Having 2 degrees in the physical sciences, which included the study of both Molecular Physics and Materials Science, I can tell you that much of the online info about “the difference between fluorine and fluoride” is wrong. Yes, it is true that fluorine gas (with the chemical formula F2), which is barely found in nature at the Earth’s surface, is different in some ways to other forms of fluorine. However, fluorine gas is just one form of the chemical element fluorine, and certainly not the most important or the only highly toxic form.
The physics (or chemistry) of fluorine and fluoride
Fluoride (chemical symbol F–) is the ionic form of fluorine (F). Similarly, nitride (N3-) is the ionic form of nitrogen (N), iodide (I–) is the ionic form of iodine (I), arsenic ions (As3-, As3+, or As5+) are an ionic form of arsenic (As, which has more than one ionic form), and radium ions (Ra2+) are the ionic form of radium (Ra), for example. Fluorine, nitrogen, iodine, arsenic, and radium 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 nitrogen, fluorine, arsenic, iodine, and radium are 7, 9, 33, 53, and 88, respectively. Again, the atomic numbers of all of the chemical elements can be found in the periodic table. Every fluorine atom has 9 protons in its nucleus and every atom with 9 protons in its nucleus is a fluorine atom, every arsenic atom has 33 protons in its nucleus and every atom with 33 protons in its nucleus is an arsenic 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, because its charge is negative) 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 scientific conventions 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 MgF2 is conventionally called magnesium fluoride, but if the naming conventions were different it could be referred to as fluorine magneside or magnesium fluorine instead. The magnesium in magnesium 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 magnesium ions are not the chemical element magnesium.
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. Fluoride is not derived from fluorine, and is not a fluorine compound, it is fluorine. The difference between the terms “fluoride” and “fluorine” is simply that the former is more specific, the latter more general. Unfortunately it is commonly claimed that those terms are mutually exclusive, that the only form of fluorine is fluorine gas, and that fluoride is a compound. 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 in compounds such as Teflon, chlorofluorocarbons, many pharmaceutical drugs, and various other chemicals. Fluoride is not a compound, it is part of various ionic compounds. Although the properties and biochemistry of compounds can be very different from the sum of their parts, the nuclei of the atoms which make up compounds remain physically distinct and intact, as opposed to merging together, for example. This is one of the reasons it makes sense to refer to particular types of ions within ionic compounds, even if they are chemically bound and therefore not “standalone” entities.
Fluoride (or fluorine) in tap water
Tap water almost invariably contains many different types of anions and cations in solution (i.e. dissolved in the water), with more than one type of cation which fluoride and the other anions could potentially bind with, so it does not really make sense to say that the free fluoride ions in tap water are part of any particular compound. This is true even if there happens to be a single, known compound which is the source of the fluoride. Students are sometimes taught that ions are always part of some compound, but that refers to work conducted in a chemistry or physics laboratory with artificially purified compounds, not the situation in the outside world. It is actually a special case of the general principle that the total negative charge of anions must be balanced by the total positive charge of cations – the number of different types of anions and cations is, in itself, irrelevant in this context as long as there is at least one of each. The fluoride in water is not necessarily entirely in the form of free fluoride ions, and as a general rule, the higher the concentration of fluoride or some other type of ion, the more likely that some of it will be undissolved. In tap water with a fluoride concentration of around 1 ppm or less, the fluoride should be mostly in the form of free fluoride ions.
Why the confusion?
The difference in the naming conventions for metallic and non-metallic ions appears to be a large part of the reason for the confusion surrounding the terms “fluorine” and “fluoride”. Another convention is that the term “fluorides” is used for compounds which contain fluorine, regardless of whether the fluorine is ionic or covalently bound. So although fluoride is not a compound, “a fluoride” is a compound – the “a” in “a fluoride” changes the meaning, which may be another source of confusion. Desperation to deny harm done by fluoride, and the superficial familiarity (and contempt) which come from using fluoridated dental products regularly, are almost certainly additional factors. The human tendency to construct false dichotomies may also play a part.
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. (The word “chronic” refers to the duration of exposure, not the severity of the health effects. Chronic exposure is longer term than acute exposure.) Fluorine is not the only toxic, naturally occurring chemical element which accumulates predominantly in bones and teeth, and is not a nutrient – lead being one example. (The similarity in accumulation may seem odd, given that fluorine, and hence fluoride, is a non-metal, whereas lead is a heavy metal. The explanation is that fluoride binds to calcium in bones and teeth, whereas lead substitutes for calcium, which is itself a metal.) Even the chemical elements which are nutrients, such as iron, calcium, and magnesium, have small margins of safety in comparison with water-soluble vitamins such as vitamin C. Vitamins are chemical compounds, not elements.
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. 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 (5–10 g, which is equivalent to 32–64 mg fluoride/kg) and sodium chloride (0.75 to 3.00 g/kg, i.e. 750 to 3000 mg/kg), 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. Bromine is another halogen, and is also neither a nutrient nor biologically similar to iodine. The biochemistry of the halogen chlorine, including its ionic form, chloride, is also very different to the other halogens. You can go through the rest of the naturally occurring chemical elements, and the groups they belong to, and the story is the same. Biochemistry relies on a knowledge of chemistry, but it is studied separately from Chemistry for a reason.
Note also that “fluorine” is sometimes used instead of “fluoride” in the literature, and if you think those terms are mutually exclusive it is bound to create confusion. The former term was actually preferred prior to 1950, or thereabouts. I encountered one fluoridationist who admitted not having read Fluorine Intoxication by Europe-based researcher Kaj Roholm (1937), but who still insisted the book is not about fluoride. The use of the “ine” or “ide” suffix according to preference is not unique to the 9th element, by the way. For example, the ionic compound potassium iodide (KI) is commonly referred to as an iodine supplement when used for that purpose.
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. Metals form positive ions (called cations) whereas fluorine only forms the negative ion (i.e. anion) F–. 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. Additionally, it should be understood that although some ionic fluoride compounds are more hazardous than others, due to differences in bioavailability and possibly other reasons in some cases (e.g. the leaching of lead by silicofluorides), naturally occurring fluoride is not inherently less harmful than fluoride from artificial sources. For example, active volcanoes give off large quantities of hydrogen fluoride (HF) gas, which is highly bioavailable and highly toxic. When you eat or drink, at least some of the fluoride which is ingested forms HF in your stomach acid, regardless of whether it came from an artificial or natural source. The study of what happens to chemicals taken into the body is known as pharmacokinetics.