AP Chemistry
Basics of Chemical Bonding
Properties of substances are largely dependent
on the bonds holding the material together.
Basics of Bonding
A chemical bond occurs when atoms or ions are
strongly attached to each other.
Ionic bonds involve the transfer of e– and the
subsequent electrostatic attractions.
-- “metal/nonmetal” (“cation/anion”)
Covalent bonds involve the sharing of e–
between two atoms.
-- “nonmetal/nonmetal”
metallic bonds: each metal atom is
bonded to several neighboring atoms
-- bonding e– (i.e., valence e–) are free to move
throughout the material
Lewis symbols show ONLY the
valence e– (i.e., the ones
involved in bonding).
C
N
S
octet rule: atoms “want” 8 v.e–
-- several exceptions (a topic for later) Gilbert Lewis
(1875–1946)
Ionic Bonding
Na(s) + ½ Cl2(g)
Na + Cl
NaCl(s)
Na+ + [ Cl ]
lattice energy: the energy required
to separate 1 mole of solid ionic
compound into gaseous ions
-- a measure of stability
NaCl(s)
Na+(g) + Cl–(g)
DHlatt = +788 kJ/mol
“Salts” are brittle
– solids with high
melting points.
DHfo = –410.9 kJ/mol
enthalpy
(i.e., heat)
of formation
In general, ionically bonded substances have...
…big, (+) lattice energies.
Because lattice energies are electrostatic in nature,
two variables are involved in how big they are:
1. the magnitude of the charges
(“wears the pants”)
2. the separation between the ions
Charge can easily be twice as large
(i.e., 2+ and 2– vs. 1+ and 1–), but the
separation never varies by that much.
Put the following in order of
increasing lattice energy:
LiBr, FeN, CdO.
LiBr < CdO < FeN
Now these:
MgS, MgCl2, MgO.
MgCl2 < MgS < MgO
With transition-metal ions, the e– lost first come
from the subshell with the largest value of n.
e.g.,
Ni = [Ar] 4s2 3d8
Ni2+ = [Ar] 3d8
Ni3+ = [Ar] 3d7
Recall that polyatomic ions are groups of atoms that
stay together and have a net charge.
-- e.g., NO3–
CH3COO–
-- their atoms are held to each other by… covalent
bonds
Cations are smaller than the neutral atoms from
which they are derived.
e.g.,
Li
1s2 2s1
Li+
1s2
-- orbitals might be completely vacated
-- e–/e– repulsion always decreases
Fe
Fe2+
Fe3+
Anions are larger than the neutral atoms from
which they are derived.
-- more e–/e– repulsion
Cl
p+,
17
17 e–
Cl–
17 p+,
18 e–
An isoelectronic series is a list of species having an
identical electron configuration.
e.g., N3–, O2–, F–, Ne, Na+, Mg2+, Al3+
(big radius)
(small radius)
EX. Which has the
largest radius?
Rb+
Sr2+
(smallest Zeff)
Y3+
(largest Zeff)