Lesson: Chapter - 13

Chemical Formulas Review: Nomenclature and Formula Writing

Naming Simple Compounds

There are four naming systems you should familiarize yourself with to succeed on the Chemistry exam. The trick is recognizing which naming system to use. Here are the guidelines:

• If the compound starts with H, it is an acid. Use the naming acids rules.
• If the compound starts with C and contains quite a few H’s and perhaps some O’s, it is organic. Use the naming organic compounds rules.
• If the compound starts with a metal, it is most likely ionic. Use the naming binary ionic compounds rules.
• If the compound starts with a nonmetal other than H or C, use the naming binary molecular compounds rules.

It is also essential that you memorize some common polyatomic ions. Polyatomic ions behave as a unit. If you need more than one of them, enclose them in parentheses when you write formulas. You need to know their names, formulas, and charges. If you learn the nine that follow, you can get many others from applying two simple patterns.

• Pattern 1: The -ates “ate” one more oxygen than the -ites and their charge doesn’t change as a result! For instance, if you know nitrate is NO₃^-,then nitrite is NO₂^-.If you know phosphate is PO₄^3-,then you know phosphite is PO₃^3-.You can also use the prefixes hypo- and per- with the chlorate series. Perchlorate, ClO₄^-,was really “hyper and ate yet another oxygen” when compared to chlorate, ClO₃^-.Hypochlorite is a double whammy: it is -ite and therefore “ate” one less oxygen than chlorate and it is hypo-, which means “below,” so it “ate” even one less oxygen than plain chlorite, so its formula is ClO^-. You can also substitute the other halogens for Cl and make additional sets of the series.
• Pattern 2: The -ates with charges less than negative 1 (that is, ions with charges of -2, -3, etc.) can have an H added to them to form new polyatomic ions. For each H added, the charge is increased by a +1. For instance, CO₃^2-can have an H added and become HCO₃^-.HCO₃^-is called either the bicarbonate ion or the hydrogen carbonate ion. Since phosphate is -3, it can add one or two hydrogens to make two new polyatomic ions, HPO₄^2-and H₂PO₄^-.These are named hydrogen phosphate and dihydrogen phosphate, respectively. If you keep adding hydrogen ions until you reach neutral, you’ve made an acid! That means you need to see the naming acids rules.
• Pattern 3: The following periodic table will also come in handy. Notice there are simple patterns for determining the most common oxidation states of the elements based on their family’s position in the periodic table. Notice the 1A family is +1, while the 2A family is +2; then skip across to the 3A family and see that aluminum is +3. Working backward from the halogens, or 7A family, the oxidation states are most commonly -1, while the 6A family is -2, and the 5A family is -3. The 4A family is “wishy-washy”: they can be several oxidation states, with the most common being +4.

Naming Acids

How do you know it’s an acid? The compound’s formula begins with an H, and water doesn’t count! Naming acids is extremely easy if you know your polyatomic ions. There are three rules to follow:

• H + element: When the acid has only an element following the H, use the prefix hydro-, followed by the element’s root name and an -ic ending. HCl is hydrochloric acid; H₂S is hydrosulfuric acid. When you see an acid name beginning with hydro-, think: Caution, element approaching! HCN is an exception since it is a polyatomic ion without oxygen, so it is named hydrocyanic acid.
• H + -ate polyatomic ion: If the acid has an -ate polyatomic ion after the H, that makes it an -ic acid. H₂SO₄ is sulfuric acid.
• H + -ite polyatomic ion: When the acid has an -ite polyatomic ion after the H, that makes it an -ous acid. H₂SO₃ is sulfurous acid.

Acids have enough H⁺ added to the anion to make the compound neutral. Supply either the acid’s name or its formula to complete the table below:

Naming Organic Compounds

How do you know it’s organic? The formula will start with a C followed by H’s. Most of the organic carbons you will encounter will be either hydrocarbons or alcohols, and luckily for you, these are the simplest of all to name. Learn the list of prefixes in the table following this section: they correspond to the number of carbons present in the compound. The following silly statement will help you remember the order of the first four prefixes since they are not ones you are familiar with: “Me eat peanut butter.” This corresponds to meth-, eth-, prop-, and but-, which correspond to one, two, three, and four carbons, respectively.

Now that we have a stem, we need an ending. There are three common hydrocarbon endings; the ending changes depending on the structure of the molecule:

• -ane = alkane (all single bonds and saturated); C_nH_2n+2; saturated: it contains the maximum number of H’s
• -ene = alkene (contains double bond, unsaturated); C_nH_2n
• -yne = alkyne (contains triple bond, unsaturated); C_nH_2n-2; polyunsaturated: it contains more than one double or triple bond

For any hydrocarbon, you can remove one H and replace it with a hydroxyl group, or —OH group, to form an alcohol. Do not be fooled—this looks like a hydroxide group but isn’t! The OH does not make this hydrocarbon an alkaline or basic compound, nor do you name it as a hydroxide! C₂H₆ is ethane, while C₂H₅OH is ethanol. Fill in the missing formulas and names for each compound in the table:

Naming Binary Ionic Compounds

How will you know a compound is ionic? You’ll know because the formula will begin with a metal cation or the ammonium cation. Formulas often end with a polyatomic anion. If only two elements are present, they are usually from opposite sides of the periodic table, like in KCl. If the metal is one of the transition metals, be prepared to use a Roman numeral to indicate which oxidation state the metal is exhibiting. Silver, cadmium, and zinc are exceptions to the Roman numeral rule! First, let’s name the ions.

Naming positive ions (usually metals)

• Monatomic, metal, cation: simply the name of the metal from which it is derived. Al³⁺ is the aluminum ion (these are often referred to as group A metals).
• Transition metals form more than one ion; Roman numerals (in parentheses) follow the ion’s name. Cu²⁺ is copper (II) ion. Exception: mercury (I) is Hg₂²⁺, that is, two Hg⁺ bonded together covalently.
• NH₄⁺is ammonium.
• Roman numerals are not usually written with silver, cadmium, and zinc. Arrange their symbols in alphabetical order—the first one is 1+ and the other two are 2+.

Naming negative ions (usually nonmetals or polyatomic ions)

• Monatomic, nonmetal, anion: add the suffix -ide to the stem of the nonmetal’s name. Halogens are called the halides. Cl^- is the chloride ion.
• Polyatomic anion: you must memorize the polyatomic ion’s name. NO₂^-is the nitrite ion.

Naming ionic compounds: The positive ion name is given first (remember, if it’s a transition metal, the Roman numeral indicating its charge is part of its name), followed by the name of the negative ion. No prefixes are used.

Naming Binary Molecular Compounds

How will you know if it’s a molecular compound? Well, it will be a combination of nonmetals, both of which lie near each other on the periodic table. Use the following set of prefixes, and don’t forget the -ide ending to the name.

If the second element’s name begins with a vowel, the a at the end of the prefix is usually dropped. N₂O₅ is dinitrogen pentoxide, not dinitrogen pentaoxide. PCl₅ is phosphorous pentachloride, not phosphorous pentchloride.

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