Molecular Polarity and Electronegativity

Earth's Polarity

• The Earth is neutral overall but has uneven charge distribution.
• One side is more positive, and the other side is more negative.
• The North and South Poles have differing charge accumulations.
• This polarity allows for the existence of compasses, which point to the magnetic North Pole.
• Navigation of ships relies on compasses.

Magnets and Their Properties

• Magnets also have poles (north and south) with opposite charges on each end.
• Magnets are neutral overall, but charge distribution gives them their properties.
• Opposite poles attract (positive to negative), and like poles repel.

Intermolecular Attractions

• Attraction between molecules is crucial; otherwise, everything would fly apart.
• Polarity is a significant source of attraction between molecules.
• Molecules with poles (positive and negative sides) are more attracted to each other.

Covalent Bonds and Polarity

• Covalent bonds involve the sharing of electrons between atoms.
• Nonpolar Bonds
  • In bonds between identical atoms (e.g., two hydrogens), electrons are shared equally.
  • The charge is centered in the middle, so the bond is nonpolar.
• Polar Bonds
  • In bonds between different atoms (e.g., hydrogen and chlorine), electrons may not be shared equally.
  • If one atom (e.g., chlorine) pulls harder on the electrons, the electrons are drawn to that side.
  • One side becomes negatively charged (chlorine side), and the other becomes positively charged (hydrogen side).
  • This creates a polar bond, similar to a magnet.

Lewis Structures and Charge Distribution

• Lewis structures can be used to represent the distribution of electrons in a bond.
• In a polar bond, electrons are closer to the more electronegative atom.
• This results in a partial negative charge (\delta^-) on one atom and a partial positive charge (\delta^+) on the other.
• This separation of charge is what defines a polar bond.

Electronegativity

• Electronegativity is a measure of an atom's ability to attract bonding electrons.
• It determines whether a bond will be polar or nonpolar.

Table of Electronegativity

• Values are relative, not absolute.
• Fluorine
  • It has the highest value and is the strongest puller of electrons.
• Francium
  • It has the lowest value and holds the least on electrons.
• By comparing electronegativity values, we can determine the polarity of a bond.
  • If the values are the same, the bond is nonpolar.
  • If the values are different, the bond is polar.
  • The greater the difference, the more polar the bond.

Examples of Determining Bond Polarity

• Bromine (Br) and Chlorine (Cl)
  • If the electronegativities are about the same, the bond is nonpolar.
• Iodine (I) and Chlorine (Cl)
  • If the electronegativities are different, the bond is polar.
  • The side with the higher electronegativity is more negative.

Representing Polarity

• Polarity can be represented with an arrow pointing towards the negative side of the bond.

Implications of Polarity

• Polarity creates a force of attraction between molecules.
• The chlorine sides of one molecule are attracted to the iodine sides of others.
• Polarity affects the state of matter (gas, liquid, solid).
• Hydrogen (H2) is a gas because there is little force of attraction between the hydrogens.
• ICl is a liquid because the forces of attraction are stronger.

Carbon Monoxide (CO) Example

• The electronegativity of carbon is 2.55.
• The electronegativity of oxygen is 3.44.
• The difference is 0.89, so the bond is polar.
• The oxygen side is negative, and the carbon side is positive.

Simulation of Electronegativity

• Simulations can show the effect of electronegativity on bond polarity.
• If two atoms have the same electronegativity, the bond is nonpolar.
• If one atom is more electronegative, the bond is polar.
• Polar molecules are affected by electric fields.

Molecules with Multiple Bonds

• Molecules can be polar even with more than two atoms.
• They must have at least one polar bond and must not be symmetric.

Symmetry

• Symmetry refers to the arrangement of atoms and bonds in a molecule.
• Symmetric molecules have no overall polarity because the bond dipoles cancel each other.
• Asymmetric molecules can be polar because the bond dipoles do not cancel.

Examples: CO2 and H2O

• CO2 (Carbon Dioxide)
  • Has polar bonds, but the molecule is linear and symmetric.
  • The polar bonds cancel each other, so the molecule is nonpolar.
• H2O (Water)
  • Has polar bonds, and the molecule is bent (not symmetric).
  • The polar bonds do not cancel, so the molecule is polar.

Tug of War Analogy

• Symmetric situation
  • Two people pulling equally hard in opposite directions.
  • No movement because the forces cancel.
• Asymmetric situation
  • Forces do not cancel, resulting in net movement.

Molecular Shapes and Polarity

• CO2 is linear, and its polar bonds cancel, making it nonpolar.
• Water is bent, and its polar bonds do not cancel, making it polar.
• The shape of a molecule is determined by its electron geometry and bond angles.
• Water is bent, with a negative side (oxygen) and a positive side (hydrogens).
• The vector sum of the bond dipoles in water results in a net dipole moment.

Rules for Molecular Polarity:

• For a molecule with more than two atoms to be polar:
  • It must have at least one polar bond.
  • It must not be symmetric, so the bonds don't cancel.

Using the Simulation

• The simulation allows exploration of molecular polarity.
• It can show the effect of bond dipoles on the overall molecule dipole.
• It demonstrates how polarity affects the behavior of molecules in electric fields.

More on Molecular Shapes and Symmetry

• Bent molecules:
  • Can be nonpolar if they have no polar bonds.
• Trigonal planar molecules:
  • Can be polar if one bond is pulling harder than the others.
• Tetrahedral:
  • If all bonds are nonpolar, the molecule is nonpolar.
  • If there are four polar bonds, they can cancel each other, making the molecule nonpolar.
  • If there are fewer than four polar bonds, the molecule will be polar.