Atoomstructuur en het Periodiek systeem

Chemische Binding (Bonding)
hfst. 8 Zumdahl
Inter-moleculaire binding
Intra-moleculaire binding
Models (Zumdahl, end § 8.7)
Models are “attempts to explain how
nature operates on the microscopic level
based on experiences in the
macroscopic world”
(Zumdahl 5th p. 370; 6th p.368; 7th p. 348).
Fundamental Properties of Models
- Models…
• are human inventions based on incomplete understanding
• and do NOT equal reality
• are (over)simplifications, and are therefore often wrong.
• become more complicated as they age.
- We must understand the underlying assumptions in a model to
prevent misuse.
(Zumdahl 5th p. 372; 6th p.370; 7th p. 350).
What models in Zumdahl Ch. 7?
• Energy and waves
• Atoms:
• The Bohr model
• Quantum Mechanical model
• Underlying assumption/insight:
• all three: quantisation of energy
• Bohr vs. Quantum-Mechanical:
• localised vs. delocalised electrons
Chemische Binding (Bonding)
Algemene Concepten (Ch. 8)
Inter-moleculaire binding
Intra-moleculaire binding
Chemische Binding (‘Bonding’)
Drie modellen voor binding:
Covalente binding
Polaire covalente binding
Overview of Bonding-types (8.2)
• Perspectief:
• vanuit de atomen in de
• Extremen:
• Ionbinding =
elektronen/lading is geheel
• Covalente binding:
elektronen paar wordt perfect
• Polaire Covalent
• zit daar precies tussen in,
lading is enigzins verdeeld
Overview of Bonding-types (8.2)
• Waarom (verklaring voor dit
• atomen willen “edelgas”
• die vertegenwoordigen voor
elk atoom een bereikbare,
lagere energietoestand
• Edelgasconfiguratie:
• atoom: buitenste s en p
orbitalen zijn gevuld met 8
elektronen (H, He: 2
• Polair Covalent
• elektronen zitten wat dichter
bij het ene atoom
Achieving Noble Gas Electron
Configurations (NGEC)
General (not always applicable) rule:
A nonmetal and representative group metal react
• The valence electron configuration of the nonmetal are filled
to achieve NGEC.
• The valence orbitals of the metal are emptied to achieve
Two nonmetals react: they share electrons to achieve NGEC
(covalent bonding or polair covalent)
NGEC in a Bond: result
Metal + nonmetal:
• Ionic bond
Two of the same non-metals:
• Covalent bond
Two different non-metals:
• polar covalent bond
Division of charge: 5th fig. 8.11; p. 368; 6th p. 366; 7th Fig. 8.12 p. 346
Bond type
• Kunnen we een eigenschap van elk atoom definiëren
en karakteriseren...
• met een voorspellende waarde t.a.v. welke binding
zich zal vormen als twee atomen A en B een binding
Electronegativity (8.2)
“The ability of an atom in a molecule to attract shared electrons to itself.”
Difference in Electronegativity
Bond Type
Polar Covalent
A molecule, such as HF, that has a center of positive
charge and a center of negative charge is said to be
polar, or to have a dipole moment.
δ+ δ-
Polar Covalent Bond:
dipole moment
• Two different non-metals:
• polar covalent bond
• Diatomic molecule:
• always
• Polyatomic molecule:
• depending on structure
• 5th fig. 8.4; p. 355-356; 6th p. 355; 7th p. 336-337
• Examples: H2O, NH3, SO3, CH4, H2S
Bond Length
Definition in the context of different models?
“The distance where the system energy is at
its minimum”
fig. 8.1: The distance between nuclei where
the quantum mechanic probability function
is at its maximum
Covalente binding
Polaire covalente binding
Ionic Bonds
When would these be formed?
When a low(er) energy state is achieved
Whereby determined?
1. Lattice energy (roosterenergie):
net energy gain or loss by electrostatic
attractions/repulsions of closely packed ions.
see 5th, figure 8.8; 7th figure 8.9 and §8.5
2. Energy involved to lose or gain electron in reaction
Ionic Bonds
Diatomic molecule:
E = 2.31* 10-19 [J.nm] (Q1. Q2) / r)
Q1 and Q2 = numerical ion charges
r = distance between ion centers (in nm)
* with a positive & negative Q, the result is…
* negative, thus a lower energy state achieved
Lattice Energy (8.5)
The change in energy when separated gaseous ions
are packed together to form an ionic solid
M+(g) + X-(g)
Lattice energy is negative (exothermic)
(energy is released from the system of ions that
combine into a lattice)
Formation ionic solid
Total enthalpy change = state property
! define suitable steps (see 5/6th Figure 8.8; 7th Fig. 8.9 )
• Sublimation of the solid metal
Li F [kJ/mol]
M(s) ⌫ M(g)
2 Ionization of the metal atoms
M(g) ⌫ M+(g) + e- [endothermic]
3 Dissociation of the nonmetal
½X2(g) ⌫ X(g)
4 Formation of X- ions in the gas phase:
X(g) + e- ⌫ X- (g) [exothermic)
5 Formation of the solid MX
M+(g) + X-(g) MX(s)
[quite exothermic]
Lattice Energy
E = k [J.nm] (Q1. Q2) / r)
Q1 and Q2 = numerical ion charges
r = distance between ion centers (in nm)
Covalente binding
Polaire covalente binding
Bond Energy
- The net energy-input to a molecule required to break a
particular bond.
- It gives us information about the strength of a bonding
Hess’s Law (Ch. 6)
The enthalpy change of an overall process is the sum of the
enthalpy changes of its individual steps.
⇒ Also to be used for calculation of reactionenthalpies from bond energies!
⇒ Why: Enthalpy = a state property!
Reaction Energies
Bond breaking requires energy (endothermic)
Bond formation releases energy (exothermic)
ΔH = Σ ΔH (bonds broken) – Σ ΔH (bonds formed)
energy required
energy released
Born Haber cycle for ammonia synthesis
N2 + 3 H2 ⇔ 2 NH3
Enthalpy change:
ΔHo = - 92 kJ/mol N2
Enthalpy change equals also
= ΔHo (break N2)
= 941
ΔHo (break H2 )*3
= 3*432
ΔHo (formation N-H)*6
= - 6*391
= -109 kJ/mol
Any difference between outcome and actual (measured) value is
caused by effects not described by this simple model (e.g. polarity)
Ammoniak - produktie
The production of the N2/H2 mixture:
Reforming + CO-shift
Chemische Binding
Inter-moleculaire binding
Intra-moleculaire binding
Intermoleculaire Binding
Gelokaliseerde electronmodellen
- Ionbinding
- Lewis structuren
- Valence Bond model
• Hybridisatie (Hfk. 9)
Gedelokaliseerde electronmodellen (hfk. 9)
- MO theorie
- Metaalbinding
Lewis Structure
- Shows how valence electrons are arranged among
atoms in a molecule.
- Reflects central idea that stability of a compound
relates to NGEC - noble gas electron configuration.
Valence electrons
- De elektronen in de orbitalen die het laatst gevuld
worden, I.e. met het hoogste quantumgetal voor het
bereiken van NGEC
- Periodiek systeem:
rij 1: H en He: 1s
totaal 2
rij 2: Li t.m Ne: 2s, 2p
totaal 8
rij 3: K t.m Ar: 3s, 3p
totaal 8
Comments About the Octet Rule
2nd row elements C, N, O, F observe the octet rule.
2nd row elements B and Be often have fewer than 8 electrons around
themselves - they are very reactive
3rd row and heavier elements CAN exceed the octet rule using empty
valence d orbitals.
When writing Lewis structures, satisfy octets first, then place electrons
around elements having available d orbitals.
Lewis-structuren schrijven
Standaard procedure (8.10):
1. Tel alle valentie electronen op van alle atomen, het gaat om
het totaal!
2. Teken een − tussen elk paar verbonden atomen (dus geen :
of .. !!!)
3. Verdeel de overige electronen, zo dat de duet regel voor
waterstof, en de octet regel voor overige wordt nageleefd.
(dit vergt ‘trial and error’)
Probeer HF, H2S, NH3, CH3OH, POCl3
Lewis structures /Formal Charge
Bij gebruik van de procedure zijn er soms meerdere
De beste Lewis-structures zijn die die de laagste energietoestand weergeven
> Molecules: Formal charge on each atom = 0;
> Ions: Formal charge on least/most
electronegative atom (positive/negative ion)
Formal Charge
The difference between the number of valence
electrons (VE) on the free atom and the number
assigned to the atom in the molecule.
We need:
1. # VE on free neutral atom
2. # VE “belonging” to the atom in the
Formal Charge
Not as good
Occurs when more than one valid Lewis structure can
be written for a particular molecule.
These are resonance structures. The actual structure is
an average of the resonance structures.