Question

Write the empirical (or molecular) formulas of compounds that the elements in the third period (sodium to chlorine) should form with (a) molecular oxygen and (b) molecular chlorine. In each case indicate whether you would expect the compound to be ionic or molecular in character.

Short answer

Oxygen: Na2O, MgO, Al2O3 (ionic); SiO2, P4O10, SO2/SO3, Cl2O (molecular). Chlorine: NaCl, MgCl2, AlCl3 (ionic); SiCl4, PCl3/PCl5, SCl2, Cl2 (molecular).

Step-by-step solution

Understanding the Problem

We need to determine the empirical or molecular formulas of compounds that elements from the third period (sodium to chlorine) form with molecular oxygen and molecular chlorine. We'll also classify each compound as ionic or molecular.

Identifying Third-period Elements

The third-period elements are: sodium (Na), magnesium (Mg), aluminum (Al), silicon (Si), phosphorus (P), sulfur (S), and chlorine (Cl). We will analyze each of these elements with molecular oxygen (O2) and molecular chlorine (Cl2).

Combining with Oxygen

For each element, we'll determine the formula when combined with oxygen (O2). Electropositive elements like Na, Mg, and Al tend to form ionic compounds with oxygen, whereas Si, P, S, and Cl form molecular oxides. - Na+O2 → Na2O (ionic) - Mg+O2 → MgO (ionic) - Al+O2 → Al2O3 (ionic) - Si+O2 → SiO2 (molecular) - P+O2 → P4O10 (molecular) - S+O2 → SO2 or SO3 (molecular) - Cl+O2 → Cl2O (molecular)

Combining with Chlorine

Next, we determine the formula when these elements combine with chlorine (Cl2). Again, metals tend to form ionic compounds while nonmetals form molecular ones. - Na+Cl2 → NaCl (ionic) - Mg+Cl2 → MgCl2 (ionic) - Al+Cl2 → AlCl3 (ionic) - Si+Cl2 → SiCl4 (molecular) - P+Cl2 → PCl3 or PCl5 (molecular) - S+Cl2 → SCl2 (molecular) - Cl+Cl2 → Cl2 (already molecular, diatomic)

Classifying Compounds as Ionic or Molecular

Elements on the left of the periodic table (Na, Mg, Al) form ionic compounds. Elements on the right, especially nonmetals (Si, P, S, Cl), form molecular compounds. This helps in predicting the nature of the compound formed with molecular oxygen or chlorine.

Summarizing Expected Compounds

The formulas of compounds formed with oxygen: Na2O, MgO, Al2O3 (ionic); SiO2, P4O10, SO2/SO3, Cl2O (molecular). With chlorine: NaCl, MgCl2, AlCl3 (ionic); SiCl4, PCl3/PCl5, SCl2, Cl2 (molecular).

Key concepts

Ionic Compounds

Ionic compounds are formed when there is a transfer of electrons between atoms, typically between a metal and a non-metal. This transfer leads to the formation of charged ions - positive (cations) and negative (anions). These oppositely charged ions attract each other to form a stable ionic bond.
In the context of third-period elements combining with oxygen and chlorine, metals like sodium (Na), magnesium (Mg), and aluminum (Al) readily lose electrons to form positive ions. Oxygen and chlorine, being non-metals, accept these electrons to complete their respective outer shells, forming negative ions.
This charge difference forms strong ionic bonds. For example:

• When sodium (Na) reacts with chlorine (Cl), each sodium atom transfers one electron to a chlorine atom, forming NaCl. This produces sodium ions (Na⁺) and chloride ions (Cl⁻).
• In the case of magnesium (Mg) with oxygen (O), magnesium transfers two electrons to form MgO, which contains Mg²⁺ and O²⁻ ions.

Ionic compounds generally have high melting points and are good conductors of electricity when dissolved in water or molten.

Molecular Compounds

Molecular compounds result from atoms sharing electrons, typically occurring between non-metals. This sharing creates covalent bonds, which hold the compound together. These compounds often form discrete molecules where atoms achieve stability by completing their valence electron shells through sharing.
For third-period elements, compounds such as phosphorus (P), sulfur (S), and chlorine (Cl) exhibit molecular characteristics when bonding with other non-metals like oxygen and themselves.
Consider these examples:

• Phosphorus forms various molecular oxides such as P₄O₁₀ with oxygen, where bonds are covalent.
• Sulfur reacts with chlorine to form molecular compounds like SCl₂, where sulfur shares electrons with chlorine atoms.

Typically, molecular compounds have lower melting and boiling points compared to ionic compounds and do not conduct electricity in solution.

Third-period Elements

The third-period elements on the periodic table range from sodium (Na) to chlorine (Cl). These elements include a combination of metals and non-metals, offering a rich variety of bonding possibilities:

• Metals: Sodium (Na), magnesium (Mg), and aluminum (Al) are metals that generally form ionic compounds with non-metals.
• Metalloids: Silicon (Si) exhibits properties of both metals and non-metals, often forming molecular compounds.
• Non-metals: Phosphorus (P), sulfur (S), and chlorine (Cl) tend to form molecular compounds through covalent bonding with other non-metals.

Each of these elements has distinct properties due to their position in the periodic table. Metals lose electrons more readily, while non-metals prefer to share or gain electrons. Understanding this distinction helps predict the type of chemical bonding and the resultant compound characteristics.

Chemical Formula Prediction

Predicting chemical formulas involves determining how elements will interact based on their electron configurations. This often means identifying whether elements will form ionic or molecular compounds.
With the third-period elements:

• Sodium and chlorine form NaCl, where sodium donates an electron to chlorine, forming an ionic compound.
• For a compound like SiO₂, silicon shares electrons with oxygen in a covalent bond, resulting in a molecular compound.

The periodic table is a great tool for predicting formulas. Elements typically strive to achieve a full outer electron shell. Metals, positioned on the left of the periodic table, usually lose electrons, forming cations. Non-metals, on the right, gain electrons or share them, forming anions or covalent bonds.
Consider charge balance in ionic compounds and shared pairs in molecular compounds when predicting formulas. Recognizing these patterns simplifies predicting the chemical formulas of compounds formed by third-period elements with molecular oxygen and chlorine.