radius, charge density decreases from top to bottom in a group.
Across a period, the proton number increases while the shielding remains the same. This leads to a decrease in the atomic radius or size of the electron cloud of an atom. Consequently, the charge density increases from left to right.
Thus, as the proton number increases and the atomic radius decreases, the charge density increases from left to right.
Defining Melting Point
The melting point is the temperature at which a substance changes from a solid to a liquid state.
More specifically, it is the temperature at which both the solid and liquid phases of a substance are in equilibrium at atmospheric pressure. The melting point of a substance can vary with pressure and is usually specified at standard pressure.
The Interplay between Charge Density and Melting Point
Substances with giant ionic structures have very high melting points. These melting points are determined by the charge of the ion and the size of the ions. Structures that contain double-charged ions have significantly higher melting points than those with single-charged ions.
| Only Single Charged Ions | Only Double Charged Ions | ||
|---|---|---|---|
| Compound | M.P. / ºC | Compound | M.P. / ºC |
| NaCl | 801 | MgO | 2800 |
| KCl | 776 | CaO | 2572 |
| LiF | 848 | MgS | >2000 |
| LiCl | 605 | CaS | 2400 |
The melting point is influenced by the size of the ion and its charge density.
The smaller the ion and the greater its charge density, the stronger the forces between the ions, leading to a higher melting point. Therefore, KCl has a lower m.p. than NaCl.
It is worth noting that lithium salts do not follow this pattern. Despite the high charge density of lithium ions (due to their small size), lithium chloride does not have a higher melting point than sodium chloride. This is because the lithium ions polarise the electron shell of the much larger negative ion, resulting in a certain degree of covalent character in the lattice that lowers the melting point.
Similarly, 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) polarises the chloride ions and forms covalent bonds.
Frequently Asked Questions
Is there a correlation between density and melting point?
The melting point is affected by the size and charge density of the ion. The greater the charge density and the stronger the forces between the ions, the higher the melting point. As a result, KCl has a lower molecular weight than NaCl.
What does it mean to have a higher charge density?
In chemistry, it can refer to the charge distribution of a particle, such as a molecule, atom, or ion, over its volume. As a result, despite having more electrons than lithium, a lithium cation will carry a higher charge density than a sodium cation due to the lithium cation’s smaller ionic radius.
What is the periodic trend of charge density in the periodic table?
The periodic trend is that charge density decreases as one moves down the periodic table because the charge remains constant, but the size increases. As the charge increases and the size decreases, the charge density increases across (as it does for Mg and Na).
Which element has the highest charge density?
The element that has the highest charge density is Osmium.
Does higher density mean a higher melting point?
A high density and a high melting point indicate a low reactivity; a low density and a low melting point indicate a high reactivity.