We've learnt that the electropositive element is written first. Then why is ammonia written as $\ce{NH3}$ ?
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3$\begingroup$ Perhaps they don't want anyone to think it is a binary acid. $\endgroup$– Brinn BelyeaCommented Sep 13, 2014 at 17:54
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$\begingroup$ Ammonia can act as an acid though. $\endgroup$– DissenterCommented Sep 13, 2014 at 19:23
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1$\begingroup$ @Dissenter: Many compounds can act as many things in various weird conditions, but in aqueous solutions ammonia doesn't generally act like an acid. $\endgroup$– supercatCommented Sep 13, 2014 at 20:34
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7$\begingroup$ See $\ce{CH4}$ , $\ce{BH3}$ etc This rule is generally applied for hydrogen when it is protic: $\ce{H2O}$, $\ce{HCl}$, IUPAC has a whole sets of rules that nobody cares to use, especially in nomenclature. $\endgroup$– GregCommented Sep 14, 2014 at 6:43
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$\begingroup$ The same is the case with phosphine. $\endgroup$– ShubCommented Aug 16, 2021 at 8:00
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3 Answers
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According to current nomenclature rules, $\ce{H3N}$ would be correct and acceptable. However some chemical formulas, like $\ce{NH3}$ for ammonia, that were in use long before the rules came out, are still accepted today.
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3$\begingroup$ Yes, hydrogen is more electropositive than nitrogen. The electronegativity of hydrogen is 2.2, while nitrogen is 3.04, see en.wikipedia.org/wiki/… $\endgroup$– ronCommented Sep 13, 2014 at 18:55
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1$\begingroup$ PS -- look at chemistry.stackexchange.com/a/538/7274 for a table which suggests that for purpose of nomenclature, hydrogen's electronegativity is considered to be between that of the first group V element (nitrogen) and the last group VI element (polonium). See also chemistry.stackexchange.com/a/975/7274 which would imply that compounds of hydrogen that are more acidic than water are written with the hydrogen first, and those which are more basic are written with the other element first. $\endgroup$– supercatCommented Sep 13, 2014 at 20:18
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6$\begingroup$ @supercat $\ce{NaH}$ and $\ce{Li3N}$ are not covalent compounds. And nomenclature does have nothing to do with chemical behaviour. Oxidation states are only a bookkeeping tool. $\endgroup$– Martin - マーチン ♦Commented Sep 14, 2014 at 10:13
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1$\begingroup$ @supercat how does oxidation state correlate with electronegativity? $\endgroup$ Commented Sep 14, 2014 at 15:20
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1$\begingroup$ The fact that hydrogen is more electropositive than nitrogen is irrelevant to the basic character of ammonia, which is due to the lone pair on nitrogen and not due to hydride ions (as it would be in LiH, for example). $\endgroup$ Commented Jan 3, 2016 at 13:50
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The atomic symbols in a formula may be ordered in various ways. For example, the electronegativity criterion is most often used in binary species (for intermetallic compounds, however, older recommendations prescribed alphabetical ordering rather than by electronegativity).
Section IR-4.4.2.1 of Nomenclature of Inorganic Chemistry – IUPAC Recommendations 2005 (Red Book) explains what is meant by the ordering principles ‘electronegativity’:
IR-4.4.2.1 Electronegativity
If electronegativity is taken as the ordering principle in a formula or a part of a formula, the atomic symbols are cited according to relative electronegativities, the least electronegative element being cited first. For this purpose, Table VI is used as a guide. By convention, the later an element occurs when the table is traversed following the arrows, the more electropositive is the element.
This table is also included in the IUPAC Technical Report Brief guide to the nomenclature of inorganic chemistry. Pure Appl. Chem. 2015, 87(9–10), 1039–1049 as well as in the corresponding four-sided lift-out document, which is available as supplementary material.
Note that hydrogen is placed between the elements of group 16 and the elements of group 15.
Therefore, all the following examples are consistent with the above convention:
$\ce{HF}$
$\ce{HCl}$
$\ce{HBr}$
$\ce{HI}$
$\ce{H2O}$
$\ce{H2S}$
$\ce{NH3}$
$\ce{CH4}$
Note that, according to this convention, the formula for the hydroxide ion should be $\ce{HO-}$ (not $\ce{OH-}$).
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$\begingroup$ Indeed, the hydroxyl radical as most commonly written (at least in the hard-core free radical chemistry community) conforms to this: $\ce{HO}^\bullet$ $\endgroup$– hBy2PyCommented Jan 4, 2016 at 12:50
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$\begingroup$ I think you quoted the wrong paragraph. That one says, elements that came first in the arrow order are cited first. I think you should rather quote IR-2.15.3.1: "In compositional names of binary compounds and corresponding formulae, the element encountered last when following the arrow in Table VI is represented first in the formula as well as the name." $\endgroup$– mhchemCommented Jun 8, 2018 at 10:59
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$\begingroup$ The table misses seven elements we now have after Rg. Cmon IUPAC, get the update in! $\endgroup$ Commented Jun 26, 2022 at 21:59
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$\begingroup$ Possibly $\ce{OH^-}$ got grandfathered in. In an ionic compound like sodium hydroxide, $\ce{NaOH}$ reflects the true bonding connections (sodium is bonded to oxygen), so the hydroxide ion appears as $\ce{OH^-}$. $\endgroup$ Commented Jun 26, 2022 at 22:02
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The answer of this question lies in the nomenclature of the late 1700’s when chemical knowledge was primitive. At the time, most chemists agreed on a class of acrid substances, for example vinegar, called “acids”. Another class of substances reacted with acids to form the “base” of compounds.
When systematizing these compounds, Lavoisier came up with the word oxygen by combining roots which translated meant “to generate acid” because he thought oxygen was used to form acid. Acid was also thought to be formed by hydrogen as well. Most of the early chemical formulas for compounds specified both hydrogen and oxygen as the acid forming component.
It was not discovered until 1832 hydrogen, unlike all of the other known elemental gases, shared a property with metal: it was electro-positive. So in the late 1800’s the nomenclature debate surfaced. Some of the old nomenclature was kept so the opposite nature of acids and bases was reflected in the nomenclature. This nomenclature was said to facilitate understanding of molecules combining and breaking apart.
As an example, notice how the nomenclature in the following reaction suggests two molecules with opposite properties are sticking together.
$$\ce{ NH3 + HCl -> NH4Cl }$$
The short answer would be early chemist developing nomenclature did not have modern understanding of hydrogen, so hydrogen often was placed in the same location of formulations as oxygen in similar compounds.
Sources:
Williamson. Remarks on Chemical Nomenclature and Notation. J. Chem. Soc. 1864, 17, 421–432.
Robert Kane, John William Draper, Elements of chemistry, page 152.
