Element \(\mathrm{M}\) is a shiny and highly reactive metal (melting point \(\left.63^{\circ} \mathrm{C}\right)\), and element \(\mathrm{X}\) is a highly reactive nonmetal (melting point \(-7.2^{\circ} \mathrm{C}\) ). They react to form a compound with the empirical formula MX, a colorless, brittle white solid that melts at \(734^{\circ} \mathrm{C} .\) When dissolved in water or when in the molten state, the substance conducts electricity. When chlorine gas is bubbled through an aqueous solution containing MX, a reddish-brown liquid appears and \(\mathrm{Cl}^{-}\) ions are formed. From these observations, identify \(\mathrm{M}\) and \(\mathrm{X}\). (You may need to consult a handbook of chemistry for the melting-point values.)

Understanding the properties of metals and nonmetals is crucial for predicting their reactions and the types of compounds they form. Metals, typically found on the left side of the periodic table, are shiny, malleable, ductile, have good heat and electricity conductivity, and tend to lose electrons to form positive ions. Nonmetals, located on the right side, are usually brittle in solid form, do not conduct heat or electricity well, and tend to gain electrons to form negative ions.

When metals and nonmetals react, they form ionic compounds. This is the result of metal atoms transferring electrons to nonmetal atoms, creating ions that are held together by strong electrostatic forces, known as ionic bonds. For example, in our exercise, when the shiny, highly reactive metal M reacts with the highly reactive nonmetal X, they form a compound with the empirical formula MX. The high melting point of the compound indicates the strong ionic bond formed between M and X.

The conductivity of a substance in its molten and aqueous states is an important characteristic that often indicates the presence of ions. Ionic compounds like MX are typically solid at room temperature and do not conduct electricity. However, when these compounds are dissolved in water or melted, they disassociate into their constituent ions, which are free to move and carry electrical charge.

In our textbook exercise, the fact that MX conducts electricity in its molten state or when dissolved in water supports the conclusion that MX is an ionic compound formed from the reaction of a metal with a nonmetal. This property is helpful for identifying the nature of the elements involved and confirming the type of compound formed through their reaction.

The empirical formula of a compound represents the simplest whole number ratio of the elements within that compound. For ionic compounds, this often equates to the ratio of cations to anions. In the process of determining the empirical formula, the valencies or charges on the ions must balance to result in a neutral compound.

In the case of the compound MX from our exercise, the empirical formula suggests that one atom of a metal M combines with one atom of a nonmetal X to form an electrically neutral compound. This information is crucial as it tells us the metal M most likely forms a +1 ion and the nonmetal X forms a -1 ion, leading to the deduction that M could be the metal, sodium, with a valency of +1, and X could be the nonmetal, fluorine, with a valency of -1.

Displacement reactions are a type of chemical reaction where an element reacts with a compound and displaces another element within that compound. These reactions typically occur when a more reactive element displaces a less reactive one. For a metal displacement, this follows the activity series of metals, and for halogens, it follows their reactivity trend down the group.

In our exercise, chlorine gas is bubbled through an aqueous solution of MX, causing a reddish-brown liquid to form and chloride ions to appear in the solution. This indicates that chlorine, being more reactive than the halogen in MX, has displaced it. The reddish-brown liquid is a hint at the identity of the displaced nonmetal, aligning with the characteristic color of fluorine gas. This type of chemical evidence is often used to conclude the identity of components in displacement reactions.