Question

Which of the following are ionic, and which are molecular? (a) \(\mathrm{PF}_{5},\) (b) \(\mathrm{NaI}\), (c) \(\mathrm{SCl}_{2}\) (d) \(\mathrm{Ca}\left(\mathrm{NO}_{3}\right)_{2}\), (e) \(\mathrm{FeCl}_{3}\) (f) LaP, (g) \(\mathrm{CoCO}_{3},(\mathbf{h}) \mathrm{N}_{2} \mathrm{O}_{4}\)

Short answer

Ionic compounds: NaI, Ca(NO\(_3\))\(_2\), FeCl\(_3\), LaP, and CoCO\(_3\) Molecular compounds: PF\(_5\), SCl\(_2\), and N\(_2\)O\(_4\)

Step-by-step solution

Review the definitions of ionic and molecular compounds

Ionic compounds are formed between a metal and a nonmetal, where there's a significant difference in electronegativity between the elements. Ionic compounds usually form when a metal transfers electrons to a nonmetal, forming positively charged cations and negatively charged anions. Molecular compounds, on the other hand, are formed by nonmetals only. They're formed when atoms share electrons to form covalent bonds.

Examine each given compound

Now we'll go through each of the compounds and determine whether they are ionic or molecular. (a) PF\(_5\): Phosphorus (P) and fluorine (F) are both nonmetals. Therefore, this compound is molecular. (b) NaI: Sodium (Na) is a metal, and iodine (I) is a nonmetal. This means NaI is an ionic compound. (c) SCl\(_2\): Sulfur (S) and chlorine (Cl) are both nonmetals. This compound is molecular. (d) Ca(NO\(_3\))\(_2\): Calcium (Ca) is a metal, and the nitrate ion (NO\(_3^-\)) is a polyatomic ion formed by nonmetals. This compound is ionic. (e) FeCl\(_3\): Iron (Fe) is a metal, and chlorine (Cl) is a nonmetal. Therefore, FeCl\(_3\) is an ionic compound. (f) LaP: Lanthanum (La) is a metal, and phosphorus (P) is a nonmetal. This compound is ionic. (g) CoCO\(_3\): Cobalt (Co) is a metal, and the carbonate ion (CO\(_3^{2-}\)) is a polyatomic ion formed by nonmetals. This compound is ionic. (h) N\(_2\)O\(_4\): Nitrogen (N) and oxygen (O) are both nonmetals. This compound is molecular.

List all the ionic and molecular compounds

Ionic compounds: NaI, Ca(NO\(_3\))\(_2\), FeCl\(_3\), LaP, and CoCO\(_3\) Molecular compounds: PF\(_5\), SCl\(_2\), and N\(_2\)O\(_4\)

Key concepts

Electronegativity

Understanding electronegativity is key to distinguishing between ionic and molecular compounds. Electronegativity is a measure of an atom's ability to attract and hold onto electrons when it forms a chemical bond. Atoms with high electronegativity, such as fluorine, oxygen, and nitrogen, tend to attract electrons more strongly than atoms with lower electronegativity, like sodium, calcium, and iron.

When two atoms with significantly different electronegativity values bond, the more electronegative atom pulls the bonding electrons closer to itself. This results in the less electronegative atom losing its electrons and becoming a cation (positively charged), while the more electronegative atom gains electrons and becomes an anion (negatively charged). This electron transfer is the hallmark of ionic bonding, which leads to the formation of ionic compounds like NaI and Ca(NO₃)₂.

In contrast, when atoms with similar electronegativity values bond together, such as two nonmetals, they share electrons more or less equally, and no significant charge separation occurs. This type of bonding, known as covalent bonding, results in the formation of molecular compounds like N₂O₄. The concept of electronegativity explains why compounds such as PF₅ and SCl₂ are molecular: the atoms involved share a similar pull for electrons, leading to a covalent bond rather than an ionic one.

Covalent Bonds

A covalent bond is a type of chemical bond where two atoms share a pair of electrons. Electrons are shared rather than transferred because the atoms involved have comparable electronegativites and neither atom fully 'wins' the electron tug-of-war. Molecules such as water (H₂O), carbon dioxide (CO₂), and the molecular compounds in the exercise, including PF₅ and N₂O₄, are held together by covalent bonds.

Covalent bonding results in stable, neutral entities called molecules. Since these shared electrons cater to the requirements of both atoms involved in the bond, molecules exhibit no significant charge separation. In our exercise, SCl₂ is also molecular because sulfur and chlorine share a similar electronegativity, leading them to form a covalent compound with shared electron pairs.

Polyatomic Ions

A polyatomic ion is a charged entity composed of two or more covalently bonded atoms. What makes polyatomic ions special is that, despite being made of atoms that share electrons covalently, the entire group of atoms carries a net charge. For instance, the nitrate ion (NO₃⁻) consists of one nitrogen atom covalently bonded to three oxygen atoms, and as a group, they hold a negative charge due to the presence of an extra electron.

Polyatomic ions, like nitrate and carbonate (CO₃²⁻), often form ionic compounds when they combine with metal cations, as seen with Ca(NO₃)₂ and CoCO₃ in our exercise. Metals with their low electronegativity are ready to lose electrons and become cations, which then attract these polyatomic anions, resulting in the formation of ionic compounds. Understanding polyatomic ions is crucial in chemistry because they can significantly alter the properties of the compounds they form, making the recognizing of these ions particularly important for students.