Textbook Notes Bond/Molecular Polarity

Bond/Molecular Polarity Notes Handout • o ▪ ▪ ▪ ▪ What is it? Atoms with more protons attract more electrons. This causes polar, nonpolar, and ionic bonds Bonds are caused by electronegativity polar - does not equal out inside atom nonpolar - equals out inside atom ionic - transfer of electrons • How can the periodic table can help, what are the trends? (Davis, R. E. (2002). Modern chemistry. Austin, TX: Holt, Rinehart and Winston.) Electronegativities increase across each period but there are exceptions. Alkali and alkaline-earth metals are the least electronegative metals Nitrogen, oxygen, and halogens are the most electronegative elements Based on the scale, typically the farther the elements are it would be ionic bonding and farther is covalent Periodic Trends Left to right, electronegativity increases visa versa Up to down electronegativity decreases Noble gases are weird because some don’t form compounds so don’t have electronegativity measurements When noble gases do form, they have high electronegativities like halogens Highest electronegativity is top right, lowest is bottom left. o o o o o ▪ ▪ o ▪ o • o o o o o o o Inter-vs. Intra- molecular forces The intramolecular bonds that hold the atoms in H2O molecules together are almost 25 times as strong as the intermolecular bonds between water molecules. Intermolecular bonds are still very important, though. The covalent bonds between the hydrogen and oxygen atoms in a water molecule are called intramolecular bonds The bonds between the neighboring water molecules in ice are called intermolecular bonds The prefix intra- comes from the Latin stem meaning "within or inside." The prefix inter- comes from the Latin stem meaning "between." Intermolecular forces are forces between molecules and intramolecular forces act within molecules Example: To remember the difference between inter and intra, I always think of 'interstate.' In every state there is an interstate road that goes across more than one state, like Interstate 80 that runs from the Pacific Ocean in California to the Atlantic Ocean in New York. • How do we determine it (with examples) 1. Draw the Lewis Structure ▪ Do as usual, but if the molecule has more than 2 atoms, it is important to distinguish between “central” and “outer” atoms in the molecule. 2. Determine the Shape ▪ ▪ ▪ First, count the number of substituents around the central atom, remembering that a nonbonding pair (lone pair) of electrons counts as a single substituent, as does any type of bonded atom. Determine the geometry according to the VSEPR model Decide on the shape of the molecule by placing substituents (atoms or lone pairs) into the geometry, and considering that the lone pairs are "invisible". Note that in linear, triangular planar, and tetrahedral geometries, all of the substituent locations are equivalent. 3. Determine the Molecular Polarity • • • • o o First, look up the electronegativity value for each atom in the molecule Remember that this value gives a relative measure of how strongly a particular kind of atom pulls on the electrons in a bond. Next, take the difference between each pair of electronegativity values in order to determine the polarity of each individual bond in the molecule. The difference reflects how much harder one atom pulls on the electrons than the other. If the difference is very small, then the electrons will be equally balanced between the two atoms (i.e., a nonpolar bond). If the difference is large, then the electrons will be much closer to the atom with the higher electronegativity, and therefore that end of the bond will be negatively charged. Finally, try to assess how all of the bond polarities in the molecule will "add up," while trying to visualize the 3-dimensional shape of the molecule. Sometimes the polarities of the bonds may cancel each other altogether, yielding a nonpolar molecule (e.g., CO2). Other times, polarities will partially cancel, but will still result in a net polarity (e.g., H2O and H2CO). Note that in H2CO, the C-H bonds have very little polarity (and much of that cancels out), but that the polar C=O bond causes the molecule to be polar. The Pauling Scale of Electronegativities (show a picture) and discuss how it is used in terms of polarity. Basically the higher the electronegativity of an atom is a relative value of that atom’s will attempt to pull electrons towards itself and away from any atoms it bonds to. The trend for electronegativity is to increase by left to right and bottom to top across the periodic table. • o o o ▪ o o o o o o How does this topic relate to each type of compound ionic and covalent? Show an example of this calculation for both ionic and covalent compounds. Non polar covalent compound- Non-polar bonding with an equal sharing of electrons. Polar bonding with an unequal sharing of electrons. The number of shared electrons depends on the number of electrons needed to complete the octet. Ionic Compound- a compound composed of ions bound together by electrostatic attraction in an ionic compound there are positive and negative ions so that the number of positive and negative ions are equal. Covalent Compound- a molecular compound where atoms share electrons through covalent bonds where the smallest The chemical formula for an ionic compound shows the ratio of ions present in any sample size the ratio of ions depends on the charge of the ion A formula unit is basically something like NaCl which is one sodium cation and one chlorine anion. An electron dot notation can be used to demonstrate the changes that take place in ionic bonding Iodine forms a diatomic non-polar covalent molecule. Molecules of oxygen, present in about 20% concentration in air are also a covalent molecules . o ▪ ▪ Bonding between sulfur and hydrogen If the electronegativity of sulfur is 2.5 and 2.1 for hydrogen then the electronegativity difference is 0.4 and then using the pauling scale we can see that he bond type is polar-covalent. If the electronegativity of sulfur is 2.5 and 0.7 for cesium the the electronegativity difference is 1.8 and then using the pauling scale we can see that the bond type is polar covalent.