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Molecular Geometry The shape of a molecule can be very important to the way it interacts with other molecules. For example, water molecules have a bent shape, and the oxygen atom, which attracts extra electrons more strongly than do the hydrogen atoms, draws the electron shells over to its side a little bit. in science textbooks, and the one on the right shows a more realistic view of the molecule. The four hydrogen atoms are at the four points of a shape called a tetrahedron, the shape below This makes the molecule polar, meaning that one side is negatively charged while the other side is positively charged. Polar molecules attach themselves to other polar molecules, and water is a good solvent as a result: the molecules cling to molecules of things like sugar, surround them, and separate them from the solid sugar. There are many other important consequences of the shapes of molecules. Many of the molecules that make up our bodies have intricate shapes on which the molecule’s function depends. They’re like little machines, and a machine needs to have a reliable structure in order to function. Pairs of Valence Electrons Below are depictions of a methane molecule, 𝐶𝐻4. The figure on the left shows the traditional ball and stick depiction that you see This configuration puts them as far from each other as they can get without leaving the carbon atom. This happens because of the repulsion of electrons from each other. Recall that in a covalent bond like the ones here, each of the two atoms share a pair of electrons. So each of the hydrogen atoms in the methane molecule above share 2 electrons with the carbon atom. The two electrons in each pair stay close together, but each pair repels the other pairs, and so the four pairs move themselves to the far corners of the carbon atom. Water molecules have this tetrahedral shape too. Below are the two depictions of a water molecule (ball & stick and a more realistic view) The two hydrogen atoms are positioned at two of the four corners of the tetrahedron. So, what are at the other corners? At the other corners are pairs of electrons that are valence electrons of the oxygen atom, but there are no hydrogen atoms there. Look at the periodic table below. Below is a depiction of a lysine molecule (the realistic view is not shown because the structure is not very clear in that view). Lysine is an amino acid, one of the 20 building blocks of all the proteins of all life forms on Earth. The red atoms are oxygens, the blacks are carbons, the blues nitrogens, and the whites are hydrogens. All but the hydrogens have their bonds arranged in the tetrahedral way. You can see this if you look closely. Covalently bonded atoms don’t always have 4 valence pairs when bonded to other atoms, and so the arrangement of the bonds is not always tetrahedral. An extreme example is shown in the figure below: a molecule of Iodine heptafluoride, 𝐼𝐹7 . Oxygen is in the sixth position of the second row, which means it has 6 electrons in its valence shell, and two empty spaces. Only two hydrogen atoms are required to provide the oxygen atom with enough electrons to complete its valence shell. The valence shell still has four pairs of valence electrons, but only two of them are pairs that are involved in bonds. All four pairs repel each other and so the water molecule is shaped in this way, giving water its important properties. The iodine atom is bonded to 7 fluorine atoms. Geometry of Metallic Crystals Metallic elements form metallic bonds between their atoms: the valence shells of individual atoms join to form a vast valence shell that surrounds the remaining ions. We thus can’t really speak of the number of valence pairs on each atom. Instead, a sea of electrons attracts the ions to each other, and they pack themselves together as closely as the forces can pull them. If you were to stack glass marbles together in a way that minimizes the space that they take up, as in the figure below, how many other marbles would each marble be touching? The answer is 12. This is typically the way that metal atoms arrange themselves when they form a solid. So, metal atoms are bonded to 12 other atoms when they’re in a solid form. This is what gives metals like steel their tremendous strength.