In this lesson, students learn some tips about how to determine whether a molecule is polar or nonpolar by question features about Lewis structures and symmetry.
By the end of this lesson, students should be able to
- determine proper VSEPR structures of molecules.
- apply rules to determine whether a molecule is polar or not.
This lesson supports students’ understanding of
Teacher Preparation: 30 minutes
Lesson: 40 minutes
- Molecular models (optional)
No specific safety precautions need to be taken for this activity.
- Examples for demonstration of technique and student practice:
a. Nonpolar examples: SO3 & BF3 (trigonal planar), CCl4 (tetrahedral) and SiO2 (linear). Polar bonds, but symmetrical, so pull is in opposite directions, therefore offset.
b. Polar examples: NH3 (pyramidal), H2O & H2S (bent). Polar bonds and asymmetrical geometry.
c. Polar: CHBr3 even though symmetrical shape (tetrahedral), a symmetrical pull of electrons because of different electronegativities: H (2.0) and Br (3.0), thus they pull on C’s electrons differently. If you draw arrows, the H side is more +.
- Students could be given the Polarity Decision Tree without the rest of the handout for quick reference.
- Answers to Your Turn: tetrahedral/109.5o; sp3
For the Student
Covalently bonded molecules do not have an overall charge (number of protons = electrons), but can have areas/regions that are more positively or negatively charged, which creates an overall polarity for the molecule.
a. Polar example, HCl: H has electronegativity of 2.2; chlorine has a higher electronegativity of 3.16, so the electrons in the shared cloud are shifted toward Cl. A polar molecule results with the chlorine side being slightly more negative than hydrogen side.
b. Nonpolar example, O2: The O’s have the same electronegativity, so the bond is nonpolar. Thus, the molecule is nonpolar.
i. If all bonds are nonpolar, so is the molecule!
Some tips about polarity:
Not all polar bonds result in a polar molecule.
a. If polar bonds are arranged symmetrically around the central atom, they offset overall for the whole molecule, therefore no regions of net negative or positive charges result.
Polar molecules must have polar bonds and 3-D asymmetry.
a. There must be “lopsided” charge distribution because of either geometry or varying levels of electronegativity between atoms.
VSEPR molecular geometry table:
a. If a molecule has equal polar bonds between atoms and no lone pairs, it is nonpolar. The molecule is symmetrical because it has no lone pairs around the central atom to make the molecule lopsided geometrically.
–Nonpolar if more than two atoms because there is nothing offsetting the pull for electrons.
–Polar if there are only two atoms because of asymmetry.
ii. Trigonal planar – nonpolar
iii. Tetrahedral – nonpolar
b. If a molecule has equal polar bonds and has lone pairs, it is polar.
i. Pyramidal – polar
ii. Bent – polar
If molecule has unequal polar bonds (multiple elements with varying electronegativity) then it is polar.
a. A molecule with lopsided bonds or electronegativity is polar.
- Polarity Decision Tree
a. Three branches:
- Kinds of bonds (Polar or Nonpolar)
- Geometry (symmetrical or assymetrical)
- Terminal Atoms (same or different)
- Use of dipole moment arrows to help visualize polarity.
a. Arrows point toward more electronegative atom.
b. Determine if arrows offset each other in 3-D (electrons are shifted to one side of atom).
- Draw the Lewis structure for SiH3Cl.
- Determine its geometry and bond angle.
- Determine the hybridization of silicon.