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Formulas and Nomenclature
Formulas
First of all, the formulas are the symbols for compounds and molecules. There are three different types of formulas, which are,
- Empirical formula
- Molecular formula
- Structural formula
Empirical formula represents the simplest unit of the molecular formula and gives the relative number of different kinds of atoms in a compound. Molecular Formula, on the other hand, represents the way the molecule exists in nature and gives the actual number of atoms of each kind. Structural formula the detail information of bonding the way the atoms are bonded to one another.
Let examine here more details about empirical and molecular formulas using the following example. The molecular formula for the hydrogen peroxide is H 2 O 2 and its empirical formula is HO. Since the empirical formula is derived from the molecular formula, the way to get it is simply divide subscripts with some common factor to come with smallest possible whole numbers. In this example, the subscripts of H 2 O 2 are divided by 2 to give HO. Note that the ratio of atoms in HO (H: O :: 1:1) is the same as in H 2 O 2 ( H:O::2:2 or 1:1). In general,
- The ratio of atoms in empirical formula is the same as in molecular formula.
Examine the following table.
Molecular formula Empirical formula Ratio of atoms
C 2 H 2 CH C:H::1: C 6 H 6 CH C:H::1: C 6 H 12 O 6 CH 2 O C:H:O::1:2: N 2 H 4 NH 2 N:H::1: P 4 O 10 P 2 O 5 P:O::2:
For many molecules, the empirical formula is the same as molecular formula. For example, water (H 2 O), ammonia (NH 3 ), carbon dioxide (CO 2 ), and (HNO 3 ) have the same empirical as well as molecular formula.
From these examples, it is evident that the molecular formula is the integral multiple of the empirical formula:
Molecular formula = n x Empirical formula
H 2 O 2 = 2 x HO , n= P 4 O 10 = 2 x P 2 O5, n= H 2 O = 1 x H 2 O, n =
Example
Deduce the empirical formula from the following molecular formula.
(a) Hg 2 Cl (^) 2, (b) HCl, (c) C 8 H 16 , (d) Na 2 S 2 O 4 , (e) Al 2 Cl 2
Answer
(a) There are 2 Hg atoms and 2 Cl atoms. Dividing the subscripts by 2 yields the empirical formula HgCl (Hg:Cl :: 1:1).
(b) This molecule is already in the lowest number ratio. Therefore, the empirical formula is the same as molecular formula, i.e. HCl ( H:Cl::1:1).
(c) There are 8 C atoms and 16 H atoms, and dividing the subscripts gives the empirical formula CH 2 ( C:H::1:2).
(d) This molecule contains 2 Na atoms, 2 S atoms, and 4 O atoms. Thus, there is a common factor, i.e., 2. Dividing the subscripts by 2 gives the empirical formula NaSO (^2) (Na:S;O::1:1:2).
(e) There are 2 Al atoms and 2 Cl atoms with common factor of 2. Dividing the subscripts by the common factor generates the empirical formula AlCl (Al:Cl::1:1).
Al
3 +
O
2 -
Al 2 O 3
In essence, what you have to do is make the positive charge number without the positive sign (+) as the subscript of the negative ion and make the negative charge number without the negative sign (-) as the subscript of the positive ion. This automatically satisfies the charge neutralization requirement.
Ionic compounds are made up of two kinds of ions, cations (positive ions) and anions (negative ions). Before you learn to name the ionic compounds, you have to learn how to name the ions. Among the ions, there are two kinds of ions; monatomic ions (containing single atom) and polyatomic ions (containing more than one atom that function as a single entity). Most of the cations, except ammonium ion (NH 4 +), are derived from the metals. Here are few examples.
Monatomic ions
Element Name Ion Name of cation
Li lithium Li +^ lithium Ba barium Ba +2^ barium Al aluminum Al 3+^ aluminum Ag Silver Ag +^ Silver
Note that name of the metal ion is the same as the name of its element. All the A group elements have fixed charges (also known as oxidation numbers), meaning, they have only one charge. For example, sodium ion is Na +. There is no other sodium ion. However, in case of B group elements, this is not the case. They do change their charges. In that case their names also include the charges on the ion in Roman numerals. For example, copper (Cu) has two charges, Cu +^ and Cu 2+. Hence the names of these ions are copper (I) ion and Cu (II) ion. The following is the list of some common cations.
Copper Cu+^ Copper (I) Cu2+^ Copper (II) Iron Fe 2+^ Iron (II) Fe 3+^ Iron (III)
Mercury Hg 2 2+^ Mercury (I) Hg2+^ Mercury (II) Lead Pb 2+^ Lead (II) Pb4+^ Lead (IV) Tin Sn 2+^ Tin (II) Sn4+ Tin (IV)
Anions are named little bit differently than cations. The ending of elemental name is changed to –ide ending:
H Æ H-
hydrogen hydride
O Æ O2- Oxygen oxide
Cl Æ Cl - chlorine chloride
Now the question is how to remember the charges? If you know the group number, pretty much you can guess the charge on the ion in that group. See the following table that gives the charges in relation to group numbers in the periodic table. Here X is any element in that column.
1A 2A 3A 4A 5A 6A 7A 8A
X+^ X2+^ X3+^ X4-^ X3-^ X2-^ X-^ No charges
example Na +^ Ba 2+^ Al 3+^ C4-^ N3-^ O2-^ Cl -
Polyatomic ions
Polyatomic ion is defined as the group of atoms function together as a single unit. Among the polyatomic cations and anions, there is only one polyatomic cation, i.e., NH 4 + (ammonium ion). But, there many polyatomic anions that you need to remember. There is no other way to remember except memorize. Some common ones are:
Chlorate ClO 3 - Chromate CrO 4 - Cyanide CN - Dichromate Cr 2 O 7 2- Hydroxide OH -
Example
Name the following compounds: (a) AgCl, (b) NaH, (c) Ca (OH) 2 , (d) Hg 2 Cl 2 , and (e) HgCl 2.
Answer
(a) The name of the metal ion is silver (Ag+) and the name of the nonmetal ion is chloride (Cl - ), and hence the name is silver chloride. (b) The name of the metal ion is sodium (Na+) and name of the nonmetal ion is hydride (H-), and hence the name is sodium hydride. (c) This compound contains a polyatomic ion. The name of the metal ion is calcium (Ca 2+) and that of polyatomic ion is hydroxide. Therefore, the name is calcium hydroxide. (d) The metal ion Hg has a 1+ charge and the name is mercury (I). The name of the nonmetal ion is chloride. Therefore, the name is mercury (I) chloride. (e) The metal ion has 2+ charges (Hg2+) and the name is mercury (II). The name of the nonmetal ion is chloride. Hence, the name is mercury (II) chloride.
So far, you have learned how to name the compound if the chemical formula is given. You should also be able to do other way around, i.e., write the proper chemical formula if the name is given. As you recall, the compound name contains the name of the metal ion followed by the name of the nonmetal ion. Followings are some simple steps:
- Write the symbol and the proper charges for the metal ion.
- Write the symbol and the charges for the negative ion.
- Neutralize the positive charges with negative charges by writing appropriate number of ions as subscripts using “criss-cross” method.
Example
Write the chemical formulas for the following compounds: (a) magnesium bromide, (b) tin (IV) oxide, (c) iron (II) phosphate, (d) silver sulfide, and (e) aluminum hydroxide.
Answer
(a) The chemical formula is MgBr (^) 2. Here magnesium is a metal ion and bromide is a nonmetal ion. The symbol for the magnesium ion is Mg2+^ because it belongs to 2A group. The symbol for the
bromide is Br -^ because it is derived from bromine element and part 7A group. The formula is written using the criss-cross method:
Mg+2^ + Br -^ Æ MgBr (^2)
NOTE: if the charge is 1+ or 1-, there is no need indicate 1 as a subscript it is understood.
(b) The chemical formula is SnO2. The symbol for the metal ion with proper charges is Sn4+^ because it has Roman numeral IV in the parenthesis that indicates it has 4+. The symbol for oxide is O- because it is derived from oxygen and belongs to 6A group. Thus according to criss-cross method,
Sn4+^ + O-2^ Æ Sn 2 O 4 Æ SnO 2
The formula Sn 2 O 4 is not acceptable because it does not contain the simple whole numbered ratio, and hence SnO 2 is the correct one.
(c) The chemical formula is Fe 3 (PO 4 ) 2. The symbol for iron (II) is Fe2+^ because Roman numeral II indicates 2+ charges. The symbol for the phosphate ion is PO 4 3-.. Thus the chemical formula after neutralizing charges is,
Fe 2+^ + PO 4 3-^ Æ Fe 3 (PO 4 ) (^2)
NOTE: if more than one polyatomic ion is involved, the polyatomic ion should be enclosed in parenthesis and the number of ions should be written as a subscript. In the above example, there are two phosphate ions and due that (PO 4 ) 2 is written.
(d) The chemical formula is Ag 2 S. The symbol for silver is Ag+^ (this is the only charge on silver) and that of sulfide is S^2 because it is derived from elemental name sulfur and belongs to 6A group. Silver ion and sulfur ion combine in 2:1 ratio to neutralize the charges. Therefore,
Ag +^ + S2-^ Æ Ag 2 S
(e) The chemical formula is Al (OH) 3. The symbol for aluminum is Al3+^ because it belongs to 3A. The symbol for hydroxide is OH -. They combine in the ratio of 1:3 to neutralize the charges. Thus,
Al 3+^ + OH-^ Æ Al (OH) (^3)
PCl 3 Phosphorous trichloride PCl 5 Phosphorous pentachloride
It is customary not use mono- for the first element if there is only atom. For example, CO is not named as monocarbon monoxide. It is simply labeled as carbon monoxide.
There are few common names still in usage. The following table gives both proper and common names:
Formula Common name Proper name
B 2 H 6 Diborane Diboron hexahydride CH 4 Methane Carbon tetrahydride SiH 4 Silane Silicon tetrahydride NH 3 Ammonia Nitrogen trihydride PH 3 Phosphine Phosphorous trihydride H 2 O Water Dihydrogen oxide H 2 S Hydrogen sulfide Dihydrogen sulfide
Example
Name following molecular compounds: (a) NF 3 , (b) Cl 2 O 7 , (c) CS 2 , (d) P 4 O 10 , (e) NO
Answer
(a) Nitrogen trifluoride (b) Dichloro heptoxide (c) Carbon disulfide (d) Tetraphosphorous decoxide (e) Nitrogen monoxide
Example
Write the chemical formulas for following moleculer compounds: (a) Carbon tetrabromide, (b) Xenon tetrafluoride, (c) Diiodine pentoxide, (d) Carbon tetrachloride, (e) Disulfur dichloride
Answer
(a) CBr 4 , (b) XeF 4 , (c) I 2 O 5 , (d) CCl 4 , (e) S 2 Cl 2
Acids and Bases
Acids
An acid is a molecular compound that liberates hydrogen ion(s) (H+^ ) when dissolved in water. The hydrogen ion is also known as proton. For example, HCl is molecular compound in the gaseous form and when dissolved in water it separates into H+^ ion and Cl -^ ion. Thus, we say that it liberates H +^ ion. To distinguish between the molecular compound in the gaseous form from the molecular compound in dissolved form, we write HCl(g) for the former and HCl(aq) for the later. Here aq means aqueous (water) solution.
There are two kinds of acids; (a) simple acids and (b) oxyacids. The simple acids contain hydrogen as the first element and nonmetal as the second element in the chemical formula. The name of the simple acid is based on the name of the nonmetal and involves three things:
- The nonmetal name is changed to –ic
- The word “hydro” used as a prefix to indicate that it is a water solution
- The word “acid” is appended
Consider the following examples.
HF(aq) Hydrofluoric acid HCl(aq) Hydrochloric acid HBr(aq) Hydrobromic acid HI(aq) Hydroiodic acid
Oxyacids contain hydrogen, nonmetal and oxygen. The nonmetal and oxygen together make up what is known as a polyatomic anion. The name of the acid depends on the name of the polyatomic anion. And the word “acid” is appended. For example,
HNO 3 Nitric acid (nitric is derived from the nitrate ion (NO 3 - ) HNO 2 Nitrous acid (nitrous is derived from the nitrite ion (NO 2 - ) H 2 SO 4 Sulfuric acid (sulfuric is derived from the sulfate ion (SO 4 2_) H 2 SO 3 Sulfurous acid (sulfurous is derived from the sulfite ion (SO 3 2-) H 3 PO 4 Phosphoric acid (phosphoric is derived from the phosphate ion ( PO 4 3-) H 3 PO 3 Phosphorous acid (phosphorous is derived from the phosphate ion (PO 3 3-)
It is important to note that –ous acid contains one less oxygen than –ic acid. In general, if the polyatomic ion name ends with –ate, it become –ic acid and if it ends with –ite, it becomes ous acid.
- -ate Æ -ic acid
- -ite Æ -ous acid
Hydrates
Hydrates are the compounds that contain specific amount of water. The amount of water molecules are indicated by the number of moles of water and are separated from the main chemical formula by a dot (.). Naming the hydrates is similar to naming ionic compounds but with one difference; the number of water molecules are indicated with Greek prefix with “hydrate” word appended. For example,
BaCl 2. 2H 2 O Barium chloride dihydrate CuSO 4 .5H 2 O Copper (II) sulfate pentahydrate CaSO 4 .4H 2 O Calcium sulfate tetrahydrate MgSO 4 .7H 2 O Magnesium sulfate heptahydrate
