Polarity Notes

Honors Chemistry Name

Remember electronegativity?

Electronegativity (trend opposite of atomic radius)

  • Definition: the tendency of an electron to attract a shared pair of electrons in a covalent bond.
  • General information: Electronegativity is a measurement of how much an atom desires another atom's electrons when electrons are shared to form a bond. Fluorine is the most electronegative element, and in general, electronegativity decreases as you go diagonally across the periodic table toward Francium.
  • Trend Across: Electronegativity increases due to increased coulombic attraction while shielding stays constant
  • Trend Down: Electronegativity decreases due increased Coulombic attraction and increased shielding, where shielding has the greater effect, resulting in less net pull on the electrons even though there are more protons in the nucleus
  • Why is this important? We will use electronegativity now to determine if a molecule is polar or non-polar. )

Steps to complete the polarity worksheet

  1. On your worksheet, determine which atom is the most electron negative and draw and arrow pointing to it. The head of the arrow should have the point directed towards the most electronegative atom and to the base of the arrow, add a line to make a cross indicating that is the positive end.
  2. Looking at the geometry and where the arrows are pointing determine the overall polarity experienced by the molecule. Then draw a large arrow to indicate the overall polarity, and label the partial positive end and the partial negative end. If you cannot draw the large arrow on top, feel free to draw it below as shown in HCl.

How do you tell if it is polar?

  • Look at the Lewis structure. If it is symmetric it is non-polar. If is it is asymmetric it's polar:
    AX3E AX2 linear AX2E2

Note: If you draw SF2 linear with both fluorine’s on opposite sides of the sulfur atom, the molecule would appear non-polar. As a general rule of thumb if the central atoms in a molecule has lone pair electrons, the molecule is polar.

Intermolecular Forces (IMF)Notes

Honors Chemistry Name

Definitions:

  • Intermolecular forces – (IMFs) attraction between 2 or more molecules
  • Intramolecular forces- (within a compound)
  • Ionic bonds – attraction between cations and anions
  • Covalent – sharing of electrons
  • Metallic – metal cations and mobile e

3 main IMFs:

  • Hydrogen bonds – when a H atoms of 1 molecule isattracted to highly electronegative atom (F,O,N)
  • Dipole-dipole – attraction between oppositely charged regions of a polar molecule
  • London Dispersion forces (Van Der Waals) - Induced dipole between nonpolar molecules

Steps for Identifying what type of intermolecular force is in a molecule:

1) Draw the Lewis structure for the molecule.

2) Examine the Lewis structure and look for H-N, H-F, or H-O bonds. If there's a hydrogen atom stuck directly to a fluorine, nitrogen, or oxygen atom, the molecule experiences hydrogen bonding.

Note: There may be some molecules that contain F, N, or O as well as hydrogen that aren't involved in hydrogen bonding - an example would be HCN, which has a Lewis structure that looks like

H-C:::N: Remember, hydrogen bonding only takes place when the H is stuck directly to F, N, or O.

3) Is the molecule polar? If it is, the molecule experiences dipole-dipole forces

4) Is the molecule nonpolar - it experiences Van der Waals forces.

Why are IMF’s important? Intermolecular forces explain how compounds behave.

  • If a compound experiences strong intermolecular forces such as hydrogen bonding, it will have a higher melting and boiling point. This is because hydrogen bonding helps to stick the molecules together, making it harder to pull them apart. Because energy is required to make this kind of change take place, more energy in the form of heat will be needed to make hydrogen-bonded molecules melt.
  • Intermolecular forces explain "like dissolves like" (molecules with similar polarities tend to dissolve one another). If you know how to find the intermolecular forces a molecule experiences, you can figure out whether one thing will dissolve another based on their polarities.

For each of the compounds, on the next page check if it is polar or non polar, and check the intermolecular forces present.

  1. How are boiling points affected by intermolecular forces?