Final answer:
A chloride ion has a charge of negative one because it has one more electron (18) than it has protons (17), giving it a net charge of -1. This comes about through electron transfer during the formation of ionic compounds.
Step-by-step explanation:
When a chlorine atom gains an electron, it becomes a chloride ion with a charge of negative one. This negative charge arises because the chloride ion now has more electrons (18b) than protons (17), as illustrated in Figure 3.1.2. The formation of a chloride ion can be seen as part of an electron transfer process, where an atom like sodium (Na) donates an electron to a chlorine atom, thus fulfilling the octet rule for both atoms involved and creating an ionic bond.
For instance, in the formation of sodium chloride (NaCl), a sodium atom gives up its one outer electron to a chlorine atom, which results in Na+ and Cl- ions that are held together by ionic bonds due to their opposite charges. When two chloride ions are present, they collectively hold a total negative charge of 2-, which must be balanced by a positively charged ion, such as iron(II) with a charge of 2+, leading to the formation of iron(II) chloride.