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Lecture 2

BIOL 112 Lecture 2: Lecture 2

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Biology (Sci)
BIOL 112
Frieder Schoeck

Lecture 2 When two atoms form a bond, it is now a molecule. Hydrogen can only form one covalent bond, never more than one covalent bond. Carbon is the most important atom. Carbon can form 4 covalent bonds because it has four electrons in the outer shell and each orbital has only one electron. Nitrogen has 5 electrons in the outermost shell, so it can form 3 covalent bonds. Oxygen has 6 electrons in the outermost shell, so it can form 2 covalent bonds. When a covalent bond is formed, there is a new kind of orbital. This new orbital has different properties. Every covalent bond is unique. Example: the CH covalent bond had a new length, unique size, and angle, with respect to the other covalent bonds. A methane atom always looks like a tetrahedron. The Oxygen2 in this example forms a double covalent bond. Ball-and-stick model is good to know something about the structure/shape about the molecule. Space filling model is rarely used, but gives the electron density. Like you would see in a microscope. Nonpolar: spend the same amount of time surrounding one atom as they do surrounding the other. Here, the electrons spend much more time with the oxygen atom (that is why they are drawn closer to it and NOT in the middle). The red outer shape also would indicate that the electrons spend a lot more time with the oxygen atom than with the Hydrogen atoms. Oxygen has a partial negative charge. Hydrogen has a partial positive charge Why do we get this? Electronegativity Oxygen has 8 protons, whereas Carbon has only 6. That means the additional protons can direct the electrons much more. Oxygen is much more electronegative than Carbon. A bond between Carbon and Oxygen would be a polar covalent bond. Carbon and Hydrogen have almost the same electronegativity. So, a bond between Carbon and Hydrogen is a non-polar covalent bond. This is surprising because hydrogen is only one proton. However, it only has one electron in its outer shell, so it balances its electro negativity. Bonds between thesameatoms (ex: Carbon and carbon, Hydrogen and Hydrogen, etc) are non polar, because both atoms, obviously, have the same electronegativity. Hydrogen always forms one covalent bond. Carbon always forms 4 covalent bonds. Nitrogen and oxygen have some variability. Hydrogen bonds: Because we have a polar covalent bond, there is an important electrostatical interaction between water molecules. You get electrostatic attraction between negative charge of oxygen atom and the positive charge of Hydrogen. TheHydrogenbondis muchweakerthanacovalentbond.Ifyouboilwater,youcandisrupthydrogen bonds. But you do not disrupt covalent bonds. However, because there are MANY hydrogen bonds, they are pretty strong. Properties of water: Meniscus: If you look in a really narrow tube, you can see that water is highest at the edges. The hydrogen bonds form between the water molecules and the polar molecules in the glass. At the surface, there is a network of hydrogen bonds. It is tight enough to allow small animals to walk on water. If you jump high enough, it can really hurt. The only reason water can move up a high tree, is because of these hydrogen bonds. The actual force that pulls up the water is evaporation, but the hydrogen bonds hold the water column together. With gasoline, you disrupt the hydrogen column. Solid water (ice) has a lower density than liqui
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