Today’s Lecture
• Regularities in chemical properties• The periodic table• Connection to quantum mechanics• Chemical bonds:
– Ionic– Covalent
• Molecules in space
The Story of Stuff: So Far
• The Big Bang made hydrogen and helium.
• Stars made heavy elements and dispersed them through supernova explosions.
• Gas clouds are filled with many different elements.
General Principle:• At low temperatures, particles tend to
prefer more binding energy and more bound particles
• At high temperatures, particles tend to prefer more spatial freedom and more unbound particles.
• In cold interstellar clouds, particles agglomerate into atoms and molecules.
The Atom in Physics and Chemistry
• Physics: electrons bound to a nucleus
• Chemistry: smallest chemical unit
Chemical Evidence for Atom• Compounds combine with small,
whole number ratios of elements• These ratios represent the number
of atoms that combine in each molecule of a compound: for example
2 H2 + O2 2 H2O• Atom: smallest unit to share in
chemistry
Crystals: Atoms packed together
• Atoms combine in particular geometrical shapes
• Reflects the geometry of how individual atoms combine
Water Salt
The Periodic Table
• Elements are arranged in columns according to chemical properties; rows according to atomic mass.
• Successes– Organized elements in rational
scheme– Predicted existence of new elements
• Shortcoming– Empirical (how, not why)
Evidence for the Physics Atom before Quantum Mechanics
• Brownian motion---jiggles of small particles in a liquid can be explained by collisions with large numbers of atoms
• Gas laws---relations between density, temperature, and pressure---can be explained by colliding atoms (or molecules)
Quantum mechanics: connecting thephysics and chemistry atom
ħ2
2m2 Ψ + VΨ = EΨ
The Schrödinger Equation
Application of Schrödinger Equation to Atom
• Predicts wave function for electron orbiting nucleus (electric force)
• Standing waves occur only for particular energies
Orbitals
Standing waves of probabilityThe chance of finding an electron is given by the square of the wave function at a certain location
Mathematical predictions from the Schrödinger equation
Comparison with the Bohr atom
Electrons in orbit around nucleus
Probability waves in constructive interference
x
y
z
How it all works
• Orbitals have different energies• Orbitals have specific shapes• Electrons in a system settle into
the lowest energy states available• Pauli Exclusion Principle
Pauli Exclusion PrincipleNo two electrons can have
the same quantum state.
Quantum state: a solution of the Schrödinger equation, which can be identified by its set of labels called “quantum numbers.”
Quantum numbers represent(for electrons)
l : Angular momentum = l x ħ (orbital motion)l = 0,1,2,3, …
ml : Alignment of l along z-axis = ml x ħ
ml = 0,+1,+2,+3,…. |ml| < l
s : Intrinsic angular momentum (“Spin”) = s x ħ s = ½
ms : Alignment of s along z-axis = ms x ħ
ms = +½, -½
Quantized Projection of ℓ
x
y
z
lml
The projection of l along the z-axis, ml, is quantized, it can take only values 0,±1ћ,
±2ћ,…±nћ
Only certain orientations for l are possible
Orbital
Name
Angular momentu
m
Number of possible l
orientations
S 0 1
P ћ 3
D 2ћ 5
F 3ћ 7
Orbital Properties
Orbital Name
Angular Momentum
#l orientatio
ns
# of electron states in orbital
S 0 1 2
P ħ 3 6
D 2ħ 5 10
F 3ħ 7 14
Nomenclature
•nl –n = principle quantum number– l is called
•S (l = 0)•P (l = 1)•D (l = 2)•F (l = 3)
Example2p
Nomenclature
•nl –n = principle quantum number– l is called
•S (l = 0)•P (l = 1)•D (l = 2)•F (l = 3)
Example2p n=2,
l = 1
Electronic Configuration
n principle quantum numberl orbital angular momentum# number of electrons in orbital
Nomenclature: nl#
Open and Closed Shells
• When all of the orbitals for a particular n (called a “shell”) are full, the shell is closed.
• When the shell has empty slots, it is open.
• Only electrons in open shells participate in chemistry.
• Atoms with closed shells are chemically inert.
Quantum Mechanics and the Periodic Table
• All atoms with the same number of electrons in open shells have similar chemistry
• Number of columns is due to the number of electrons allowed in orbitals
Orbital Properties
Orbital Name
Angular Momentum
#l orientatio
ns
# of electron states in orbital
S 0 1 2
P ħ 3 6
D 2ħ 5 10
F 3ħ 7 14
Chemical Bonds
• Atoms tend to minimize their energy by obtaining a closed-shell configuration
• Two possibilities– Lose or gain electrons (ion=charged
atom) “Ionic bond”– Share electrons with other atoms “Covalent bond”
Chemical Bonds: Ionic
• Ions --- atoms that have gained or lost electrons beyond their neutral state
• Positive ions’ charge balances negative ions
• Shape of crystal results from packing together ions of different sizes
Example: Salt = Sodium Chloride
How do sodium and chlorine most easily obtain a closed-shell structure?
How does atom attain a closed shell?
• Sodium has one extra electron, so it loses one.
• Chlorine needs one extra electron, so it gains one.
Which atoms form ionic bonds?• Elements in first (second) column have
one (two) loosely bound electron(s).• These atoms lose electrons and form
positive ions. • Elements in last (next to last) column
require one (two) electron(s) to complete a closed shell
• These atoms lose electrons and form negative ions.
Salts
• Na (sodium) + Cl (chlorine)– Na+ + Cl- NaCl
• Ba (barium) + F (fluorine)– Ba++ + 2F- BaF2
• Cs (cesium) + I (iodine)– Cs+ + I- CsI
Chemical Bonds: Covalent
The wave function of an electron from one atom overlaps that of an electron from a different atom
Bonding orbital
Negative charge screens one nucleus from the other, and attracts nucleus
+ +-Constructive Interference
+ +
Anti-bonding orbital
Destructive Interference
Negative charge screen is absent, nuclei “see” each other, repel each other, attracted to negative charge opposite the nucleus
Building Diatomic Molecules
1s
2sHydrogen
H2 exists
2 bonding electrons
0 antibonding electrons
Bonding
Anti-bonding
Anti-bonding
Bonding
1s
2sHelium
He2 does not exist
2 bonding electrons
2 antibonding electrons
Bonding
Anti-bonding
Anti-bonding
Bonding
1s
2sLithium
Li2 exists
4 bonding electrons
2 antibonding electrons
Bonding
Anti-bonding
Anti-bonding
Bonding
1s
2sBeryllium
4 bonding electrons
4 antibonding electrons
Be2 does not existBonding
Anti-bonding
Anti-bonding
Bonding
Diatomic Molecules
• The following molecules have more bonding than anti-bonding electrons– H2, Li2, B2, C2, N2, O2, F2
– These molecules exist in nature• The following molecules have
equal numbers of anti-bonding and bonding electrons– He2, Be2, Ne2, …– These do not exist in nature
Successes of Schrödinger Atom
• Explains patterns in periodic table• Explains chemical properties of
elements• Explains structure of crystals and
molecules
Molecules in Space
• Supernova explosions enrich the interstellar gas with heavy elements
• They become incorporated into gas clouds
• Gas clouds can form molecules– Mostly H2
– But many, many other molecules are seen
Interstellar Molecules: Two Atoms
AlF AlCl C2 CH CH+ CN CO CO+ CP CS CSi HCl HF H2 KCl NH NO NS NaCl OH PN SF SO S0+ SiN SiO SiS
Carbon monoxideHydroxyl radicalInterstellar SiN
Interstellar Molecules: Three Atoms
C3 C2H C20 C2S CH2 HCN HCO HCO+ HCS+ HOC+ H20 H2S HNC HNO MgCN MgNC N2H+ N20 NaCN OCS S02 c-SiC2 CO2 NH2 H3+ SiCN
Water!
Interstellar Molecules: Four Atoms
c-C3H l-C3H C3N C30 C3S C2H2 CH2D+? HCCN HCNH+ HNCO HNCS HOCO+ H2CO H2CN H2CS H30+ NH3 SiC3
FormaldehydeAmmonia