Atoms, Molecules, and Ions
Chapter 2
2.6 Molecules, Compounds, and
Formulas
 A molecule is the smallest identifiable unit
into which a pure substance can be divided
and still retain the composition and chemical
properties of the substance.
Molecules, Compounds, and
Formulas
 How do molecules differ from their
composite elements?
 molecular formula
– gives composition (ex. Ethanol C2H6O)
 condensed formula
– elements are written as groups (CH3CH2OH)
 structural formula
– shows how elements are attached
Molecular Models
 ball and stick model
 space filling model
2.7 Ionic Compounds: Formulas,
Names, and Properties
 ionic compounds consist of charged atoms,
or ions
 cations and anions
– metals generally lose electrons
– nonmetals generally gain electrons
Ions
 monatomic – How can you determine
charge?
 Group A elements have predictable patterns
of behavior, but Group B elements do not!
 polyatomic ions
– see chart p 74 in book – MEMORIZE!!
Ionic Formulas and Names
 cation is written first followed by the anion;
use parentheses if more than one
polyatomic ion is present
 name of the metal plus the word “cation”
– Ba+2 is barium cation
 if metal can form more than one ion, then
use Roman numerals to indicate the charge
– Hg+ is mercury (I)
Ionic Formulas and Names
 the ending on an anion is changed to –ide
– F- is fluoride (halide ions)
 polyatomic ions keep their ending
– SO4-2 is sulfate (oxoanions)
– prefixes tell how much oxygen is present
– Chlorine oxoanions are good examples!
Properties of Ionic Compounds
 electrostatic forces hold ions together
– force of attraction or repulsion between ions is
Coulomb’s law
 The force between two charged particles is
proportional to the magnitude of the charges
(attraction between ions increases as ion charge
increases)
 The force is inversely proportional to the square of
the distance (attraction decreases as distance
between the ions increases).
Properties of Ionic Compounds
 ionic compounds are three dimensional
arrays of atoms called a crystal lattice
 high melting points due to the great
attraction between ions
2.8 Molecular Compounds:
Formulas, Names, and Properties
 can be gases, liquids, or solids
 The first nonmetal is named, the second is
named with an –ide ending.
– HCl hydrogen chloride
 The number of atoms of a given type is
indicated by using a prefix.
– N2O dinitrogen monoxide
 Some have common names
– CH4 methane
2.9 Atoms, Molecules, and the Mole
 The mole is the chemical unit measuring
amount of substance.
 1 mole = 6.0221415 x 1023 particles
– Avogadro’s number
 molar mass is the mass in grams of one
mole of an element (g/mol)
Atoms, Molecules, and the Mole
 use the formula of a compound to determine
the molar mass
 use molar mass to convert to moles
(amount)
 use molar mass to convert to molecules
2.10 Describing Compound
Formulas
 Molecular composition
– formula of compound
– mass of each element per mole of compound
– mass of each element relative to the total mass
of the compound (mass percent)
Empirical and Molecular Formulas




convert mass percent to mass
convert mass of each element to moles
find the mole ratio of elements
the simplest, whole-number ratio is the
empirical formula
 If the molar mass is determined
experimentally, then you can also figure out
the molecular formula.
Practice Problems
 The empirical formula of acetic acid is
CH2O. If its molar mass is 60.05 g/mol,
what is the molecular formula of acetic acid?
 Isoprene is a liquid compound that can be
polymerized to form synthetic rubber. It is
composed of 88.17% carbon and 11.83%
hydrogen. Its molar mass is 68.11 g/mol.
What are its empirical and molecular
formulas?
Practice Problems
 Camphor is found in “camphor wood,” much
prized for its wonderful odor. It is composed
of 78.90% carbon and 10.59% hydrogen.
The remainder is oxygen. What is its
empirical formula?
 Analysis shows that 0.586 g of potassium
metal combines with 0.480 g of O2 gas to
give a white solid having a formula of KxOy.
What is the empirical formula of the
compound?
Hydrated Compounds
 Compounds in which molecules of water are
associated with the ions of the compound
are called hydrated compounds.
– Copper (II) chloride dihydrate is CuCl2∙2H2O
which is the same as CuCl2(H2O)2
 anhydrous means a substance without
water
Homework for Chapter 2
 After reading sections 2.6 – 2.11, you
should be able to do the following
problems…
 P. 107 (112-121)