Chemistry - Bonding and VSEPR

VSEPR Theory

Valence Shell Electron Pair Repulsion

helps predict the 3D shape of molecules (covalent compounds) by assessing a Lewis Structure. Idea that Pairs of Electrons, in bonds or as lone pairs, Repel each other. Happens b/c all electrons are negative and like charges repel. The shape of a molecule can be predicted by assuming that all pairs of electrons are spread as far from e/o as possible.

Formal Charge formula

Lewis dot diagram

angle bond diagram

ionic bond

• occur between a metal and nonmetal

• NaCl

covalent bond

• occur between two nonmetals

• CO₂ , H₂O

metallic bond

• occur within pure metals or metal alloys

  • brass, bronze, steel, solder, pewter, amalgam

when can atoms have more than 8 electrons on the central atom?

when the atom is below the first row on the PT

roman numerals on ions

some elements form cations with multiple charges. electrons do not form ANIONS with more than one charge.

ex.

copper (I) oxide (Cu⁺ and O^2-) - Cu₂O

copper (II) oxide (Cu²⁺ and O^2-) - CuO

avg. bond order

total # bonds in resonance / # of resonance structures

OR
total bonds / total bond locations.

formal charge

determines distribution of electrons in a molecule.

[# valence e^- in neutral atom] - [# non-bonding e^-] - ½ [# bonding e^-]

resonance structures

when there is a random and unequal distribution of formal charges.

oxyanions

polyatomic anions that contain oxygen

octet-rule exceptions

Grp 1: (H, Li) need 2 to satisfy

Grp 2: (Be) need 4 to satisfy

Grp 13: (B) need 6 to satisfy

everything else needs at least 8. elements below 2nd row can have more than 8 on a central atom.

Bond polarity

Cause of bond polarity

unequal distribution of electron density. Difference in Electronegativity. >0.3

Examples of Polar Bonds

all ionic (NaCl)

OH, CF, CN, SF, NBr, CO.

Examples of Nonpolar Bonds

CC, NN, ClCl, OO

Dipole moment

(#) measure of molecular polarity. unit = debye.

polarity 0=Dipole moment

Examples of Polar Molecules

ASYMMETRICAL

>0D - H2O, CO, NO, HF, NH3, SF4, CH3I, CH2O

Examples of Nonpolar Molecules

SYMMETRICAL

0D - all hydrocarbons (CxHy), X₂, N2, Br2, Cl2, CO2, SF6

Vector Sum = 0

2 same @ 180

3 same @ 120

4 same @ 109.5

sum of all repulsions equal 0. if there are 2 arrows point ← → and both have the same pull of ‘x’, then it would meet the vector sum rule. If they do not cancel, they are polar.

polarity arrows are the same as vectors.

CO2 Yes

COS No

Polar compounds dissolve best in polar solvents

Nonpolar compounds dissolve best in nonpolar solvents

London Dispersion Forces (LD)

• very weak

• occurs between non/polar mols

• due to induced, temp dipole moments

  • electrons are concentrated to one end of mol

• more total e, the stronger the LD forces and stronger the IMFs.

Hydrogen Bonds (H-bond)

• occur mostly in polar molecules

• HAS TO HAVE: F, O, N

  • FONee bonds

• strongest IMF

• e are not shared or transferred

• large electronegativity difference and small size of the atoms result in a very polar bond.

Dipole-Dipole forces (d-d)

• occur between any POLAR mols

• the more polar, the stronger the d-d force

• occur b/c the pos end is attracted to the neg end of another mol

• stronger than LD

Ion-dipole attractions

• occur when ions are solvated in water

• water mols form favorable attractions with ions that help to break up the crystal structure

• b/c water has a noticeable dipole moment, there is a more negative end of the mol at O and H end is more pos

• I-d strength increases when the ion-water distance is shorter and when the magnitude of the ionic charge is greater.

what can we do with IMFs?

• make predictions about melting pts and boiling pts

• predict which substance is more volatile

• predict relative solubility (like dissolves like)

volatility

The tendency of a substance to vaporize; higher volatility means the substance can easily become a gas at lower temperatures.

Forces in ionic bonds

electrostatic attraction

all ionic compounds are solids at room temp

ions need mobility to carry charge, meaning you need to change their phase for them to move.

unit cell of ionic compound

the smallest division of the repeating crystal pattern

solid ionic compounds are brittle b/c…

like charges repel when you press/slide a compound. the charges align with a like charge, causing them to repel and flake off.