In every redox reaction, there is a species whose oxidation number increases when the reaction occurs. Is the species whose oxidation number increases: a) being oxidized or reduced? b) referred to as the oxidizing agent or the reducing agent?

Oxidation and reduction are two halves of a type of chemical reaction known as a redox reaction, which is integral to understanding how substances interact in a chemical process. Oxidation involves the loss of electrons by a molecule, atom, or ion. In contrast, reduction is the gain of electrons. It's essential to remember that these two processes always occur together; when one species is oxidized, another is reduced.

For a clearer understanding, consider the simple mnemonic 'OIL RIG' - Oxidation Is Loss and Reduction Is Gain. Oxidation is not strictly limited to oxygen involvement, as it initially was thought, but broadly refers to electron shifts. Moreover, oxidation numbers, which we will explore in further detail, help scientists track these electron shifts and determine the oxidation state of each element in a compound during a chemical reaction.

Oxidation numbers, also known as oxidation states, are assigned to elements in chemical compounds to keep track of electron distribution. The oxidation number is a theoretical charge on an atom if the compound were to be broken down into its constituent atoms, assigning shared electrons to the more electronegative element. Several rules help assign oxidation numbers:

• The oxidation number of an element in its elemental form is always zero.
• For monatomic ions, the oxidation number is equal to the charge on the ion.
• Oxygen generally has an oxidation number of -2 in compounds, except in peroxides or bonded to fluorine.
• Hydrogen typically has an oxidation number of +1 when bonded with nonmetals and -1 with metals.
• The sum of oxidation numbers in a neutral compound is zero, and in a polyatomic ion, it is equal to the ion charge.

By determining oxidation numbers, one can identify which atoms are oxidized and which are reduced in a redox reaction. Changes in these numbers during the reaction reflect the movement of electrons.

An oxidizing agent, or oxidant, takes electrons from another substance, thereby undergoing reduction itself. A common example of an oxidizing agent is molecular oxygen (O_2) in combustion reactions. Reducing agents, or reductants, donate electrons leading to their own oxidation. Examples include metals like sodium (Na) and magnesium (Mg), often used in reactions to produce hydrogen gas from acids. An easy way to remember this is that an oxidizing agent 'oxidizes' something else and is 'reduced' in turn, while a reducing agent 'reduces' another substance and is 'oxidized' as a result.

In the context of our exercise, the substance whose oxidation number increases is the one losing electrons or being 'oxidized.' Consequently, it also acts as a 'reducing agent' because it donates electrons to another species, allowing the other substance to be 'reduced.' This conceptual bridge links the changes in oxidation states with the reactive role of substances in redox reactions, making it clearer which species is experiencing oxidation or reduction and what their corresponding agent roles are.