POLAR AND NONPOLAR MOLECULES •Molecular polarity can be determined by symmetry. Symmetry can be defined as a balance or regular arrangement of the atoms within the molecule. •If a molecule is symmetrical in shape, it is said to be… •NONPOLAR-----without poles. •If a molecule is not regular, it is said to be… •POLAR-----containing (+) and (-) poles. Let’s take a look at some specific molecules… This line is called the Axis of Symmetry Cl-|-Cl We can draw a line down the middle of this molecule..... The left side looks like the right. The molecule is symmetrical and is therefore a NONPOLAR MOLECULE. The molecule… H-|-Cl The left contains Hydrogen and the right contains Chlorine. Nonsymmetrical and therefore POLAR. H H C H H Is this a nonpolar or a polar molecule? POLAR NONPOLAR CH4 is methane gas that we use to fuel our Bunsen Burners. It is a very symmetrical molecule and is therefore a nonpolar molecule. INCORRECT This molecule is symmetrical O H H Is this molecule nonpolar or polar? POLAR NONPOLAR Water molecules are angular in shape due to the two bonded hydrogens and the two unbonded pairs of electrons on the oxygen. This angular shape, along with the electronegativity of the oxygen, causes the molecule to be polar. CORRECT! O=C=O Is this molecule nonpolar or polar? POLAR NONPOLAR INCORRECT Carbon dioxide has two double covalent bonds that form a linear molecule. It is very symmetrical and therefore nonpolar. This molecule is symmetrical N H H H POLAR Polar or nonpolar, that is the question? NONPOLAR Ammonia has a trigonal pyramidal shape due to the unbonded pair of electrons on the nitrogen. It has definite polar characteristics because it is not totally symmetrical. Yet it is not as strong of a polar bond as water due to the lower electronegativity of the nitrogen atom. CORRECT! SYMMETRICAL ON N NONPLOAR A I YM E C S M TR L POLAR Molecules that are polar are also said to be dipoles. Dimeans 2. –pole means it has (+) and (-) electrostatic poles. Thus the name DIPOLE. Because dipoles have charged ends, they can attract each other. This is called…DIPOLEDIPOLE ATTRACTION. Molecules that are polar are also said to be dipoles. Dimeans 2. –pole means it has (+) and (-) electrostatic poles. Thus the name DIPOLE. Because dipoles have charged ends, they can attract each other. This is called Dipole-dipole attraction. The stronger the dipoles, the stronger the force of attraction. Because a dipole has charged ends, it can attract the ions in an ionic compound also. This is called the iondipole attraction. Network Solids • Network Solids are also called covalent crystals. Covalent bonds extend from 1 atom to another in one repeating pattern. • Because of this network of covalent bonds, such substances do not have distinct molecules. The entire mass of a network solid is considered a single macromolecule. A few Network Solids: • Diamonds: These are a large group of carbon atoms bonded in a three-dimensional crystal. • Silicon dioxide: This is silicon and 2 oxygen atoms covalently bonded, also known as mineral quartz. • Silicon Carbide: Carbon atoms covalently bonded with silicon, also known as carborundum. (used as an abrasive) HYDROGEN BONDING Compounds such as water and ammonia have hydrogen atoms that are bonded to smaller atoms of relatively high electronegativity. Let’s look at water as an example. The high attraction of oxygen for hydrogen’s electron partially exposes the nucleus of hydrogen. In other words, electrons that form the bond between the hydrogen and oxygen move a little closer to the oxygen. This causes the hydrogen to be partially positive and the oxygen to be partially negative. HYDROGEN BONDING Notice how the water molecules are attracted to each other. This is an example of hydrogen bonding. MORE POWER The fluorine atom has a higher electronegativity (a greater attraction for an electron) than an oxygen atom. Therefore the electrons in an H-F bond are pulled closer to the fluorine and this makes the hydrogen bond between H-F molecules stronger. London Dispersion Forces At any instant in time, the electrons in a particle may be unequally distributed. This means one side of the molecule will have a charge for just an instant. This slight charge will affect the neighboring molecule and force that molecules electrons into an unequal pattern as well. This produces a slight attraction between the molecules. London Dispersion Forces • The slight attraction between molecules can then affect the other molecules near them. • This is the weakest force of the Van der Waal’s forces. • This weak force can be increased by an increase in the number of electrons. • This weak force will be decreased if the molecules are further apart.