Phosphorous vs. phosphorus

  • Phosphorus denotes the chemical element (with the symbol P and the atomic number 15) present in a few types of minerals found on Earth. It comes from a Latin word for morning star, and it is so named because it glows when exposed to oxygen.1

    Phosphorous (note the o in the last syllable) is an adjective meaning (1) of, relating to, or containing phosphorus, and (2) of or relating to phosphor (phosphor being a general term for various luminescent substances).2 In its first sense, phosphorous is synonymous with phosphoric.

    We don’t have the scientific knowledge necessary to sort through the many words beginning with phosph- (some related to phosphorus and some unrelated). The point of this post is that phosphorus is the noun and phosphorous is its corresponding adjective.

    The adjective is often used in place of the noun—for example:

    The less polluting pig, called the “enviropig,” has a gene that allows it to better digest phosphorous in its food. [New York Times]

    Fertilizers contain three primary plant nutrients: mostly nitrogen and phosphorous with smaller amounts of potassium. [National Geographic]

    The document said one of the main reasons for the murky colour of the Manawatu River and high levels of phosphorous was the large amount of sediment finding its way into the water. []

    In fact, this happens so often that we should probably just consider phosphorous an alternative spelling of phosphorus. Among the dozen or so dictionaries we checked, though, not one has noted this.


    Phosphorous is so often used as a noun that examples of its use as an adjective are hard to find. We gave up after two:

    And they’d used phosphorous grenades which burn a man alive. [Telegraph]

    A study at the University of Michigan … found that Ann Arbor’s ban on phosphorous fertilizers for grass led to a 28 percent drop in the pollutant’s levels in nearby Huron River. [Scientific American]

    See also

    Aluminum vs. aluminium


    1. “Phosphorus” in the OED (subscription required)
    2. “Phosphorous” in the OED



    1. “Phosphorous” (chemistry) is also used to denote that there is a valency or oxidation number of +3 on the phosphorous or, more commonly, that there are specifically three oxygen atoms bound to the phosphorus atom. For example, phosphorous acid = phosphonic acid = [HPO(OH)2] and the oxidation number of P is +3 and there are 3 O directly attached to the P atom. Also, hypophosphorous acid = phosphinic acid = [H2PO(OH)] where “hypo-” and “-ous” mean that that compound has “one less than three” oxygens bound to the central atom, P, so P has two O bound to it and has an oxidation number of +1. The latter (number of O) definition is generally used, although the “+3” definition is technically correct even if the number of oxygens is not three.

      This “-ous” terminology has largely fell to the wayside in favor of the “-onic” and “-inic” terms for acids containing phosphorus. This may be due to the fact that phosphorus and phosphorous are homonyms.

      The notion that the “-ous” is the British spelling is erroneous, but probably stems from the “u” seen in a lot of British spellings of words. Examples of alternative spellings of elements include aluminium (IUPAC, used outside of North America) vs. aluminum (not IUPAC but still used in US and Canada, even in the scientific community) and sulfur (IUPAC, used in the US) and sulphur (not IUPAC, but often used outside the US). Grammarist has an article over each of these. It is also of interest that the spelling used in naming towns and rivers is “sulphur” (e.g. Sulphur River in TX and AR and the town of Sulphur, OK), which has contributed to the confusion.

      [The information on phosphorous came from a graduate-level inorganic chemistry course, the professor of which is an inorganic chemist who explores phosphorus ligands on metals. There was also a letter in Nature (doi:10.1038/426119c) which discusses the noun-adjective confusion, but does not address the alternative chemical definition of phosphorous, probably due to the writer’s profession of biologist, not chemist. ]

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