Final answer:
The oxidation number of a chlorine ion (Cl-) is -1, as halogens generally have a -1 oxidation state when they are in the form of anions. In the Lewis dot diagram, chlorine would be shown with seven dots around its symbol, plus one additional dot representing the extra electron gained to form the chloride ion.
Step-by-step explanation:
Writing the Oxidation Numbers for Chlorine Ions
To write the oxidation numbers for chlorine ions, we must first understand the Lewis dot diagram of a chlorine atom. A chlorine atom (Cl) has seven valence electrons, depicted as dots surrounding the element symbol. When this atom becomes an anion (Cl-), it gains one additional electron to complete its octet, resulting in a chloride ion with a -1 charge.
In the case of the chlorine ion (Cl-), the oxidation number is equal to the charge of the ion. Therefore, the oxidation number of Cl- is -1. This is because halogens like chlorine typically gain one electron to form an anion with a -1 charge. Rules of assigning oxidation numbers indicate that in ionic compounds, like MgCl2, the magnesium atom would have an oxidation number of +2 while each chlorine atom would have an oxidation number of -1.
Furthermore, the sum of oxidation numbers in a molecule must equal the charge of the molecule. For the chlorine elemental form (Cl2), the oxidation number is 0 for each chlorine atom according to rule 1 which states that elements in their pure form have an oxidation number of 0.