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- Tema 2. Enllaç químic i estructura molecular
- 2.1 Tipus d’enllaç químic. Enllaç iònic. Enllaç covalent. Estructures de Lewis.
Ressonància. Polaritat i electronegativitat. Paràmetres d’enllaç: longitud
d’enllaç i energia d’enllaç. (CAPÍTOL 2 DEL LLIBRE DE REFERÈNCIA)
- 2.2 Geometria molecular. Model de les repulsions entre parells d’electrons de
la capa de valència (VSEPR). Teoria de l’enllaç de valència: enllaços sigma i pi.
Hibridació dels orbitals atòmics.
- 2.3 Teoria dels orbitals moleculars. Molècules diatòmiques homonuclears i
heteronuclears. Molècules poliatòmiques. Teoria de bandes de sòlids
Chemical bond
- Chemical bond: union among atoms (Atkins)
GREC: “Interacció entre dos o més àtoms el resultat de la qual és la formació d'un compost,
és a dir, d'un agregat que és estable dins un cert interval de temperatures i pressions”
- A chemical bond is formed if the resulting arrangement of the nuclei and valence electrons has lower energy than the total energy of the separated atoms
- Several models for the chemical bond:
- ionic: “complete transfer of electrons”: LiCl
- covalent: sharing electrons in individual molecules: HCl
- metallic: “cations bonded by a sea of electrons”
- … and many others
http://www.xtal.iqfr.csic.es/Cristalografia/parte_01_2-en.html
Ionic bond
- The ionic model is useful to describe binary compounds formed by:
- A metal element from s block
- A non-metal element
- Ionic compounds (e.g. NaCl) are not molecules ionic solid
- In general, an ionic solid is a regular arrangement of cations and anions
- Ionic solids are a particular type of crystalline solids (Tema 3)
- Crystalline solids are those where atoms, ions or molecules are arranged in a regular way
- There are four types of crystalline solids :
- Ionic solids: NaCl, CaF 2 , …
- Molecular solids: ice (H 2 O), glucose, …
- Networks: diamond, graphite, SiO 2 , …
- Metal: copper, silver, …
http://yourkamagraguide.com/files4/ice-crystal-shape.html http://www.e6cvd.com/cvd/page.jsp?pageid=
http://www.chemguide.co.uk/at oms/structures/ionicstruct.html
Example of a molecular solid Example of a network
Electronic configuration of ions (1): cations
- It is possible to predict the molecular formula of a binary ionic compound from the electronic configuration of the constitutive cation and anion
- For elements of s block:
- Loosing electrons produces a cation
- In general, cations have an electronic structure like ns^2 np^6 (octet)
- Examples: Na+, Mg2+, Al3+,… C4+?
- Metal atoms lose s - and p -electrons to obtain noble gas electron configurations
Be: 1 s^22 s^2 Be2+: 1 s^2
Electronic configuration of ions (3): Lewis symbols
- One of the best known modern chemists
- He proposed a very useful model for the covalent bond that enables the
explanation of many organic and inorganic reactions
- He proposed a nomenclature to follow the course of electrons through a
reaction:
- Lewis symbols correspond to the valence configuration of an atom or ion
- Example: formation of CaCl 2 :
Gilbert N. Lewis (USA, 1875-
Paired electrons
Unpaired electron
The formation of NaCl(s) from Na(s) and Cl2(g)
http://www.youtube.com/watch?v=2mzDwgyk6QM
En quin estat físic està inicialment el sodi? Quina és la temperatura d’un bec Bunsen? És prou alta per tenir Na(g)? De quin color és la flama? Per què té aquest color? De quin color és el clor (“chlorine”)? Tenim Cl o Cl 2? Per què el sòlid final és de color blanc?
The formation of ionic bonds: key points
- The most important contribution to the energy scheme previously shown is the
ionization energy of one of the two elements involved in the ionic bond
- Only metal elements with low ionization energies form monoatomic cations
- All cations interact with all anions:
- The ionic bond has no direction (and the covalent bond?)
- The ionic bond is a characteristic of a crystal, not of a pair of ions
- The ionic solid is a regular arrangement of cations and anions
- Can we propose a stoichiometry from this arrangement? How?
- Ionic model: simple description of the ionic bond in terms of ions and Coulomb’s law
- Not always valid, best results for s block why?
http://www.chemistry.wustl.edu/~courses/genchem/Tutorials/Water/Solubility.htm
Ionic model: lattice energy
- Lattice energy:
- Definition: energy difference between ions grouped in a solid and widely separated ions
in gas phase:
NaCl(s) + lattice energy Na+(g) + Cl-(g) lattice energy = 787 kJ·mol-
- Nomenclature: U, E, DHL…
- It is always positive (from ionic solid to separated ions in gas phase)
- Depending on the definition it may be also negative!
- We will come back again to this definition in Chapter 7
- Lattice energy is strongly influenced by two terms:
- electrostatic interaction among charges (electrostatic or Coulomb’s potential energy)
- repulsion among atomic orbitals (because atoms are not point charges!)
Ionic model: Coulomb’s energy
- 1D lattice:
- 3D lattice:
- Which variables affect to Ep?, how do they affect to EP?
- What about MgO and MgS?
π d
z z N e
E P A A
0
2 1 2
π d
zz N e
EP A
0
2 1 2
2 ln 2
Electrostatic potential energy (EP) (also known as Coulomb’s energy) is the sum of attractive and repulsive electrostatic interactions
Ionic model: steric effects
- Why ions are not close enough so that d is almost zero?
- Overlap steric effects repulsion
- As for orbitals, the repulsion decays exponentially:
- Potential energy = Coulomb’s energy + steric effects
E e d d* 34.5pm
d
P
r=d
A
d
d
π d
N z z e
U
CA CA
A A B
2
d *^ = 34.5 pm dCA = distance from cation to anion
Born-Meyer equation:
http://catalog.flatworldknowledge.com/bookhub/4309?e=averill_1.0-ch08_s
Donat el caràcter i la finalitat exclusivament docent i eminentment il·lustrativa de
les explicacions a classe d'aquesta presentació, l’autor s’acull a l’article 32 de la Llei
de propietat intel·lectual vigent respecte de l'ús parcial d'obres alienes com ara
imatges, gràfics o altre material contingudes en les diferents diapositives
- Tema 2. Enllaç químic i estructura molecular
- 2.1 Tipus d’enllaç químic. Enllaç iònic. Enllaç covalent. Estructures de Lewis.
Ressonància. Polaritat i electronegativitat. Paràmetres d’enllaç: longitud
d’enllaç i energia d’enllaç.
- 2.2 Geometria molecular. Model de les repulsions entre parells d’electrons de
la capa de valència (VSEPR). Teoria de l’enllaç de valència: enllaços sigma i pi.
Hibridació dels orbitals atòmics.
- 2.3 Teoria dels orbitals moleculars. Molècules diatòmiques homonuclears i
heteronuclears. Molècules poliatòmiques. Teoria de bandes de sòlids
Lewis structures to visualize the covalent bond
- Lewis structures are based on Lewis symbols
- The arrangement of atoms in these structures is such that the atoms fill their electron shell in order to achieve a noble gas electron configuration (octet rule) - H: 1s^1 - 1 electron in valence shell It needs 1 additional electron and reach [He] - F: [He] 2s^2 2p^5 - 7 electrons in valence shell It needs 1 additional electron and reach [Ne]
- Valence of a given atom (s. XIX): now, it is the number of bonds that this atom can form
- H can share only one electron valence of H = 1
- F can share one lonely electron valence of F = 1
Lonely pairs: very important to explain the geometry (VSEPR) and strength of the bond (Chapter 3)
Which one is the largest atom?
Bond order
- It is the number of chemical bonds between a pair of atom
Triple bond
Double bond
Single bond
In which case we will require a greater input of energy to break chemical bonds?
May we have intermediate bond orders?
Ep! Veurem més endavant (TOM) que aquesta estructura no reflecteix correctament la realitat
