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We've learnt that the electropositive element is written first. Then why is ammonia written as $\ce{NH3}$ ?

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    $\begingroup$ Perhaps they don't want anyone to think it is a binary acid. $\endgroup$ – Brinn Belyea Sep 13 '14 at 17:54
  • $\begingroup$ Ammonia can act as an acid though. $\endgroup$ – Dissenter Sep 13 '14 at 19:23
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    $\begingroup$ @Dissenter: Many compounds can act as many things in various weird conditions, but in aqueous solutions ammonia doesn't generally act like an acid. $\endgroup$ – supercat Sep 13 '14 at 20:34
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    $\begingroup$ 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. $\endgroup$ – Greg Sep 14 '14 at 6:43
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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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  • $\begingroup$ Are the hydrogens in ammonia really more electropositive than the nitrogen? Combining H+ ions with anions other than OH- yields compounds which behave as acids in aqueous solutions, but NH3 behaves like a base. $\endgroup$ – supercat Sep 13 '14 at 18:46
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    $\begingroup$ Yes, hydrogen is more electropositive than nitrogen. The electronegativity of hydrogen is 2.2, while nitrogen is 3.04, see en.wikipedia.org/wiki/… $\endgroup$ – ron Sep 13 '14 at 18:55
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    $\begingroup$ 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. $\endgroup$ – supercat Sep 13 '14 at 20:18
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    $\begingroup$ @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 Sep 14 '14 at 10:13
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    $\begingroup$ @supercat how does oxidation state correlate with electronegativity? $\endgroup$ – Dissenter Sep 14 '14 at 15:20
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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.

Table VI Element sequence, Red Book 2005

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: $\ce{HO}^\bullet$ $\endgroup$ – hBy2Py Jan 4 '16 at 12:50
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    $\begingroup$ See also, my answer to this question $\endgroup$ – Ben Norris Jan 6 '16 at 23:26
  • $\begingroup$ 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." $\endgroup$ – mhchem Jun 8 '18 at 10:59
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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.

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