6. Element & atom
• Chemical element: substance that cannot be broken down
into simpler substances & made of only one type of atom
• Chemical element has its own chemical symbol
• Atom: smallest constituent unit of ordinary matter that has
properties of a chemical element.1
6
1 ‘Atom’. Compendium of Chemical Terminology (IUPAC Gold Book) (2nd ed.). IUPAC. Retrieved 2015-04-25
8. Atomic structure
• Atom: nucleus (center) & electron
shells (outside)
• Nucleus: protons & neutrons
• Electron shells consist of electrons
• Most of atom is empty space !!!
8
nucleus
electron
shells
proton neutron
electron
Particle Symbol Mass /kg Relative
mass
Charge /C Relative
charge
Proton p 1.673 × 10–27 1 +1.602 × 10–19 +1
Neutron n 1.675 × 10–27 1 0 0
Electron e 9.109 × 10–31 1/1836 (~0) –1.602 × 10–19 –1
• Most of mass of atom concentrated in nucleus !!!
9. Atomic & mass number
• Proton/atomic number (Z): number of protons
• Each element has a unique Z
• Nucleon/mass number (A): number of protons & neutrons
• Isotopes: atoms with same Z & different A
• Isotopes have same chemical properties
9
10. • Relative atomic mass ( ): weighted mean of mass
numbers of all naturally occurring isotopes
• Example:
Mg: 78.99% 24Mg 10.00% 25Mg 11.01% 26Mg
masses 23.99 24.99 25.98
Isotopes
10
11. Isotopes
11
• Determination of accurate Ar from mass spectrometry
Mass spectrometer
Mass of each isotope
(mass/charge ratio)
How much of each
isotope (relative
abundance)
Isotopic
mass
Abundance
/%
20 90.9
21 0.3
22 8.8
Mass spectrum
12. Electronic structures of atoms
• Schrödinger equation:
• Solution of the equation provides:
Wavefunctions → atomic orbitals (AOs)
Energy E associated with particular
12
Why ?
13. AOs & Quantum Numbers
• AOs defined by 3 quantum numbers:
Principal quantum number ( ): related to size & energy AO
Orbital quantum number ( ): determines shape of orbital
Magnetic quantum number ( ): related to orientation of AO
13
principal
orbital
magnetic
15. Ground State Electronic Configuration
• aufbau (building-up) principle: lowest energy AOs filled first
• Pauli exclusion principle: maximum of 2 electrons in an AO
• Hund’s rule: maximum total spin
Example: ground state electronic
configuration of N (Z = 7), Cu (Z = 29)
15
16. Classic Periodic Table
16
• Most successful classification by Dmitri Mendeleev in 1869
Dmitri Ivanovich Mendeleev
(1834 –1907)
17. Classic Periodic Table
17
• Arrangement of elements in order of atomic weight
• Elements with similar chemical properties in same group
Annalen der Chemie und Pharmacie, VIII, Supplementary Volume for 1872, page 511
18. Modern Periodic Table
18
• Arrangement of elements in order of atomic number
row ~ period
column ~ group
19. The Periodic Table & Electronic Configuration
19
• Period: elements with same electron shells
• Period number: number of electron shells
• Group: elements with same valence electronic
configuration
• Group number: number of valence electrons with ‘1–18’ or
‘I – VIII’ numbering system
21. Basic concepts
• A molecule is an electrically neutral group of two or more
atoms held together by chemical bonds.1
• Classification of chemical bonds:
Ionic bonds: electron transfer between atoms to form ions
Covalent bonds: electron sharing between atoms
Metallic bonds
21
1 ‘Molecule’. Compendium of Chemical Terminology (IUPAC Gold Book) (2nd ed.). IUPAC. Retrieved 2015-04-25
22. Molecular structure & Bonding
1. The Octet Rule
2. Lewis Diagram
3. Molecular Shapes & the VSEPR Model
4. Valence Bond (VB) theory
5. Molecular Orbital (MO) theory
22
23. The octet rule
• Atoms gain, lose or share electrons to give an outer shell
with 8 electrons (an octet)
• Elements in Period 2 strictly obey the octet rule
23
If atoms is not only 8 electron in outer
shell ?
24. The octet rule
• Example: MgO
Mg (Z = 12) 1s2 2s22p6 3s2
Mg – 2e → Mg2+ (1s2 2s22p6)
O (Z = 8) 1s2 2s22p4
O + 2e → O2– (1s2 2s22p6)
→ MgO ionic compound
24
O
x
x
x
x
x
x
x Mg
x
x
x +
x
x
O2–
x
x
x
x
x
x
Mg2+
x
x
x
x
x
x
25. The octet rule
• Example: O2
O (Z = 8) 1s2 2s22p4
→ O2 covalent compound
25
O
x
x
x
x
x
x
O
+ O
x
x
x
x
x
x O
26. Molecular structure & Bonding
1. The Octet Rule
2. Lewis Diagram
3. Molecular Shapes & the VSEPR Model
4. Valence Bond (VB) theory
5. Molecular Orbital (MO) theory
26
27. Lewis Electron-dot Diagrams
• Using dots to describe number & arrangement of valence
electrons in molecules
• Example:
• Bonding pairs: pairs of electrons involved in bonding &
represented by a single line (–)
• Non-bonding electrons: electrons not involved in bonding
Atoms Molecule
28. Lewis Electron-dot Diagrams
• Constructing Lewis structure:
1. Determine overal number of valence electrons
2. Write arrangement of atoms bonded together
3. Distribute electrons in pairs so that each atom has an octet
• Example: CO2, SCN–
28
29. Resonance structures
• More than one valid Lewis structure with a given atomic
arrangement → resonance structures
• Resonance indicated by double-headed arrow
• Actual structure ~ a resonance hybrid of all resonance
structures
• Example: CO3
2–
29
a given chemical formula
resonace structure describe the position of
elctrons in compound.
30. Higher electron counts
• Number of electrons around central atom over 8 →
expanded shell & hypervalent molecules
• Frequently observed for elements of 3rd & higher periods
due to the d orbitals
• Example: PCl5, SF6
30
31. Molecular structure & Bonding
1. The Octet Rule
2. Lewis Diagram
3. Molecular Shapes & the VSEPR Model
4. Valence Bond (VB) theory
5. Molecular Orbital (MO) theory
31
32. VSEPR Model & Molecular Shapes
• Molecular shape: 3D arrangement of atoms that constitute a
molecule
• Experimentally determined by spectroscopic methods
• Valence-Shell Electron-Pair Repulsion (VSEPR) model
• Simple but useful to predict shape of small molecules of
main group elements
32
33. VSEPR Model & Molecular Shapes
• Valence-Shell Electron-Pair Repulsion (VSEPR) model
• Assumption: valence electron pairs adopt arrangements
that minimize repulsion between them
33
Electron
pairs
2 3 4 5 6
Geometry
linear
trigonal
planar
tetrahedral
trigonal
bipyramidal
octahedral
Standard
bond angle
180° 120° 109.5° 90°, 120° 90°
34. VSEPR Model & Molecular Shapes
• Valence-Shell Electron-Pair Repulsion (VSEPR) model
• Electron–electron repulsions decrease in sequence:
lone-lone pair > lone-bonding pair > bonding-bonding pair
triple -single bond > double -single bond > single -single bond
34
VSEPR model describe the 3D shape in space so
of compound
39. Molecular structure & Bonding
1. The Octet Rule
2. Lewis Diagram
3. Molecular Shapes & the VSEPR Model
4. Valence Bond (VB) theory
5. Molecular Orbital (MO) theory
39
40. Valence Bond (VB) theory
• Principle: chemical bonds ~ overlap of valence AOs on two
different nuclei so that both nuclei share pair of electrons
• Localized electron model (localized AOs)
• Homonuclear diatomic molecules
40
x
y
z
↑↓
↑↓ ↑↓
↑↓ ↑↓
F F
σ bond
H H
↑↓
σ bond
H2 F2
F (Z = 9) 1s2 2s2 2p5
41. Valence Bond (VB) theory
• Homonuclear diatomic molecules
41
x
y
z
O (Z = 8) 1s2 2s2 2p4
O2
π bond
↑↓
↑↓ ↑↓
↑ ↓
O O
σ
N (Z = 7) 1s2 2s2 2p3
N2
π
↑↓
↓ ↑
↑ ↓
N N
σ
π
observe empty orbital
in atom and
42. VB theory & Hybridization of AOs
Polyatomic molecules – Hybridization of AOs
• Hybridization of AOs: combinations of AOs to
mathematically obtain new AOs for molecular shapes in
terms of σ-bonds
• Number of hybrid orbitals = number of AOs mixed
• Label of hybrid orbitals reflects contributing AOs
• Type of hybrid orbitals varies with types of AOs mixed
42
46. Hybridization related to d-orbitals
sp3d sp3d2
s
p p p
sp3d
d d d d
d d d d
d
s
p p p
sp3d2
d d d d
d d d d
46
E E
47. Double & Triple bonds in VB theory
C C
↓↑
↓↑
↓↑
↓↑
↓↑
↓↑ ↓↑
↓↑
↑
↑
↓
↓
↑ ↓
47
48. Molecular structure & Bonding
1. The Octet Rule
2. Lewis Diagram
3. Molecular Shapes & the VSEPR Model
4. Valence Bond (VB) theory
5. Molecular Orbital (MO) theory
48
49. Molecular Orbital (MO) theory
• Generalization AO description of atoms to MO description of
molecules
• MOs: space spread whole molecule, in which a single
electron can occupy → delocalized electron model
• Linear Combination of Atomic Orbital (LCAO) approximation
49
when 2 atom bond form bond, and valence atomic
orbital is wave function and the property of
wwave fuction is the same
when 2 atom bond form bond, and valence atomic
orbital is wave function and the property of
wwave fuction is the same
50. Molecular Orbital (MO) theory
• MO arises from interactions between AOs if interactions are:
Allowed if symmetries of AOs are compatible
Efficient if region of overlap is significant
Efficient if AOs are relatively close in energy
• Number of MOs equal number of contributing AOs
50
s s px py px s pz s
51. Molecular Orbital (MO) theory
• Homonuclear diatomic molecules
destructive
interaction
↑↓
↑
H H
H2
51
E
anti-bonding MO
constructive
interaction
bonding MO
↑
53. Orbital energies for main group elements
53
J. B. Mann, T. L. Meek, L. C. Allen, J. Am. Chem. Soc., 2000, 122, 2780
54. MO diagram of X2 (X = O, F, Ne)
X X
X2
2s 2s
2p 2p
54
E
55. MO diagram of X2
X X
X2
2s 2s
2p 2p
2s 2s
2p 2p
X X
X2
55
E
(X = O, F, Ne) (X = Li, Be, B, C, N)
56. Molecular Orbital (MO) theory
• Heteronuclear diatomic molecules
antibonding MOs close to
less electronegative atom
bonding MOs close to
more electronegative atom
56
X Y
XY
less
electronegative
more
electronegative
E
57. MO diagram of CO
2s
2s
2p
2p
C O
CO
(-32.38 eV)
(-15.85 eV)
(-10.66 eV)
(-19.43 eV)
↑↓
↑↓
↑↓
↑ ↑
↑ ↑
↑↓
↑↓
↑↓ ↑↓
↑↓
57
E
58. Molecular Orbital (MO) theory
• Very unequal energies → resulting MOs with energies &
shapes closer to original AOs
58
∗
X Y
XY
E
less
electronegative
more
electronegative
59. MO diagrams of HF & LiF
𝜎
𝜎
𝜎∗
𝜋
2s
1s
2p
H F
HF
(-40.2 eV)
(-18.7 eV)
(-10.7 eV)
↑↓
↑
↑↓ ↑↓ ↑
↑↓
↑↓ ↑↓
↑↓
𝜎
𝜎
𝜎∗
2s
2s
2p
Li F
LiF
(-40.2 eV)
(-18.7 eV)
(-5.4 eV)
↑↓
↑
↑↓ ↑↓ ↑
↑↓
↑↓ ↑↓
↑↓
𝜋
59
E
E