A molecular compound is made up of discrete units called molecules, which
typically consist of a small number of non-metal atoms held together by covalent
bonds. Molecular compounds are represented by chemical formulas, symbolic
representations that, at minimum, indicate: the elements present and the relative
number of atoms The 2ch element.
Example: H 2
Two H Lack of subscript means 1 atoms of O per
atoms per molecule
The empirical formula is the simplest formula for a compound where its subscripts
are reduced to their simplest whole number ratios. Generally, the empirical formula
does not tell one a significant about of information about a given compound. Acetic
acid (2 4 2 ) and glucose 6C12 6 ) both have the empirical formul2 CH O.
The molecular formula is based on an actual molecule of a compound. In some
cases, the empirical and molecular formulas are identical, such as formaldehyde
(CH2O). In other cases, the molecular formula is a multiple of the empirical formula.
Both the empirical and molecular formulas provide information on the combining
rations of the atoms in the compounds, however, they show nothing on how the
atoms are attached relative to each other.
The structural formula shows the order in which atoms are bonded together in a
molecule and by what types of bonds. For example acetic acid:
H C C O H
The covalent bonds in the structural formula are represented by lines or dashes and
a double bond is represented by a double dash. Condensed Structural Formula
This formula is a representation of the molecular structure of a compound which is
written in one line and is an alternate way of showing how the atoms of a molecule
are connected. For example with acetic acid: CH COOH or CH CO H.
3 3 2
It can also be used to show how a group of atoms is attached to another atom. For
example, methylpropane (C 4 10
CH C3 CH 3 can be written as CH 3H (CH ) C33 or asCH (CH ) 3 3
The parentheses indicate that that ligand is connected to the molecule at that given
Line angle Formula
The line angle formula contains lines that represent chemical bonds. For example,
the male hormone molecule, testosterone.
An ionic compound is made up of positive and negative ions joined together by
electrostatic forces of attraction. The atoms of metallic elements tend to lose one or
more electrons when combining with non-metal atoms. The non-metal atoms, in
return, gains one or more electrons in the process. When the metal loses an
electron during transfer, it becomes a cation (positive ion) and the non-metal atom
becomes an anion (negative ion).
Consider NaCl. An ionic crystal of NaCl shows that one Na ion is surrounded by six
Cl ions. We cannot pick any one of these Cl ions and exclusively associate it with a
given Na , however, since the ratio of chloride ions to sodium ions is 1:1, and so a combination of one sodium ion and one chloride ion as a formula unit. The formula
unit of an ionic compound is the smallest electrically neutral collection of ions. The
ratio of atoms in the formula unit is essentially the same as in the chemical formula.
Since a formula unit is only a fragment of a vast network of ion (crystal), a formula
unit cannot be distinguished as distinct. Therefore, it is incorrect to label a formula
unit as a molecule.
Ions such as Na , Mg 2+ and Cl are monatomic, meaning that each consists of a
single ionized atom. Alternately, and ion made up of two or more atoms are called
polyatomic ions (e.g. NO ). (St3dy polyatomic ions)
The Mole Concept and Chemical Compounds
Formula mass: the mass of a formula unit in atomic mass units. It is appropriate to
use the term formula mass, but for molecular compounds, the formula unit is
actually a molecule, so the term molecular mass is used. The molecular mass is
the mass of a molecule in atomic mass units (amu).
Mole of a Compound
The molar mass is the mass of one mole of compound – one mole of molecules of a
molecular compound and one mole of formula units of an ionic compound.
Problem: An analytical balance can detect a mass of 0.1 mg. What is the total
number of ions present in this minimally detectable quantity of MgCl ? 2
Solution: Use molar mass to convert from mass to number of moles of MgCl . Then 2
use Avogadro constant as a conversion factor to convert moles to number of
formula units (fu). Finally, there are three ions (1 Mg 2+and two Cl ) per formula unit
of magnesium chloride.
? ions =
[0.1mg MgCl x (2 g MgCl / 1002 mg MgCl ) x (1 mol2MgCl / 95 g MgCl )2x (6.0 x 2
10²³ fu MgCl /21 mol MgCl ) x 23 ions / 1 fu MgCl )] 2
= 2 x 10 ions
Problem: The volatile liquid ethyl mercaptan, C H S, i2 o6e of the most odoriferous
substances known. It is sometimes added to natural gas to make gas leaks
detectable. How many C H S molecules are contained in a 1.0 µL sample? (d=0.84
? molecules C H 2 =6 -6
[1.0 µL x (1 x 10 / 1µL) x (1000mL / 1L) x (8.4 g C H S / 1 m2) 6 (1 mol C H S / 62.1 2 6
g C 2 S6 x (6.022 x 10²³ molecule C H S / 12m6l C H S] 2 6
= 8.1 x 10 molecules C H S2 6
Some Familiar Molecular Formulas
H , O , N , F , Cl , Br , I , P , S
2 2 2 2 2 2 2 4 8
Composition of Chemical Compounds
Consider Halothane C HBrCl2 3 Mole ratio: nc/ n halothane Mass ratio: m /cm halothane
M(C H2rClF )= 3M + M +CM H BR + M +ClM F
=(2 x 12.01) + 1.01 + 79.90 + 35.45 + (3 X 19.00)
= 197.38 g / mol
Problem: How many moles of F atoms are in a 75.0 mL sample of halothane (d =
1.871 g / mL)?
? mol F
= 75.0 mL C HBr2lF x (1.371g C HBrClF /21 mL C HB3ClF ) x (12mol C HB3ClF / 2 3
197.4g C HBrClF ) x (3 mol F/ 1 mol C HBrClF )
2 3 2 3
Calculating Percent Composition From a Chemical Formula
1. Determine the molar mass of the compound. This is the denominator
in the equation.
2. Determine the contribution of the given element to the molar mass.
This product of the formula subscript and the molar mass of the
element appears in the numerator of the equation.
3. Formulate the ratio of the mass of the given element t