Chemistry 1111: Test 2 - Ch. 10

Front 1

10.1 Molecular Geometry

Back 1

> 3D arrangement of atoms in a molecule.
> Molecular geometry affects its physical and chemical properties (melting point, boiling point, density, types of reactions it undergoes).
> Can predict molec-Geo if we know # of e- surrounding a central atom in Lewis Structure.

> e- pairs in Val Shell of an atom repel one another.

> In a polyatomic molecule (two or more bonds between central atom and surrounding atoms) repulsion between e- in a different bonding pairs causes them to remain as far apart as possible.

> VSEPR Model: accounts for geometric arrangements of e- pairs around a central atom in terms of electronic repulsion between e- pairs.

Front 2

General Rules of VSEPR Model

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1) as far as e- pari repulsion is concerned - Double & Triple bonds can be treated like single bonds.

2) If a molecule has two or more resonance structures, can apply VSEPR model to any one of them. Formal charges are not usually shown.

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Table 10.1

Arrangement of e- pairs about a central atom

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1)Linear: 2 e- pairs / 180deg eg) BeCl2, HgCl2

2) Trigonal Planar (flat triangle) / 120deg : 3 e- pairs. eg) BF3

3) Tetrahedral (3-side pyramid) / 109.5 deg
: 4 e-pairs. eg) CH4

4) Trigonal Biplanar (dbl tri-pyramid) /
90deg & 120deg : 5 e- pairs
eg) PCl5

5) Octahedral (dbl quad-pyramid)
90 deg & 90 deg / : 6 e-pairs eg) SF6

> mutual repulsion, the e- pairs stay far away as possible
>MOlecules where central atom has no lone pairs have one of these 5 arrangements of bonding pairs.

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Molecules where the Central Atom Has 1 or more Lone Pairs

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> If central atom has both lone & bonding pairs, there are 3 types of repulsive forces
- those between bonding pairs, those between lone pairs, those between a bonding pair and a lone pair.

Lone-pair vs. Lone-pair Repulsion >
Lone-pair vs. bonding-pair Repulsion >
Bonding pair vs Bonding-pair Repulsion

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Guidelines for Applying the VSEPR Model

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- applies to all types of molecules

1) Write Lewis structure of the molecule, considering only e- paris around the central atom (the atom that is bonded to more than one atom)

2) Count # of e- pairs around the central atom (bonding pairs and lone pairs).
- Treat dbl and triple bonds like single bonds.
- Refer to table 10.1 to predict overall arrangement of e- pairs.

3) Use table 10.1 and 10.2 to predict geometry of molecule

4) In predicting bond angles, note that a lone pair repels another lone pair or a bonding pair more strongly than a bonding pair repels another bonding pair.
- No easy way to predict bond angles accurately when central atom possesses one or more lone pairs.

Front 6

10.2 Dipole Moments

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> Shift in electron density is symbolized by placing an arrow to indicate the direction of the shift.
> The shift point the arrow to the more negative side (more e-). More charge.

> A quantitative measure of polarity of a bond is its Dipole Moment, which is the product of charge Q and distance r between charges:
Dipole moment = Q x r

> Dipole moments are expressed in debye units (D)
-conversion factor is: 1D = 3.336x10 -30 C m

C= coulomb. M = meter.

Front 7

Dipole moments (part 2)

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> diatomic molecules containing atoms of diff elements (HCl, CO, NO) have Dipole Moments, and are called: POLAR MOLECULES

> Diatomic molecules containing atoms of same element (H2, O2, F2) are NONPOLAR MOLECULES, they don't have dipole moments.

> Molecules made up of 3 or more atoms, both polarity of bonds and molecular geometry determine whether there is a dipole moment.

Front 8

10.3

Valence Bond Theory

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> Valence Bond theory assumes that the e- in a molecule occupy atomic orbitals of the individual atoms.

> Molecular Orbital theory assumes the formation of molecular orbitals from the atomic orbitals.

Front 9

Hybridization

Back 9

Hybridization: term applied to mixing of atomic orbitals in an atom (usually a central atom) to generate a set of hybrid orbitals.