Question

The melting point of ionic solids depends on the magnitude of the electrostatic attractions that hold the solid together. Draw ionic Lewis structures for \(\mathrm{NaF}\) and \(\mathrm{MgO}\). Which do you think has the higher melting point?

Short answer

MgO should have a higher melting point than NaF due to the stronger electrostatic attraction between the Mg2+ and O2- ions as compared to the Na+ and F- ions.

Step-by-step solution

Draw the Lewis structure for NaF

To draw the Lewis structure for NaF, first determine the valence electrons. Sodium (Na) has one valence electron and fluorine (F) has seven. Sodium gives up its one electron to fluorine, resulting in a Na+ cation and an F- anion. Represent Na+ by Na with no dots around it (as it has lost its valence electron) and for F-, draw F with eight dots representing its valence electrons (seven from fluorine itself and one gained from sodium).

Draw the Lewis structure for MgO

Magnesium (Mg) has two valence electrons and oxygen (O) has six. Magnesium gives up its two electrons to oxygen, leading to a Mg2+ cation and an O2- anion. Represent Mg2+ by Mg with no dots around it (as it has lost its two valence electrons) and for O2-, draw O with eight dots representing its valence electrons (six from oxygen itself and two gained from magnesium).

Predict the higher melting point

To predict which compound has a higher melting point, consider the charges of the ions; Mg2+ and O2- have higher charges than Na+ and F-. The electrostatic attraction between ions with higher charges is stronger; therefore, MgO, with a Mg2+ cation and an O2- anion, should have a higher melting point than NaF, which contains Na+ and F- ions with lower charges.

Key concepts

Understanding Lewis Structures

When beginning to explore the intricacies of chemical bonding, one of the most fundamental tools is the Lewis structure. This schematic diagram aids in visualizing the distribution of valence electrons around atoms, thereby illuminating the bond formation within molecules and ionic compounds.

For example, in the Lewis structure of NaF, sodium (Na) is depicted without electrons around it, signifying it as a Na+ cation. Fluorine (F) is shown surrounded by eight electrons, indicating a full octet and its existence as an F- anion. The transfer of an electron from sodium to fluorine is the key to their ionic bond formation, which the Lewis structure aptly represents.

In an ionic context, drawing Lewis structures also underscores the nature of the ionic solid. By clearly showing the loss and gain of electrons, students can almost 'see' the charged entities and start to relate to the next concept: electrostatic attractions.

The Role of Electrostatic Attractions

The melting point of ionic solids can be largely attributed to electrostatic attractions, which are the forces holding oppositely charged ions together. These forces are incredibly strong due to the nature of ionic bonds, where complete transfer of electrons results in fully charged ions.

For instance, in NaF and MgO, the cations Na+ and Mg2+ respectively are attracted to the anions F- and O2-. However, the strength of the attraction differs between the two compounds. MgO boasts a Mg2+ and an O2-, both with charges of +2 and -2, leading to a stronger attraction between them as compared to the +1 and -1 charges in NaF. This intensified attraction in MgO means that more energy, hence a higher temperature, is required to disrupt the ionic lattice and melt the compound, resulting in a higher melting point.

Significance of Valence Electrons

Valence electrons are at the heart of understanding chemical reactivity and bond formation. These are the electrons located in the outermost shell of an atom and are the ones involved in bonding. In the context of ionic compounds like NaF and MgO, valence electrons define the charge of the resulting ions after they are transferred from one atom to another.

Returning to our examples, sodium has one valence electron which it donates to fluorine, which has seven. This electron transfer forms a Na+ ion with no valence electrons and an F- ion with eight valence electrons (a full octet). Magnesium, having two valence electrons, transfers both to oxygen, which has six valence electrons. The resulting Mg2+ and O2- ions have no and eight valence electrons respectively. The complete transfer solidifies the ionic bond and shapes the strength of the electrostatic attractions, directly influencing the physical properties, like melting points, of the ionic solids.