There is a lot of confusion surrounding the relationship between the terms “fluoride” and “fluorine”. As the title says, fluoride is the ionic form of fluorine. Similarly, nitride is the ionic form of nitrogen, iodide is the ionic form of iodine, arsenic ions are an ionic form of arsenic (which has more than one ionic form), and mercury ions are the ionic form of mercury, for example. Fluorine, nitrogen, iodine, arsenic, and mercury are all chemical elements, a full list of which can be found in the periodic table. Every atom, including every atom which is also an ion and regardless of whether or not the atom is part of a chemical compound, is an atom of some particular chemical element. The characteristic which distinguishes an atom of one element from that of another element is the number of protons in the nucleus of the atom. That number is called the atomic number. The atomic numbers of fluorine, nitrogen, iodine, arsenic, and mercury are 9, 7, 53, 33, and 80, respectively. Again, the atomic numbers of all of the chemical elements can be found in the periodic table. At risk of labouring the point, every fluorine atom has 9 protons in its nucleus and every atom with 9 protons in its nucleus is a fluorine atom, every nitrogen atom has 7 protons in its nucleus and every atom with 7 protons in its nucleus is a nitrogen atom, every iodine atom has 53 protons in its nucleus and every atom with 53 protons in its nucleus is an iodine atom, every arsenic atom has 33 protons in its nucleus and every atom with 33 protons in its nucleus is an arsenic atom, every mercury atom has 80 protons in its nucleus and every atom with 80 protons in its nucleus is a mercury atom, and the pattern is the same for all of the other elements.
A fluoride ion is an atom with 9 protons in its nucleus, making it a fluorine atom. It is an “ion” because the number of electrons is not equal to the number of protons, which is also the reason for the “ide” suffix. More specifically, each fluoride ion (or anion to be more precise) has 10 electrons, giving it a net electrical charge (also known as a valence) of -1. The negative sign and the magnitude of “1” which make up that “-1” are conventions in chemistry which were adopted for the sake of convenience. The “ide” suffix used for the ionic forms of fluorine, nitrogen, iodine, and other non-metals is also merely a convention, and it could have been decided to use it for metals and metalloids instead, or not at all. For example, the chemical CaF2 is conventionally called calcium fluoride, but if the naming conventions were different it could be referred to as fluorine calcide or calcium fluorine instead. The calcium in calcium fluoride is just as much in its ionic form as the fluorine is, but just happens to not be given its own suffix or prefix or anything else to indicate that fact, and it makes no more sense to say that fluoride ions are not the chemical element fluorine than it does to say that calcium ions are not the chemical element calcium. The ionic nature of the chemical compound calcium fluoride, and of the calcium which forms part of that compound, can be inferred from the presence of the word “fluoride”, with its ion-indicating suffix. Nevertheless, the difference in the naming conventions for metallic and non-metallic ions does appear to be part of the reason for a common misunderstanding.
By now it should be very clear that the terms “fluoride” and “fluorine” are not mutually exclusive. Not only are they not mutually exclusive, but fluoride is fluorine; every fluoride ion is in fact a fluorine atom. The difference between the terms is simply that “fluoride” is more specific, “fluorine” more general. Unfortunately it is commonly claimed that those terms are mutually exclusive, and that the only form of fluorine is fluorine gas. As previously stated, “fluorine” refers to any atom with 9 protons in its nucleus. It is a general term which includes fluoride, fluorine gas, and covalently bound fluorine (such as that found in Teflon, chlorofluorocarbons, many pharmaceutical drugs, and various other chemicals). The term “fluorides” is used for compounds which contain fluorine, regardless of whether the fluorine is ionic or covalently bound, which may be another source of confusion. Desperation to deny harm done by fluoride, and the extreme electronegativity and reactivity of fluorine gas, are almost certainly additional factors.
Why it matters
So why does this matter? One reason is that chemical elements are not biodegradable, and that includes the ionic forms of elements. The fact that fluoride cannot be broken down by the human body, or any other living organism, has implications for its bioaccumulation and chronic toxicity. Fluorine is not the only toxic, naturally occurring chemical element which accumulates mostly in bone and is not a nutrient. Another reason is that legitimate concerns about fluoride toxicity are claimed to be a case of mistaken identity. Those who make such claims appear to think that we do not understand the difference between fluorine gas and fluoride and have incorrectly assumed that fluoride is hazardous just because fluorine gas is hazardous, but in reality they are either deliberately lying about fluoride toxicity or have incorrectly and irrationally assumed that just because fluorine gas and fluoride are not the same thing, fluoride must be more or less harmless. They often liken fluoride to the chloride in table salt, but if they bothered to compare the lethal doses of sodium fluoride and sodium chloride, or to discover other relevant facts, they would know how silly that is. They also claim that fluoride is biologically similar to iodine merely because they are both halogens, which is equally silly.
It is also important that those of us who are pro-choice on taking fluoride get our facts straight, even if the argument against forced-fluoridation does not depend on those particular facts. Fluorine (and consequently its ionic form, fluoride) is a non-metal, not a heavy metal or any other kind of metal. The fluoridation chemicals which are used for forced-fluoridation are industrial grade, and come with contaminants which include heavy metals, but the fluoride itself is not metallic. It should also be understood that although some fluoride compounds are less toxic than others, due to differences in bioavailability and possibly other reasons in some cases, naturally occurring fluoride is not inherently less harmful than fluoride from artificial sources.