Chemical Nomenclature

In clinical settings, "Sodium Chloride" and "Sodium Chlorite" are two very different substances. It important to understand the difference.

1. Common and Systematic Naming

Many substances are so deeply integrated into our daily routines that we refer to them by familiar "nicknames" or "Common names" much like we do with close friends. A single compound often carries multiple common or trivial names; for instance, the white crystals in your shaker are known as "salt." If you were to ask for it by its more formal name, sodium chloride, at a backyard barbecue, you might get a few puzzled looks.

However, the situation becomes truly absurd if you attempt to use its full systematic name in a casual setting:

"Could you please pass the ionic halite lattice of sodium and chlorine ions?"

NaCl Lattice Structure

2. Elements

Some elements were not really "discovered", but have been known since ancient times; many of these have symbols that are derived from the Latin names of the elements.

Element Name	Symbol	Latin Name
Antimony	Sb	Stibium
Copper	Cu	Cuprum
Gold	Au	Aurum
Iron	Fe	Ferrum
Lead	Pb	Plumbum
Mercury	Hg	Hydrargyrum
Potassium	K	Kalium
Sodium	Na	Natrium
Tin	Sn	Stannum

3. Transition Metals (The Roman Numeral Rule)

Because metals like Iron (Fe) or Copper (Cu) can have different charges, we specify the charge in the name:

FeCl₂: Iron (II) chloride (Uses Fe²⁺)
FeCl₃: Iron (III) chloride (Uses Fe³⁺)_

Ion Symbol (Lower Charge)	Ion Symbol (Higher Charge)	Element Name
Cu⁺	Cu²⁺	Copper
Fe²⁺	Fe³⁺	Iron
Sn²⁺	Sn⁴⁺	Tin
Pb²⁺	Pb⁴⁺	Lead

Chemical Symbol	Common (Trivial) Name	Formal (Systematic) Name
Cu⁺	Cuprous ion	Copper(I) ion
Cu²⁺	Cupric ion	Copper(II) ion
Fe²⁺	Ferrous ion	Iron(II) ion
Fe³⁺	Ferric ion	Iron(III) ion
Sn²⁺	Stannous ion	Tin(II) ion
Sn⁴⁺	Stannic ion	Tin(IV) ion

4. Binary Molecular Compounds (Greek Prefixes)

For nonmetal compounds, use prefixes to show the number of atoms:

Formula	Prefixes Used	Name
SO₂	di = 2	Sulfur dioxide
N₂O₅	di = 2, penta = 5	Dinitrogen pentoxide

_di = 2

tri = 3
tetra = 4
penta = 5
hexa = 6

Examples:

N₂O₄ - dinitrogen tetroxide [note the missing a preceding the vowel]
N₂O - dinitrogen oxide [more commonly, nitrous oxide]
SF₆ - sulfur hexafluoride
P₄S₃ - tetraphosphorus trisulfide [more commonly, phosphorus sesquisulfide]
Na₂HPO₄ - disodium hydrogen phosphate
H₂S - hydrogen sulfide [we skip both the di and mono]
CO - carbon monoxide [mono- to distinguish it from the dioxide]
CaSO₄·½H₂O - calcium sulfate hemihydrate [In this solid, two CaSO₄ units share one water of hydration between them; more commonly called Plaster of Paris]

It will be apparent from these examples that chemists are in the habit of taking a few liberties in applying the strict numeric prefixes to the more commonly known substances.

These two-element compounds are usually quite easy to name because most of them follow the systematic rule of adding the suffix -ide to the root name of the second element, which is normally the more "negative" one. Several such examples are shown above. But as noted above, there are some important exceptions in which common or trivial names take precedence over systematic names:

H₂O (water, not dihydrogen oxide)/
H₂O₂ (hydrogen peroxide, not dihydrogen dioxide)
H₂S (hydrogen sulfide, not dihydrogen sulfide)
NH₃ (ammonia, not nitrogen trihydride)
NO (nitric oxide, not nitrogen monoxide)
N₂O (nitrous oxide, not dinitrogen oxide)
CH₄ (methane, not carbon tetrahydride)

3. Clinical Acids to Memorize

These acids are frequently encountered in respiratory therapy and metabolic studies:

HCl (aq): Hydrochloric acid (Stomach acid)
H₂CO₃: Carbonic acid (Formed by CO₂ in the blood)
HNO₃: Nitric acid
H₂SO₄: Sulfuric acid

4. Hydrates

Some salts can be hydrated, meaning that they have one or more water molecules as part of their crystal structure.

For example, CuSO₄∙5H₂O is the salt copper (II) sulfate with 5 water molecules attached in the crystal.

The Greek prefix for the number 5 is ‘penta’, so this would be a pentahydrate … copper (II) sulfate pentahydrate.