Chemistry Corner

Names and Formulas used in General Chemistry

Mastering names and formulas takes practice �?the more you do, the more you remember and the easier it gets! This is a skill that will help to open up the subject of chemistry to you, because after all, chemical names are an integral part of the language of chemistry. The following notes are really intended to be more of reminders, as your textbook should have this information in much better detail and organization. Practice exercises can be found under "Exercises" in the menu on the left. The rules below pertain to the English names, but even so, there are variations, such as "sulfate" (American spelling) and "sulphate" (British spelling), and "aluminum" (American) and "aluminium" (British).

Memorize (some things in chemistry simply have to be memorized!) the names and charges of common ions such as chloride, nitrate, sulfate, sulfite, etc. This will happen automatically with practice. Use the periodic table to determine the charges of common single atom (monatomic) cations and anions: Group(charge) 1A(1+), 2A(2+), 3A(3+), 7A(1�?, 6A(2�?, 5A(3�?for N, P, and As). You can many times easily deduce the charge of an ion that you're not sure about from another formula with the same ion in it. Now, what was that sulfate ion's charge? But, I happen to have this formula from another problem: Na₂SO₄. Since Na ion is +1 (Group 1A) and there are two of them in the formula, SO₄ ion must therefore be �?, SO₄^{2�?/SUP>. Easy! Incidentally, when you write the charge with the formula of an ion, the convention nowadays is like this: Na⁺, Ca²⁺, PO₄^{3�?/SUP> rather than Na⁺¹, Ca⁺² or PO₄^�?. Oxidation numbers of atoms (sometimes but not necessarily the actual charge of the atom), are normally written +1, +2, �?, etc.}}

Ionic Compounds

Formulas of ionic compounds are by their nature empirical, or simplest, formulas, which have the simplest whole numbers of positive and negative ions that give a charge-balanced formula. The formula of sodium chloride is always NaCl, never "Na₂Cl₂." If an equation has two sodium chlorides in it, we would use a coefficient of two: 2 NaCl. A case that looks like an exception, but is not, is mercurous chloride, or mercury(I) chloride, which has the formula Hg₂Cl₂ instead of HgCl. This is because the mercurous ion occurs as an unusual diatomic metal cation, Hg₂²⁺ with a covalent bond between the two Hg⁺ ions. Remember that when naming ionic compounds (in English), the positive ion is always named first, and likewise with the formulas also. Sodium chloride, not "chloride sodium;" NaCl, not "ClNa." Note the "ide" ending of single atom negative ions: sodium chloride and not "sodium chlorine."

Molecular Compounds

Names of molecular compounds (which do not contain positive and negative ions and usually contain only nonmetallic elements) can contain the prefixes mono, di, tri, tetra, penta, etc. Normally, names of ionic compounds do not use these prefixes to indicate the number of positive or negative ions. CaCl₂ is calcium chloride, not "calcium dichloride". However, aluminum chloride, AlCl₃, is sometimes called aluminum trichloride which is not incorrect in this case because it actually is a molecular compound (it has very polar aluminum-chlorine covalent bonds) even though it looks like it should be ionic since it contains metal and nonmetal elements typical of ionic compounds. MnO₂ is often called manganese dioxide rather than manganese(IV) oxide for the same reason. Usually, it is best to play it safe with compounds like these and use the ionic names: FeCl₃ is ferric chloride or iron(III) chloride rather than "iron trichloride." However, PCl₃ is commonly called phosphorus trichloride rather than "phosphorus(III) chloride" since this is clearly a molecular compound (since only nonmetallic elements are present). The (III) in the Stock name, if used, would indicate the oxidation state rather than the actual charge of the phosphorus atom in this case.

Acid Names

Acid names should be used when specified or usually when the acid is dissolved in water. For example, hydrogen chloride, HCl (g), which is a gaseous molecular compound, becomes hydrochloric acid, HCl (aq) a strong acid which exists as completely separated H⁺ and Cl^{�?/SUP> ions in aqueous solution. H₂SO₄ is normally always named by its acid name sulfuric acid, not "dihydrogen sulfate;" likewise HNO₃ is nitric acid and not "hydrogen nitrate." H₂S (g) is hydrogen sulfide or dihydrogen sulfide, a gaseous molecular compound. In aqueous solution, H₂S (aq), the acid name hydrosulfuric acid (a weak acid) will normally be used, especially in the context of acid behavior. Another common weak acid is acetic acid, HC₂H₃O₂, which is found in vinegar. In this case, as with other common acids such as sulfuric acid, the acid name is always used; "hydrogen acetate" is not correct. (Acetic acid belongs to a class of organic compounds called carboxylic acids. These have their own special nomenclature rules as do the many other classes of organic compounds.)}

Charges

Many positive ions can have more than one charge and in those cases the charge must be specified in the name. You can indicate the positive charge with a Roman numeral in parentheses (known as the Stock number after the German chemist Alfred Stock), or by using the common name endings ous (for the lower common charge) or ic (for the higher common charge). Since iron can form +2 or +3 ions commonly, the names of ionic iron compounds must be unambiguous in this regard. FeCl₂ is iron(II) chloride or ferrous chloride, not "iron chloride." On the other hand, since the calcium ion, for example, always has the same charge (+2), we call CaCl₂ simply calcium chloride and not "calcium(II) chloride" because we know automatically that the charge of calcium ion is +2.

Oxyanions

Remember, with the oxyanion names that the "ite" ion simply has one less oxygen than the "ate" ion, the charge is still the same. Na₂SO₄ = sodium sulfate, Na₂SO₃ = sodium sulfite. These translate into the acid names sulfuric acid, H₂SO₄, and sulfurous acid, H₂SO₃. Also, if the oxyanion has one more oxygen than the "ate" ion, add the prefix per to the ate ion name, and if there is one less oxygen than the "ite" ion, add the prefix hypo to the ite name. You probably have the chlorate series in your textbook as an example:

ClO₄^{�?/SUP> = perchlorate ion (one more oxygen than chlorate)}

ClO₃^{�?/SUP> = chlorate ion}
ClO₂^{�?/SUP> = chlorite ion
ClO^{�?/SUP> = hypochlorite ion (one less oxygen than chlorite)}}

And the corresponding acid names:

HClO₄ = perchloric acid

HClO₃ = chloric acid
HClO₂ = chlorous acid
HClO = hypochlorous acid

IUPAC Names

Systematic names from the IUPAC rules (International Union of Pure and Applied Chemistry) are sometimes used to name complex inorganic compounds (the IUPAC rules are the standard for organic compounds). Here are some examples:

NaClO₄ = sodium tetraoxochlorate(VII)

Na₂SO₄ = sodium tetraoxosulfate(VI)

Na₂SO₃ = sodium trioxosulfate(IV)

NaAlCl₄ = sodium tetrachloroaluminate(III)

While the last name is in common use, the other three compounds (and most other inorganic compounds) are almost always named according to the common rules outlined above - sodium perchlorate, sodium sulfate, and sodium sulfite. Other examples of IUPAC names of inorganic compounds may be found by searching the ChEBI (Chemical Entities of Biological Interest) database.

Names & Formulas Exercises –�?gt;

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