Final answer:
The assertion that halogens always have an oxidation number of 1- in metallic halides is false. Halogens like Cl, Br, and I can have varying oxidation numbers depending on the compounds they form. Fluorine is an exception as it consistently has an oxidation number of -1. This statement is false
Step-by-step explanation:
The statement that in metallic halides the halogen always has an oxidation number equal to 1- is false. While it is true for some halogens like fluorine, which always has an oxidation number of -1 in its compounds due to its high electronegativity, other halogens such as chlorine, bromine, and iodine can have positive or negative oxidation numbers depending on the compounds they form. For example, in NaCl, chlorine indeed has an oxidation number of -1, however, in the chlorate ion (ClO3-), chlorine has an oxidation number of +5.
The sum of the oxidation numbers in a neutral molecule must be zero. In the case of polyatomic ions like chlorate, the sum of the oxidation numbers must equal the overall charge of the ion. Therefore, the oxidation states of halogens can vary based on what other elements they are bonded to in the compound.
When halogens react with metals to form halides, they usually exhibit a negative oxidation number, showcasing their tendency to gain electrons due to having seven valence electrons in their outer shell. However, when bonding with oxygen, as is the case in oxyanions like chlorates, bromates, and iodates, halogens can obtain positive oxidation numbers. Additionally, the reactions involving the halide ions (Cl-, Br-, and I-) show their capability as reducing agents, which is consistent with their negative oxidation states in such contexts.
Hence, the statement is false