Question

Define oxidation number

Short answer

An oxidation number is a number assigned to atoms in a molecule, ion, or chemical compound that represents its "charge" or oxidation state, indicating the degree of electron transfer that has occurred in a chemical reaction. It follows a set of rules, such as: (1) The oxidation number of an isolated atom is always zero; (2) the oxidation number of a monoatomic ion is equal to its charge; (3) the sum of oxidation numbers of all elements in a compound should add up to zero; (4) some elements have specific, well-known oxidation numbers; (5) the more electronegative element is assigned a negative oxidation number while the less electronegative element is assigned a positive oxidation number, and; (6) the sum of oxidation numbers in a polyatomic ion must equal the overall charge of the ion.

Step-by-step solution

Rule 1: Oxidation number of an isolated atom

The oxidation number of an isolated atom, also known as its elemental state, is always zero. For example, the oxidation number of an oxygen (O) atom in elemental form is 0.

Rule 2: Oxidation number of charged atoms (ions)

The oxidation number of a monoatomic ion is equal to its charge. For example, the oxidation number of sodium (Na) in sodium chloride (NaCl) is +1, while the oxidation number of chloride (Cl) is -1.

Rule 3: Oxidation numbers of elements in a compound

In a chemical compound, the oxidation numbers of all elements should add up to zero. For example, in water (H2O), the oxidation number of hydrogen (H) is +1 and that of oxygen (O) is -2. The sum of oxidation numbers in H2O is therefore (+1 * 2) + (-2 * 1) = 0.

Rule 4: Known oxidation numbers for certain elements

Some elements have well-known, consistent oxidation numbers in their compounds. Here are a few of these elements and their respective oxidation numbers: - Hydrogen (H): +1 when combined with non-metals and -1 when combined with metals - Oxygen (O): -2 in almost all compounds, except in peroxides where it is -1, and in superoxides, where it is -1/2 - Alkali metals (Group 1 elements): +1 - Alkaline earth metals (Group 2 elements): +2 - Halogens (Group 17 elements): -1 in their binary compounds with metals, but can have positive oxidation numbers in their compounds with oxygen or other non-metals

Rule 5: Learning from the electronegativity

When assigning oxidation numbers to elements in a compound, the more electronegative element is assigned a negative oxidation number while the less electronegative element is assigned a positive oxidation number. Electronegativity describes the tendency of an atom to attract electrons to itself in a chemical bond.

Rule 6: Sum of oxidation numbers in a polyatomic ion

In a polyatomic ion, the sum of the oxidation numbers of all the atoms in the ion should equal the overall charge of the ion. For example, in the sulfate ion (SO4^2-), the sum of oxidation numbers of sulfur (S) and oxygen (O) atoms should equal -2, the overall charge of the ion.

Key concepts

Chemical Compounds

Chemical compounds are substances formed when two or more different types of atoms chemically bond together. These compounds can exist in various forms, such as solids, liquids, or gases. Compounds are characterized by having a fixed ratio of elements, which determines their chemical formula and properties.
For instance, water (\( H_2O \)) is a chemical compound composed of two hydrogen atoms bonded to a single oxygen atom. The chemical bonds that hold the atoms together can be ionic or covalent, depending on how the electrons are shared or transferred between the atoms. The structure and type of bond in a compound significantly influence its physical and chemical properties, such as melting point, boiling point, and solubility.
Understanding chemical compounds is vital for studying chemical reactions, such as oxidation-reduction reactions, where changes in oxidation numbers help track electron transfers.

Monoatomic Ions

A monoatomic ion is an ion consisting of a single atom with a positive or negative charge. The charge of a monoatomic ion is determined by the loss or gain of electrons, associated with achieving a stable electronic configuration, often resembling the nearest noble gas. For instance:

• Cations: These are positive ions formed by losing electrons. Sodium (\( Na^+ \)) is a typical cation, where it loses one electron to achieve a stable electronic configuration.
• Anions: These are negative ions formed by gaining electrons. Chloride (\( Cl^- \)) is a classic example of an anion, where it gains an electron to fill its valence shell.

Monoatomic ions are crucial in forming ionic compounds, where they combine to balance the overall charge. The understanding of their oxidation numbers is straightforward, as it directly correlates to the ion's charge. For example, in sodium chloride (\( NaCl \)), sodium has an oxidation number of +1, while chloride has -1, ensuring the compound's neutrality.

Electronegativity

Electronegativity is a chemical property that indicates an atom's tendency to attract electrons towards itself in a chemical bond. Atoms with high electronegativity values, like fluorine, oxygen, and nitrogen, have a strong pull on electrons. This concept is crucial in determining the type of bond between atoms and their oxidation states.
In molecules, the atom with higher electronegativity is often assigned a negative oxidation number, whereas the less electronegative atom gets a positive oxidation number. For example, in water (\( H_2O \)), oxygen is more electronegative than hydrogen, assigning it an oxidation number of -2, while hydrogen gets +1.
Electronegativity values also allow chemists to predict the polarity of bonds and molecules. Polar molecules, where one end is slightly negative and the other slightly positive, often have significant differences in electronegativity between their constituent atoms. Understanding electronegativity helps in anticipating the behavior of molecules in different chemical reactions.

Polyatomic Ions

Polyatomic ions are ions made up of two or more atoms covalently bonded, that carry an overall positive or negative charge. These ions function as a single unit in chemical compounds and reactions. Some common examples include sulfate (\( SO_4^{2-} \)), nitrate (\( NO_3^- \)), and ammonium (\( NH_4^+ \)).
The oxidation numbers of the atoms within a polyatomic ion must sum to the charge of the ion. For instance, consider the sulfate ion (\( SO_4^{2-} \)): Each oxygen atom typically has an oxidation number of -2, and the sulfur atom has an oxidation number that balances the overall charge to -2.
Polyatomic ions are critical in the study of chemistry as they frequently appear in various chemical processes, including acid-base reactions and redox processes. Their unique structures and charges allow them to participate in forming stable ionic compounds, which showcase widespread applications in chemical industries.