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Unformatted text preview: Biology 112 Lecture 1: Small Molecules: Chemical Bonds- Radioactive isotopes can be used as tracers in medicine. - Isotopes of the same element have a different number of neutrons, e.g. 14C has 6 protons and 8 neutrons - Isotopes have identical chemical properties, but are usually unstable, i.e. radioactive - To link atoms together, i.e. to form chemical bonds only electrons are important. 1. Covalent Bond - A covalent bond is formed by sharing a pair of electrons between two atoms. - In hydrogen molecules (H 2 ), a pair of electrons share a common orbital and spend equal amounts of time around each of the two nuclei. - The nuclei stay some distance from each other due to mutually repelling positive charges. - Covalent bonds are very strong. - Each covalent bond has a predictable length and angle, which makes it possible to predict the three-dimensional structures of molecules. - A double covalent bond occurs when atoms share two pairs of electrons. (H 2 C=CH 2 )- A covalent bond is made of 2 electrons - These 2 electrons are shared, i.e. one electron comes from each atom - Atoms can form bonds until their outermost shell is filled - Electrons are not always shared equally between covalently bonded atoms. - The attractive force that an atom exerts on electrons is called electronegativity. - When a molecule has nuclei with different electronegativities, an electron spends most of its time around the nucleus with the greater electronegativity. 2. Hydrogen Bond- Hydrogen bonds form when a hydrogen atom is sandwiched between two electron- attracting atoms (usually oxygen or nitrogen). - The partial positive charge of the hydrogen atom (δ+) is attracted to the partial negative charge (δ–) of oxygen or nitrogen. - Hydrogen bonds do not share electrons. Lecture 2: Small Molecules: Bonds, Acids and Bases 3. Ionic Bond - Ionic bonds involve a complete transfer of one or more electrons. - Ions are formed when an atom loses or gains electrons. - Positively charged ions are called cations. - Negatively charged ions are called anions.- Ionic bonds are formed by the electrical attraction between ions with opposite charges. - Table salt has chloride and sodium ions, held together by ionic bonds. - When salt is introduced into water, the partial charges of the water molecules can easily interfere with (replace) the ionic bonds. - Polar molecules tend to be hydrophilic. Substances that are ionic or polar often dissolve in water due to hydrogen bonds. - Nonpolar molecules are called hydrophobic because they tend to aggregate with other nonpolar molecules in water (hydrophobic interaction). 4. Hydrophobic Interaction- Water forces nonpolar molecules together, because doing so minimizes their disruptive effects on the hydrogen-bonded water network 5. van der Waals Interaction- Nonpolar molecules are also attracted to each other via relatively weak attractions called van der Waals forces (transient dipoles)....
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This note was uploaded on 12/25/2011 for the course BIOL 112 taught by Professor Lasko during the Winter '08 term at McGill.
- Winter '08