Final answer:
The sum of all oxidation numbers in a neutral molecule must be zero, and for ions, it must equal the charge of the ion. Halogens have negative oxidation numbers in halides but can have positive numbers when combined with oxygen.
Step-by-step explanation:
When assigning oxidation numbers in chemistry, it is essential to follow a specific set of rules to ensure that the sum of the oxidation numbers equals the net charge on the molecule or polyatomic ion. For a neutral compound, this sum must be zero. For example, in NaCl, the sodium (Na) atom has an oxidation number of +1, while chlorine (Cl) carries an oxidation number of -1, resulting in a net charge of zero, which is consistent across all neutral compounds.
Halogens such as chlorine (Cl), bromine (Br), and iodine (I) typically have negative oxidation numbers when they form halide compounds, such as -1. However, when these elements are combined with oxygen, they often have positive oxidation numbers. This is exemplified in the chlorate ion (ClO3-), where chlorine has an oxidation number of +5, while each oxygen atom has an oxidation number of -2, adding up to the ionic charge of -1.
The rules for assigning oxidation numbers are critical for understanding redox reactions, which are based on the transfer of electrons and the changes in oxidation states of the elements involved. Conservation of charge is a fundamental principle, thus confirming that in any chemical reaction, the sum of the oxidation numbers must be equal to the overall charge of the molecule or ion.