Aim: How to describe the polarity of bonds and molecules Do Now: Draw and label in the image below what the molecule is and what the bond is bond molecule

Define the following terms in your own words Bond Molecule

What does polarity mean? The property of having opposite poles (positive and negative end) In chemical bonding, polarity has to do with the distribution of charge or electrons in a bond (between two atoms) or in a molecule (group of atoms)

Review of electronegativity Electronegativity is defined as the ability of an atom to attract electrons when the atoms are in a compound. Explain in terms of electronegativity why Li2O contains an ionic bond, while CO2 has covalent bonds.

Are electrons always shared “equally” between two atoms in a covalent bond? Discuss the question above in pairs. During your discussion think about what the word electronegativity means and consider the electronegativities of the two atoms involved in H-Cl. Write down what you think.

Polar and Nonpolar Bonds There are two types of covalent bonds Nonpolar Covalent Bonds (equal share of electrons between two atoms) Polar Covalent Bonds (unequal share of electrons between two atoms)

Polar Covalent Bond In a polar covalent bond, the shared electrons spend more time around the more electronegative atom The more electronegative atom has a stronger hold on the shared electrons; has a partial negative charge The less electronegative atom has weaker hold on the shared electrons; has a partial positive charge

How to show a bond is polar Not a whole charge just a partial charge d+ means a partially positive (on less electronegative atom) d- means a partially negative (on more electronegative atom) Why does chlorine have the partial negative charge? The Cl pulls harder on the electrons, the electrons spend more time near the Cl d+ d- H Cl

Nonpolar Covalent Bond A Nonpolar Covalent Bond is an equal sharing of electrons between two bonded atoms

Classification of Bonds You can determine the type of bond between two atoms by calculating the difference in electronegativity values between the elements The bigger the electronegativity difference the more polar the bond. Type of Bond Electronegativity Difference Nonpolar Covalent 0  0.4 Polar Covalent 0.5  2.0 Ionic Greater than 2.0

Summary of Polarity of bonds Non-Polar Covalent Bond Polar Covalent Bond Ionic Bond Between two nonmetal atoms Between metal ion and nonmetal ion Equal sharing of electrons Unequal sharing of electrons Complete transfer of electrons EN diff. (0 -0.4) EN diff. (0.5 – 2.0) EN diff. greater than 2.0 H-H H-Cl 𝑁𝑎 +1 𝐶𝑙 −1 No charges on atoms Partial charges on atoms Full charges on atoms d+ d-

Symmetric vs Asymmetric molecules Think about what the terms symmetric and asymmetric mean. Next, determine if the following molecules are symmetric or asymmetric. Asym Symmetric Asym Asym Symmetric

Symmetrical Molecules In chemistry, symmetry means everything around the central atom is the same (element, shared e- pairs, unshared e- pars) Symmetric molecules are always nonpolar molecules There is an even or symmetric distribution of charge throughout the molecule. Molecule does not exhibit positive and negative ends Examples: CO2 BF3 CCl4

Asymmetrical Molecules In chemistry, asymmetry means that not everything around the central atom is the same (can be shared e-, unshared e-, and/or element) Asymmetric molecules are polar molecules There is an uneven or asymmetrical distribution of charge Molecule exhibits positive and negative end Examples: H2O HCl NH3

Water as a Polar Molecule

Dipole Polar molecules are also referred to as dipoles Dipole is a molecule that has two poles or regions with opposite charges

Determine which end of the polar molecule (dipole) is the negative end, and which is the positive end. In a polar molecule, the side of the molecule with the more electronegative atom is the negative end, and the opposite side is the positive end.

Practice Dot Structure Bond Type (use EN difference) Molecule Type (look at molecular shape) Water H2O Methane CH4 Carbon Dioxide CO2

Answer the following questions How do you determine if a bond is nonpolar, polar, or ionic? How do you determine if a molecule is polar or nonpolar? Why would a molecule with polar bonds not necessarily be a polar molecule? Explain in terms of symmetry.

Summary In a polar bond, one atom is more electronegative than the other. In a nonpolar bond, both atoms have similar electronegativities. An asymmetric molecule is a polar molecule. A symmetric molecule is always a nonpolar molecule.