Chem 314 Supplement material
Beauchamp
1
The structure of atoms…provides a basis for the structure of molecules
What is the relative volume of an electron cloud (Ve) compared to volume of a nucleus (Vn)?
eval
p,n
ecore
Ve
Vn
=
4π
3
re
rn
3
= (1.33)(3.14)(100,000) 3 = 4 x 1015 =
4,000,000,000,000,000
1
Electron clouds determine
the overall volume of atoms.
The size of an atom is the size of its electron cloud.
What is the relative mass of the electrons compared to the mass of nuclear particles?
d=1
d = 100,000
mass protons (or neutrons)
mass electrons
1840
1
Protons and neutrons
determine the mass of an atom.
The mass of an atom is mostly the mass of its incredibly small nucleus.
p = protons = This number is constant for a particular type of atom and defines an element. If there are six protons,
the element has to be carbon.
n = neutrons = This number can vary in an element; it defines an isotope. Some isotopes are stable and some are
radioactive. Carbon-12 has six protons and six neutrons and is stable. Carbon-13 has six protons
and seven neutrons and is stable. Carbon-14 has six protons and eight neutrons and is unstable (and
radioactive) with a half life of almost 6000 years.
e = electrons = The number of electrons can vary. It can increase or decrease, depending on an atom's position in
the periodic table.
If electrons = protons? (same number of electrons and protons)
associated term
neutral atom
If electrons < protons? (deficiency of electrons)
posistively charged cation
If electrons > protons? (excess of electrons)
negatively charged anion
Valence electrons (eval) are the outermost layer of electrons. They determine the bonding patterns of an
atom and the usual goal is to attain a Noble gas configuration. This is accomplished by losing electrons
(becoming cations) or gaining electrons (becoming anions) or sharing electrons in covalent bonds (in
neutral molecules or in complex ions).
Core electrons (ecore) are electrons in completely filled inner-shells. They are held too tightly for bonding
(sharing with another atom) and they are not usually considered in the bonding picture. They are important
because they cancel a portion of the nuclear charge (called shielding) so that the valence electrons only see
an effective nuclear charge, Zeffective.
The effective nuclear charge, Zeffective, is the net positive charge felt by the valence electrons (bonding and
lone pair electrons). It can be estimated by subtracting the number of core electrons from the total nuclear
charge.
approximate positive charge
Zeffective = (Ztotal) - (# core electrons) = felt by the valence electrons
Z:\classes\314\314 Special Handouts\1 recent updates\1. 314 supplement at orbs, IP, bonds.doc
Chem 314 Supplement material
Beauchamp
2
s and p atomic orbitals (…most important to organic students)
+z
+z
1s atomic orbital
has a spherical
shape, no nodes.
This is the orbital
used by hydrogen
in bonding.
a single 2p atomic orbital,
artificially separated from
the other two 2p orbitals,
dumbell shape at 90o angle
to the other two p orbitals,
there is a single node at the
nucleus
2s atomic orbital
spherical shape,
one node
all 2p atomic orbitals
together, 2px, 2py, 2pz,
when completely filled
the 2p orbitals have
spherical symmetry
node = a region in space where the probability of finding electron density goes to zero.
d orbitals look more complicated (…only occasionally invoked in organic chemistry)
dyz
z
z
dxz
y
x
y
x
lobes in
the yz plane
z
dxy
dx2-y2
z
dz2
y
x
y
lobes in
the xy plane
y
x
x
lobes in
the xz plane
z
lobes in
the xy plane
along axes
lobes along
the z axis
We rarely have occasion to
discuss d orbitals, but we will
use them briefly in discussions
of sulfur, phosphorous and a
few transition metals. Even in
those discussions we will
mainly look at them as bigger
versions of p-like orbitals.
Atomic Configuration
p
d
s
n=3
p
s
n=2
s
+Z
n=1
+Z = total nuclear charge = number protons
Zeff = residual positive charge felt by the outer
most valence electrons = (Ztotal - core electrons)
1. Aufbau Principle - Orbitals are filled with electrons
in order of increasing energy, from the innermost
orbitals (lower energy) towards the outermost orbitals
(higher energy).
2. Pauli Exclusion Principle - Only two electrons
may occupy any orbital and those electrons must
have opposite spins (applicable to s, p, d and f orbitals).
3. Hund's Rule - Electrons entering a subshell
containing more than one orbital will spread
themselves out over all of the available orbitals
with their spins in the same direction, until the
subshell is over half filled.
Z:\classes\314\314 Special Handouts\1 recent updates\1. 314 supplement at orbs, IP, bonds.doc
Chem 314 Supplement material
Beauchamp
3
1. Ionization potential as a measure of an atom’s electron attracting power.
ionization
potential
(kcal/mole)
electron
is lost
Atom
Atom
+
electron
Atom
efinal energy
greater PE (less stable)
Potential
Energy
Ionization always has an
energy cost to strip an
electron from an atom.
∆ Energy =
Atom
starting energy
lower PE (more stable)
Energy to ionize an electron from neutral atoms = IP1 (units are kcal/mole). Compare rows and compare columns.
Group 1A
H
Li
Na
K
+314
+124
+118
+99
Zeff = +1
Group 2A
Group 3A
Be +215
Mg +177
Ca +141
B +192
Al +138
Ga +138
Zeff = +3
Zeff = +2
H
H C
H
H H
H C
C
H H
Group 6A
C +261
Si +189
Ge +182
N +335
P +242
As +226
Zeff = +5
O +315
S +239
Se +225
Zeff = +4
electron
ionized
+260
electron
ionized
+229
electron
ionized
+201
N
C
H
H C
H N
H
H
electron
ionized
+335
electron
ionized
N
F
Cl
Br
Zeff = +6
+402
+300
+273
Zeff = +7
fluorine
H N
+261
electron
ionized
O
+315
electron
ionized
H O
+296
H
H H
H C
Group 8A
(He) +568
( Ne) +499
(Ar) +363
(Kr) +323
Zeff = +8
Group 7A
oxygen
nitrogen
carbon
C
Group5A
Group 4A
H
C
H C
H H
H
O
electron
ionized
+285
electron
ionized
O
F
+402
H O
H
H C
O
H
How does the ionization potential of the halogen atoms vary down a column? The entire halogen family
has a constant Zeffective of +7.
F
electron
ionized
+402
F
+
e-
Cl
electron
ionized
+298
Cl
+
e-
Br
electron
ionized
+272
Br
+
e-
I
electron
ionized
+241
I
+
e-
Larger
ionization
potential
Z:\classes\314\314 Special Handouts\1 recent updates\1. 314 supplement at orbs, IP, bonds.doc
F
Chem 314 Supplement material
Beauchamp
4
Electronegativity defines the relative attraction an atom has for electrons in chemical bonds with other atoms.
Mulliken Model of
Electronegativity ≈ Average of ionization potential (IP) and electron affinity (EA).
χ≈
attracting power of atoms for
(IP + EA)
= electrons in chemical bonds
2
Electronegativity will determine nonpolar, polar and ionic characteristics of bonds, and when shapes are
included it determines the same attributes in molecules.
Electronegativity =
Table of electronegativities.
Group 1A
H
Li
Na
K
Group 2A
2.2
1.0
0.9
0.8
Be
Mg
Ca
Zeff = +1
H
1.5
1.2
1.0
B
Al
Ga
Zeff = +2
1.8
1.5
1.6
C 2.5
Si 1.9
Ge 1.9
Zeff = +3
H
N 3.0
P 2. 2
As 2.0
Zeff = +4
O
hydrogen gas,
a single bond
Group5A
Group 4A
Group 3A
Group 6A
O
S
Se
Zeff = +5
3.5
2.6
2.4
Zeff = +6
O
N
Group 7A
F
4.0
Cl 3.2
Br 3.0
I 2.7
Zeff = +7
Group 8A
(He)
(Ne)
(Ar)
(Kr)
-
Zeff = +8
N
nitrogen gas,
a triple bond
oxygen gas,
a double bond
χa symbolizes the electronegativity of atom a.
bond
atom
a
χa
∆χ
atom
b
χb
=
χa _ χb
∆χ
=
χa _ χb ≤ 0.4
∆χ
=
χa _ χb
< 1.4-2.0 classify bond as polar covalent
∆χ
=
χa _ χb
≥ 1.4-2.0
classify bond as nonpolar covalent
classify bond as ionic
The symbols + and - represent qualitative charge separation forming a bond dipole. Alternatively, an arrow can be
drawn pointing towards the negative end of the dipole and a positive charge written at the positive end of the dipole.
Two qualitative pictures of a bond dipole. B is assumed to be more electronegative than A.
µ
δδ+
or
A
B
A
B
...........
d
µ=
amount of
charge
separated
d
x
distance
between
charges in cm
= units of Debye (D = 10-18 esu-cm)
µ = (e)(d) = dipole moment
e = electrostatic charge (sometimes written as q)
The abosolute value of a unit charge on
an electron or proton is 4.8x10-10 esu
d = distance between the opposite charges
This is often given in angstrums, but converted to cm
for use in calculations (1A = 10-8 cm, A = angstrum)
Z:\classes\314\314 Special Handouts\1 recent updates\1. 314 supplement at orbs, IP, bonds.doc
Chem 314 Supplement material
Beauchamp
5
Ionic bonding
"explosion?"
with transfer
of electrons
Na
Cl
Cl
(curved arrows
Na show electron
movement)
Cl2 (gas)
mp = -101oC
bp = -35oC
Na (metal)
mp = 98oC
bp = 883oC
Na
Cl
Cl
Na
+
Our arbitrary rules
classify this difference
in electronegativity as
ionic.
+
+
+
+
+
+
+
+
+
+
+
+
+
χa _ χb = 3.2 - 0.9 = 2.3
=
+
+
+
∆χ
NaCl (salt)
mp = 801oC
bp = 1413oC
+
+
table salt lattic structure,
both Na and Cl ions
attaine a Noble gas configuration
+
Lattice structure - depends on
the size and charge of the ions.
+
Each ion is surrounded on many sides by oppositely charged ions. To
introduce the disorder of a liquid (melt) or a gas (boil) requires a very
large input of energy (mp indicates the amount of energy required to
breakdown the ordered lattice structure and boiling point indicates the
amount of energy required to completely remove an ion pair from the
influence of the lattice structure). Ionic bonds (ionic attractions on all
sides) can only be broken at a great expense in energy.
Examples
Melting point (oC) Boiling point (oC) ∆χ
NaCl
801
1465
2.3
1275 sub
2.5
Na2O
NaOH
318
1390
2.5?
>2200
1.4*
AlN
674
sub
1.4*
FeCl2
306
315 dec
1.2*
FeCl3
MgCl2
714
1412
1.9*
MgO
2800
3600
2.1
1184
2.1?
Mg3(PO4)2
* = exception to our electronegativity rules about bond polarity
sub = sublimation
Notice the very high melting
points and boiling points of
ionic substances.
Covalent Bonding (Molecules)
H
H
H
If two hydrogen atoms should
find one another, they would
from a diatomic molecule with
a tremendous release of energy,
about 104 kcal/mole.
H
The line symbolizes a two-electron, purecovalent bond based on the calculation below.
∆χ =
χa _ χb = 2.2 - 2.2 = 0
Z:\classes\314\314 Special Handouts\1 recent updates\1. 314 supplement at orbs, IP, bonds.doc
Chem 314 Supplement material
Beauchamp
6
H
H
C
H
H
N
H
O
H
H
F
H
H
polar covalent bond
based on the
calculation below
pure covalent bond
based on the
calculation below
polar covalent bond
based on the
calculation below
polar covalent bond
based on the
calculation below
_
∆ χ = χa χb
_
∆ χ = χa χb
_
∆ χ = χa χb
_
∆ χ = χa χb
∆ χ = 2.5 - 2.2 = 0.3
∆ χ = 3.0 - 2.2 = 0.8
∆ χ = 3.5 - 2.2 = 1.3
∆ χ = 4.0 - 2.2 = 1.8
single bonds
H
H
H
H
H H
C
H
H
H
H
H
H
C
C
H
H
H
H
N
N
H
H
H
H
C
N
H
H
H
H
H
C
O
H
H
H
C
F
H
H
O
N
H
H
O
O
H
F
N
H
O
F
F
F
F
double bonds
H
H
C
H
H
C
C
H
H
C
N
H
H
O
N
H
H
N
N
O
O
O
H3C
H
two double bonds
H
H
H
C
C
C
C
triple bonds
C
N
C
C
H
H
H
H
H
H
C
H
H
C
H
C
C
H
H
C
N
H
C
H
C
N
H
CH2
H2C
H2C
H2
C
H2C
C
H2
CH2
H
C
C
H2C
O
NH
CH2
H2C
N
H
rings
H2
C
O
C
H
H
C
O
O
H
C
C
H2C
C
C
H2
H
Z:\classes\314\314 Special Handouts\1 recent updates\1. 314 supplement at orbs, IP, bonds.doc
C
H
H
N