ATOMIC STRUCTURE and BONDING
Atom
Nucleus
Orbiting Electrons
Protons
Neutrons
Positive
charge
Electrically
neutral
Definitions
Molecule: Group of atoms bonded together by strong primary
bonds (e.g. Diatomic molecule, O2, H2,…, Compounds, CH4, H2O )
Atomic number (Z): Number of protons in the nucleus or
equivalently number of electrons for a neutral atom.
Avagdro’s Number (NA) : Number of atoms or molecules per
mole and it is equal to 6.023 x 1023 (atoms/mole)
Atomic mass: The sum of masses of protons and neutrons with in
the nucleus for an avagdro’s number of atoms.
NA (atoms/mole)* [1.67x10-24 (mass of protons) *number of protons
+ 1.67x10-24 (mass of neutrons ) * number of neutrons] = units is
g/mole
* As an example one mole of iron contains 6.023 x 1023 atoms and
has a mass of 55.847 gramsÆ atomic weight
BOHR ATOM
orbital electrons:
n = principal
quantum number
1
2
n=3
Adapted from Fig. 2.1,
Callister 6e.
Nucleus: Z = # protons
= 1 for hydrogen to 94 for plutonium
N = # neutrons
Atomic mass A ≈ Z + N
2
Electron Configuration
Stable Electron Configuration: The outer most shell is
completely filled. The s and p states for the outermost shell are
filled by a total of 8 electrons
SURVEY OF ELEMENTS
• Most elements: Electron configuration not stable.
Element
Atomic #
Hydrogen
1
Helium
2
Lithium
3
Beryllium
4
Boron
5
Carbon
6
...
Neon
10
Sodium
11
Magnesium
12
Aluminum
13
...
Argon
18
...
...
Krypton
36
Electron configuration
1s 1
(stable)
1s 2
1s 22s 1
1s 22s 2
Adapted from Table 2.2,
1s 22s 22p 1
Callister 6e.
1s 22s 22p 2
...
1s 22s 22p 6
(stable)
1s 22s 22p 63s 1
1s 22s 22p 63s 2
1s 22s 22p 63s 23p 1
...
1s 22s 22p 63s 23p 6
(stable)
...
1s 22s 22p 63s 23p 63d 10 4s 24 6
(stable)
• Why? Valence (outer) shell usually not filled completely.
Valence: Number of electrons in the outer most combined sp level.
•Electronegativity: Tendency of an atom to gain an electron and
become a negative charged ion .
Elements that have almost completely filled outer energy level
are strongly electronegative.
• Electropositivity: Tendency to give up electrons and become
positive charged ions.
Elements that have nearly empty outer levels readily give up
electrons.
give up 1e
give up 2e
give up 3e
• Columns: Similar Valence Structure
H
Li Be
Metal
Nonmetal
Intermediate
accept 2e
accept 1e
inert gases
THE PERIODIC TABLE
He
Ne
O
F
Na Mg
S
Cl Ar
K Ca Sc
Se Br Kr
Rb Sr
Te
Y
Cs Ba
I
Adapted
from Fig. 2.6,
Callister 6e.
Xe
Po At Rn
Fr Ra
Electropositive elements:
Readily give up electrons
to become + ions.
Electronegative elements:
Readily acquire electrons
to become - ions.
6
ELECTRONEGATIVITY
• Ranges from 0.7 to 4.0,
• Large values: tendency to acquire electrons.
H
2.1
He
-
Li
1.0
Be
1.5
F
4.0
Ne
-
Na
0.9
Mg
1.2
Cl
3.0
Ar
-
K
0.8
Ca
1.0
Br
2.8
Kr
-
Rb
0.8
Sr
1.0
I
2.5
Xe
-
Cs
0.7
Ba
0.9
At
2.2
Rn
-
Fr
0.7
Ra
0.9
Ti
1.5
Cr
1.6
Fe
1.8
Smaller electronegativity
Ni
1.8
Zn
1.8
As
2.0
Larger electronegativity
Adapted from Fig. 2.7, Callister 6e. (Fig. 2.7 is adapted from Linus Pauling, The Nature of the
Chemical Bond, 3rd edition, Copyright 1939 and 1940, 3rd edition. Copyright 1960 by Cornell
University.
7
PRIMARY ATOMIC BONDING
IONIC BONDING: Metallic elements give their valence electrons
to non metallic elements, doing so the atoms acquire stable
configuration
• Occurs between + and - ions.
• Requires electron transfer.
• Large difference in electronegativity required.
• Example: NaCl
Na (metal)
unstable
Cl (nonmetal)
unstable
electron
Na (cation)
stable
+
Coulombic
Attraction
Cl (anion)
stable
EXAMPLES: IONIC BONDING
• Predominant bonding in Ceramics (metallic and non m
NaCl
MgO
H
He
CaF
2.1
2
Be
O
Li
F
Ne
CsCl
3.5 4.0
1.0 1.5
Na
0.9
K
0.8
Rb
0.8
Cs
0.7
Fr
0.7
Mg
1.2
Ca
1.0
Sr
1.0
Cl
3.0
Ti
1.5
Cr
1.6
Ba
0.9
Fe
1.8
Ni
1.8
Zn
1.8
As
2.0
Br
2.8
I
2.5
At
2.2
Ar
Kr
Xe
Rn
-
Ra
0.9
Give up electrons
Acquire electrons
Bonding energy is large, thus known to have high melting temperatures
(e.g. MgO Æ Tm=2800 C), ionic materials are known to be hard and
brittle, electrically and thermally insulative.
COVALENT BONDING
Stable electron configuration is achieved by sharing of electrons
• Requires shared electrons
• Example: CH4
C: has 4 valence e,
needs 4 more
CH4
H: has 1 valence e,
needs 1 more
H
Electronegativities
are comparable.
H
C
H
shared electrons
from carbon atom
H
shared electrons
from hydrogen
atoms
* Covalently bonded material are strong but posses poor ductility
10
and poor electrical and thermal conductivity.
EXAMPLES: COVALENT BONDING
H2
H
2.1
Li
1.0
Na
0.9
K
0.8
Rb
0.8
Be
1.5
Mg
1.2
Ca
1.0
Cs
0.7
Sr
1.0
Ba
0.9
Fr
0.7
Ra
0.9
•
•
•
•
column IVA
H2O
C(diamond)
SiC
Ti
1.5
Cr
1.6
Fe
1.8
Ni
1.8
Zn
1.8
Ga
1.6
C
2.5
Si
1.8
Ge
1.8
F2
He
O
2.0
As
2.0
Sn
1.8
Pb
1.8
GaAs
F
4.0
Cl
3.0
Br
2.8
I
2.5
At
2.2
Ne
Ar
Kr
Xe
Rn
-
Adapted from Fig. 2.7, Callister 6e. (Fig. 2.7 is
adapted from Linus Pauling, The Nature of the Chemical Bond, 3rd edition, Copyright
1939 and 1940, 3rd edition. Copyright 1960 by Cornell University.
Molecules with nonmetals
Molecules with metals and nonmetals
Elemental solids (RHS of Periodic Table)
Compound solids (about column IVA)
Cl2
METALLIC BONDING
Metals have one or two or at the most three valence electrons. These
valence electrons are given away forming a sea of electrons
between posotive charged ions.
• Arises from a sea of donated valence electrons
(1, 2, or 3 from each atom).
+
+
+
+
+
+
Adapted from Fig. 2.11, Callister 6e.
+
+
+
• Primary bond for metals and their alloys
12
SECONDARY BONDING (Van der waal Bonding)
Weak electrostatic attraction arising from polarized molecules.
Polarized molecule: A molecule whose structure cause a portion of
the molecule to have a negative charge while other portion have a
positive charge.
Molecule induced dipole
SECONDARY BONDING (Van der waal Bonding)
Arises from interaction between dipoles
• Fluctuating dipoles
asymmetric electron
clouds
- secondary +
+
bonding
-
Adapted from Fig. 2.13, Callister 6e.
• Permanent dipoles-molecule induced
-general case:
+
-
-ex: liquid HCl H
Cl
-ex: polymer
secondary
bonding
secondary
bonding
secon
d
a ry b
ondin
+
-
H Cl
g
Hydrogen bonding:
A special type of
secondary bonding,
exist when hydrogen
is one of the dipole.
It is the strongest
secondary bonding
type