Question

Assign oxidation states to all of the atoms in each of the following. a. \(\mathrm{MgO}\) b. \(\mathrm{Fe}_{2} \mathrm{O}_{3}\) c. \(\mathrm{PCl}_{3}\) d. \(\mathrm{N}_{2} \mathrm{O}_{5}\)

Short answer

In the given compounds, the oxidation states are as follows: a. \(\mathrm{MgO}\): - Oxidation state of \(\mathrm{Mg} = +2\) - Oxidation state of \(\mathrm{O} = -2\) b. \(\mathrm{Fe}_2 \mathrm{O}_3\): - Oxidation state of \(\mathrm{Fe} = +3\) - Oxidation state of \(\mathrm{O} = -2\) c. \(\mathrm{PCl}_3\): - Oxidation state of \(\mathrm{P} = +3\) - Oxidation state of \(\mathrm{Cl} = -1\) d. \(\mathrm{N}_2\mathrm{O}_5\): - Oxidation state of \(\mathrm{N} = +5\) - Oxidation state of \(\mathrm{O} = -2\)

Step-by-step solution

a. Assign oxidation states to \(\mathrm{MgO}\).

MgO is an ionic compound composed of \(\mathrm{Mg^{2+}}\) and \(\mathrm{O^{2-}}\) ions. Therefore: - Oxidation state of \(\mathrm{Mg} = +2\) - Oxidation state of \(\mathrm{O} = -2\)

b. Assign oxidation states to \(\mathrm{Fe}_2 \mathrm{O}_3\).

\(\mathrm{Fe}_2\mathrm{O}_3\) is an ionic compound composed of \(\mathrm{Fe^{3+}}\) and \(\mathrm{O^{2-}}\) ions. Therefore: - Oxidation state of \(\mathrm{Fe} = +3\) - Oxidation state of \(\mathrm{O} = -2\)

c. Assign oxidation states to \(\mathrm{PCl}_3\).

\(\mathrm{PCl}_3\) is a covalent compound. Consider the most electronegative atom, which is Cl with an oxidation state of -1. Since there are three Cl atoms (each with an oxidation state of -1), the total negative charge is -3. To balance this negative charge, P must have an oxidation state of +3. Therefore: - Oxidation state of \(\mathrm{P} = +3\) - Oxidation state of \(\mathrm{Cl} = -1\)

d. Assign oxidation states to \(\mathrm{N}_2\mathrm{O}_5\).

\(\mathrm{N}_2\mathrm{O}_5\) is a covalent compound. As mentioned earlier, the oxidation state of O is usually -2. There are five O atoms (each with an oxidation state of -2), giving a total negative charge of -10. As there are two N atoms, each N must have an oxidation state of +5 to balance the negative charge. Therefore: - Oxidation state of \(\mathrm{N} = +5\) - Oxidation state of \(\mathrm{O} = -2\)

Key concepts

Ionic Compounds

Ionic compounds are formed when metals and non-metals chemically bond with each other. This happens through the transfer of electrons from one atom to another, creating ions. The metal loses electrons to become a positively charged ion, known as a cation. The non-metal gains electrons to become a negatively charged ion, known as an anion.
In ionic bonding, the overall goal is to reach a stable electronic configuration, typically the nearest noble gas configuration. The resulting compound is electrically neutral, which means that the total positive and negative charges must balance out.

• Example: In extrm{MgO}, magnesium (Mg) donates two electrons to oxygen (O), resulting in extrm{Mg}^{2+} and extrm{O}^{2-}, making extrm{MgO} an ionic compound.
• Another example is extrm{Fe}_2 extrm{O}_3, where iron (Fe) becomes extrm{Fe}^{3+} and combines with extrm{O}^{2-} ions to balance the charge.

Understanding ionic compounds is important for determining the oxidation states of atoms in chemical formulas. These states can be inferred based on the known tendencies of elements to form cations or anions.

Covalent Compounds

Covalent compounds form when two non-metals share electrons, bonding together to achieve a stable electronic configuration. These shared electrons result in a strong bond between the atoms, creating molecules that are distinct from ionic compounds.
In covalent bonding, the term 'molecule' is used to describe the bonded atoms. Since the electrons are shared, the bond is directional, and this directionality can influence the molecule's shape and properties. Covalent compounds do not consist of ions, which is why their overall charges remain neutral.

• Example: extrm{PCl}_3 is a covalent compound. Here, phosphorus (P) shares electrons with three chlorine (Cl) atoms. Though Cl is more electronegative, resulting in each extrm{Cl} having an oxidation state of -1, the total negative charge is balanced by extrm{P}'s oxidation state of +3.
• Another example is extrm{N}_2 extrm{O}_5, where nitrogen (N) shares electrons with oxygen (O), leading to distinct oxidation states but remaining a covalent bond.

Covalent bonding often results in complex structures, which can affect physical properties like boiling and melting points.

Oxidation Numbers

The oxidation number or oxidation state is a concept that helps us understand how electrons are distributed in a molecule, particularly during a chemical reaction. It acts as a bookkeeping tool to track electron transfer.
Oxidation numbers offer insight into which atoms are oxidized or reduced. The number is assigned based on a set of rules:

• The oxidation state of an atom in its elemental state is always zero.
• For monatomic ions, the oxidation state is equal to the charge of the ion.
• Oxygen usually has an oxidation state of -2, except in peroxides or when bonded to fluorine.
• Hydrogen is typically +1 when bonded to non-metals and -1 when bonded to metals.
• The sum of oxidation numbers in a neutral compound is zero, whereas in a polyatomic ion, it equals the ion's charge.

For instance, in extrm{Fe}_2 extrm{O}_3, knowing that oxygen generally has an oxidation state of -2 allows us to deduce that iron must be +3 for the compound to be neutral. Oxidation numbers serve as a foundation for more advanced chemical concepts.

Chemical Formulas

Chemical formulas are symbolic representations of chemical compounds, showing the elements involved and the number of atoms of each. They provide a concise way to convey complex information about a compound's composition.
There are several types of chemical formulas:

• Empirical formulas give the simplest whole number ratio of atoms in a compound.
• Molecular formulas provide the exact numbers of each type of atom present in a molecule.
• Structural formulas depict the specific connections between atoms.

Understanding these formulas involves knowing the chemical symbols and their quantities. For example, extrm{MgO} indicates one magnesium atom bonded to one oxygen atom, forming an ionic compound. In contrast, extrm{PCl}_3 reveals that one phosphorus atom is covalently bonded to three chlorine atoms.
By examining chemical formulas, students can infer the type of bonding, predict the compound's properties, and assign oxidation states. Mastery of reading and interpreting chemical formulas is a fundamental skill in chemistry, enabling students to explore more intricate chemical reactions.