This gives composition: stoichiometry

This gives composition: stoichiometry

• This gives composition: stoichiometry

• Law of the defined proportions (1807) or the law of constant composition)

• Defined does not mean unique: NO2, NO and N2O

• Law of multiple proportions. The relative number of different atoms is always a simple integer This is a quantum law.

• Few exceptions: CuyS.
• As defined originally the ratio of atomic “weights” was approximate due to isotopic mixures.
• A consequence about connectivity: in AB2, B has twice neighbors than A (assuming only A-B bonds)

a measure of the number of chemical bonds formed by the atoms of a given element.

• a measure of the number of chemical bonds formed by the atoms of a given element.

• This definition is ambiguous but historically very helpful.

HOCl or HClO ?

• HOCl or HClO ?

H2C=O a double bond is necessary

• H2C=O a double bond is necessary

• N2 a triple bond is necessary

• Never write a quadruple bond! (except for Cr2)

• The chemical formula of a saturated hydrocarbon (alcane) is CnH2n+2. For others, the number of unsaturations may be found from y in the formula CnH2n+2-2y.

• This is easily generalized:

• Replace every monovalent atom by H
• Ignore divalent atoms
• Replace NH by CH2

A developed formula is inadequate to describe the electronic environment of the atoms in the molecule. Even before we talk about electron, from 1845, Auguste Laurent wrote "a system of such formulas is too absolute and if adopted, it would prevent finding a wealth of valuable reports. We do not know how atoms are really, but we already know that in such a compound the atoms are arranged like in another ... I can say that compounds have the same or different structures without knowing anything of their structure."

• A developed formula is inadequate to describe the electronic environment of the atoms in the molecule. Even before we talk about electron, from 1845, Auguste Laurent wrote "a system of such formulas is too absolute and if adopted, it would prevent finding a wealth of valuable reports. We do not know how atoms are really, but we already know that in such a compound the atoms are arranged like in another ... I can say that compounds have the same or different structures without knowing anything of their structure."

Valence electrons are the least stable ones, core electrons the most stable ones.

• Valence electrons are the least stable ones, core electrons the most stable ones.

• Ionization potential for Valence electron is in the range of 10 eV or less, that for core electron in the range of 100 eV or more

• Why to be interested in the least stable electrons? …because they are involved in chemistry; the others don’t; they are too stable.

A compound is stable when two rules are satisfied:

• A compound is stable when two rules are satisfied:

• the octet rule

• and the electric neutrality.

• If it not possible to satisfy both, obey first the octet rule unless excessive deviation of charge neutrality.

• Molecules better have only electron pairs (closed shell molecules). No dots.

There are no general recipe

• There are no general recipe

• For compounds containing multiple elements with only one atom in the formula, the central atom is the least electronegative single atom that is not hydrogen. For instance, in thionyl chloride (SOCl2), the sulfur atom is the central atom.

• Do not connect similar atoms between them except C or Si (see Pauling remarks: A-B stronger than (A-A+B-B)/2)

We are interested in the local environment of a given atom. How many electrons lie around on atom?

• We are interested in the local environment of a given atom. How many electrons lie around on atom?

• Warning! Be careful that the sum of the electrons for all the atoms is larger than the total number of valence electrons. Some atoms belong to two neighborhood and are counted twice!

• A molecule is stable when for all of its atoms, the number of electrons in the atomic environment is 2, 8 or 18 (the number of valence electrons of the rare gas atom that follows).

• Two is for hydrogen. The eighteen electron rules apply for transition metals.

Do not couple dots.

• Do not couple dots.

• Start counting the total number of valence electrons.

• Try to build a structure respecting the octet rule

• If you fail, start from there and move electrons to adjust.

It is strictly defined using symmetry (QM)

• It is strictly defined using symmetry (QM)

In some cases, several structures are necessary to represent together a molecule.

• In some cases, several structures are necessary to represent together a molecule.

• If these structures are equivalent (symmetry related) the account for the molecule together.

• If these structures are different, one is more appropriate; however the others are informative

At variance with the octet rule, the counting of formal charge is a partition of electron. The sum of the electrons should be the total number of the valence electrons of the system. The sum of the charge should be the total charge of the system (0 for molecules, A value for ions).

• At variance with the octet rule, the counting of formal charge is a partition of electron. The sum of the electrons should be the total number of the valence electrons of the system. The sum of the charge should be the total charge of the system (0 for molecules, A value for ions).

• These are two informative visions of the electron distribution.

A formal charge is a partial charge on an atom in a molecule assigned by assuming that electrons in a chemical bond are shared equally between atoms, regardless of relative electronegativity or in another definition the charge remaining on an atom when all ligands are removed homolytically.

• A formal charge is a partial charge on an atom in a molecule assigned by assuming that electrons in a chemical bond are shared equally between atoms, regardless of relative electronegativity or in another definition the charge remaining on an atom when all ligands are removed homolytically.

The total number of valence electron is 4+6=10.

• The total number of valence electron is 4+6=10.

• One Lewis structure satisfies the octet rule and not satisfies the electron neutrality whereas the other does the contrary.

• The dipole moment is very weak; it corresponds to the charged formula. Since O is more attractive to electrons than C, the formal charges however come out very small.

This is very important. However there are many ways that differ; all are informative

• This is very important. However there are many ways that differ; all are informative

• Isoelectronic system Isolobal

• HClO HOCl
• N2 CO

• 3 main ways of distributing electrons:

• Formal charges, democratic
• Oxidation numbers, not democratic: everything for the most powerful atom
• Electronegativity, intermediate

Partial charges. If A is stronger than B (more electronegative) A has more electrons.

• Partial charges. If A is stronger than B (more electronegative) A has more electrons.

• Not to consider in cases of presence of formal charges.

• There are many different scales: Pauling, Mulliken, Allred and Rochow, Sanderson, Allen…

• Decomposition of dipole moments may generate partial charges on the atoms (however decomposition is also an oversimplification)…

1932

• 1932

• The covalent bond between two different atoms (A–B) is stronger than would be expected by taking the average of the strengths of the A–A and B–B bonds.

• The difference in electronegativity between atoms A and B is given by:

• where the dissociation energies, Ed, of the A–B, A–A and B–B bonds are expressed in eV, the factor (eV)−½ being included to ensure a dimensionless result.

It is also a partition of electronic density. The electrons from the bonds are attributed to the most electronegative atom. The model is extreme and purely ionic.

• It is also a partition of electronic density. The electrons from the bonds are attributed to the most electronegative atom. The model is extreme and purely ionic.

• We can count electrons on the atoms in this doing so and oxidation numbers are the corresponding charge, q-d.

They have been defined relative to O (F would have been more appropriate)

• They have been defined relative to O (F would have been more appropriate)

• 4 rules:

• N is 0 in elements whatever the allotropic form is.

• For O, N=-2 (except when bound to itself: O2, O3,HOOH, peroxides)

• H is +1 or -1 according to electronegativities

• N for others is deduced so that the total sum is nil (molecules) or equal to the total charge (ions)

nmax -nmin = 4-(-4) = 8 for C

• nmax -nmin = 4-(-4) = 8 for C

• nmax -nmin = 5-(-3) = 8 for N

• nmax -nmin = 6-(-2) = 8 for S

• nmax -nmin = 7-(-1) = 8 for Cl

The valence is the maximum number for oxidation numbers ignoring sign

• The valence is the maximum number for oxidation numbers ignoring sign

• For electropositive atoms, it is the highest

• 2 for Mg since they usually lose electrons

• For the electronegative atoms, it is the smallest: -3 (giving 3) for N since these atoms usually gain electrons

VSEPR

• VSEPR

• Conformations and configurations

• Chirality

• Symmetries

Improvements:

• Improvements:

• The lone pairs are closer to the nuclei than the bonding pairs; so the angle between them is larger than expected and the angle between bonds decreases; count 2 degrees per lone pair for that decrease

• H-N-H = 109-2=107 H-O-H = 109-2x2=105

• Multiple bonds have density that perturbs??.

• There is no need to refine further the model or to apply it to exotic cases; its main interest is simplicity!

H3O+ NH3 CH3-

• H3O+ NH3 CH3-

• Compounds with the same electron count have the same structure and often behave the same.

• Of course charges differ and modify reactivity (modify the balance between the same set of iterations)

In standard condition a double bond is rigid and it is not possible to convert an isomer to another without breaking or reforming a bond.

• In standard condition a double bond is rigid and it is not possible to convert an isomer to another without breaking or reforming a bond.

• Names for configuration

• Old Cis and Trans. This is ambiguous

• New Z and E. Differents ligands are classified according to Z (R. S. Cahn, C. Ingold, V. Prelog)

Instead of D and L nomenclature, asymmetric carbon atoms are named according to Z (R. S. Cahn, C. Ingold, V. Prelog).

• Instead of D and L nomenclature, asymmetric carbon atoms are named according to Z (R. S. Cahn, C. Ingold, V. Prelog).

• Classify the atoms according to decreasing Z, place the smallest behind and see whether they are ordered clock-wise (R) or not (S).

Some bond properties are transferable from one compound to another: dissociation energies, bond lengths, dipole moments, spectroscopic shifts… It is then useful to decompose molecules into bond contributions. Such reasoning seems reasonable knowing Lewis structure that electrons are attributed to bonds. It is productive leading to the concept of substituent.

• Some bond properties are transferable from one compound to another: dissociation energies, bond lengths, dipole moments, spectroscopic shifts… It is then useful to decompose molecules into bond contributions. Such reasoning seems reasonable knowing Lewis structure that electrons are attributed to bonds. It is productive leading to the concept of substituent.

• However, this is conceptually wrong: a major feature for a molecule is its symmetry and this requires considering the global molecule (several bonds together related by symmetry).

If the center of mass of the positive charge of a neutral molecule differs from that of the negative charges, there is a dipole moment that is a molecular property.

• If the center of mass of the positive charge of a neutral molecule differs from that of the negative charges, there is a dipole moment that is a molecular property.

• It is a vector from – to + whose magnitude is qxd expressed in C.m.

• Warning: Never talk of dipole moment for charged species (not an intrinsic property; it varies with the origin). Charge is then the significant property.

Two charges +e and -e separated by a distance of 1 Å lead to 4.8 Debyes.

• Two charges +e and -e separated by a distance of 1 Å lead to 4.8 Debyes.

• (1 Debye equals 3.34 10-30 C.m).

• Easy to use using proportionality.

A simple model is to attribute dipoles to bonds and sum up the contributions for each of them (sum of vectors)

• A simple model is to attribute dipoles to bonds and sum up the contributions for each of them (sum of vectors)

• This leads to partial charges (fraction of e). The total moment is on a symmetry axis of the molecule

There are 5 kinds of operations

• There are 5 kinds of operations

• 1. Identity

• 2. n-Fold Rotations

• 3. Reflection

• 4. Inversion

• 5. Improper n-Fold Rotation

If one of the many conformation has a mirror symmetry, the compound is inactive on light.

• If one of the many conformation has a mirror symmetry, the compound is inactive on light.

Answer : Everywhere

• Answer : Everywhere

• Food taste

• Clothes colors

• Perfumes smell

• Medecine health

• Gas energy

• Materials…

• Environment

Two main answers :

• Two main answers :

• Cooking

• philosophy

The combinatorial ability of elements was so little obvious property that it did not appear as an intrinsic property by observation but was revealed by a chemical analysis that destroyed the compound showing a property outside it.

• The combinatorial ability of elements was so little obvious property that it did not appear as an intrinsic property by observation but was revealed by a chemical analysis that destroyed the compound showing a property outside it.

• Auguste Laurent (1845) wrote « that chemistry had become a science of bodies that do not exist "; meaning that it was a property that was only revealed by destruction “ when these bodies no longer existed."

Many have been discovered before the discovery of electrons:

• Many have been discovered before the discovery of electrons:

• Lewis formula (1916), Kekulé formula (1856)

• Periodic table.

• Of course, this is much easier using Q.M.

WE ARE PERHAPS NOT FAR REMOVED FROM THE TIME WHEN WE SHALL BE ABLE TO SUBMIT THE BULK OF CHEMICAL PHENOMENA TO CALCULATION.

• WE ARE PERHAPS NOT FAR REMOVED FROM THE TIME WHEN WE SHALL BE ABLE TO SUBMIT THE BULK OF CHEMICAL PHENOMENA TO CALCULATION.

• -- JOSEPH LOUIS GAY-LUSSAC MEMOIRES DE LA SOCIETE D'ARCUEIL, 2, 207 (1808)