The Fundamental Chemistry of Life - Part 1

1.1 The Fundamental Chemistry of Life (p. 8-18) Isotopes • all atoms of an element have the same number of protons, but they may not all have the same number of neutrons • atoms of a single element that possess different numbers of neutrons are called isotopes of that element • most elements in nature exist as mixtures of different isotopes • Carbon (C), for example, has three isotopes, all containing six protons • the isotopes of elements behave exactly the same as the main element in chemical reactions because the number of protons and electrons are the same • the nuclei of some isotopes of an element are unstable and tend to break down, or decay, giving off particles that are detected as radioactivity • the decay process transforms an unstable, radioactive isotope--called a radioisotope--into an atom of another element • over 99% of the carbon found in nature exists as 12C, carbon-12 • the rarest carbon isotope is carbon-14 with ____ neutrons • unlike the other two isotopes, carbon-14 is unstable • it decays, giving off particles and energy • as it decays, one neutron splits into a high-energy electron and a proton • the 14C isotope will break down into a 14N atom (with 7 neutrons, 7 electrons and 7 protons) • some radioactive isotopes are more unstable than others, and ∴ they decay more readily • for any given isotope, however, the rate of decay is constant (i.e. carbon-14 has a half-life of 5730 years) • it is independent of chemical reactions or environmental conditions (like temperature and pressure) • radioactive decay may damage molecules in living cells, which harm the organism • but, some radioisotopes are useful because of their steady rate of decay--they provide scientists with information about the age of organic materials, rocks, and fossils • i.e. a sample of carbon containing 1 gram of C-14 today would contain 0.5 gram of C-14 after 5730 years, 0.25 gram ________ years from now, and so on • as well, radioisotopes are involved in many different medical applications • radioactive tracers are radioisotopes that are used to follow a specific chemical through a chemical reaction • for example, Melvin Calvin used radioactively labeled carbon to determine the reactions in photosynthesis • as well, they are used in the field of nuclear medicine to help with the diagnosis and treatment of diseases • i.e. injecting radioactive iodine-131 in patients in order to detect thyroid problems (the thyroid is the only gland that actively absorbs iodine) Scientist wearing a dosimeter Electron Arrangements • the chemical properties of an atom are determined by their electron arrangement since it is the electrons that are directly involved in chemical reactions • RECALL: the # of electrons in an atom = the # of protons in the nucleus of the atom • atoms are electrically neutral since protons (+vely charged) and electrons (-vely charged) are equal in number • electrons move around the nucleus of an atom in shells more commonly called orbitals • electron orbitals are grouped into energy levels, which are also called energy shells • the lowest energy shell (numbered 1, 2, 3, etc.) is closest to the nucleus • a maximum of two electrons are found in the lowest energy shell • the second and third energy shells hold up to 8 and 18 electrons, respectively • valence electrons are the electrons in an atom's outer energy shell (or valence shell) • when this valence shell is not completely full with its maximum number of electrons, the atom is chemically reactive (i.e. Hydrogen) • when the shell is completely full, the atoms are chemically inactive, or inert--like the noble gases (i.e. Helium) • unfilled valence shells are less stable than filled ones • so, these atoms will try to become stable by losing, gaining or sharing electrons to complete their valence shell • the elements predominantly found in living things (H, C, N and O) have unfilled valence shells, and will participate in chemical reactions with other atoms • atoms that become stable by sharing electrons created hybridized orbitals in which the valence electron orbitals overlap • sharing is the most common way that atoms bond to form biological molecules Chemical Bonds 1) Ionic Bonds • ionic bonds form between metals and nonmetals • metals tend to lose electrons and form positively charged cations • nonmetals tend to gain electrons and form negatively charged anions • ions are the general term given to atoms that have lost or gained electrons • ions of opposite charge are attracted to each other • ions are strongly attracted to water molecules, so ionic compounds will dissociate and easily dissolve in water • ionic compounds also tend to form crystalline solids with high melting temperatures 2) Covalent Bonds • covalent bonds form when two nonmetal atoms share one or more pairs of valence electrons • the resulting molecule that is formed is called a molecular compound • i.e. the covalent bond between two hydrogen atoms to form the molecule H2 is the simplest example of this sharing mechanism • the strength of a covalent bonds depends on the electronegativity, which is the measure of atom's attraction for additional electrons • bond strength also depends on the number of shared electrons Examples: H2 single covalent bond O2 double covalent bond N2 triple covalent bond Polar and Nonpolar Covalent Bonds • atoms differ in their affinity for electrons, a property called electronegativity • in general, electronegativity increases left to right in a row of the periodic table (due to greater desire to gain electrons) and decreases down the columns of the table (due to electrons being further from the nucleus which wants to grab them) • for atoms that differ greatly in electronegativity, electrons are not equally shared • the shared electrons are more likely to be found near the atom with greater electronegativity and less likely to be located near the atom of lower electronegativity • in this case, although the molecule is still electrically neutral the distribution of charge is not uniform • the unequal distribution results in regions of partial negative charge near the more electronegative atom and regions of partial positive charge near the less electronegative atom • such bonds are termed polar covalent bonds, and the molecules polar molecules When drawing polar molecules, these partial charges are usually symbolized by the lowercase Greek letter delta (δ). The table of electronegativities can be used to determine whether a bond is nonpolar, polar or ionic. By calculating electronegativity difference (ΔEn), the difference in electronegativity between two atoms participating in a covalent bond, you can determine the type of bond. ΔEn = 0 ⇒ Nonpolar covalent Ex. Oxygen gas (O2) O O ΔEn > 0 but < 1.7 ⇒ Polar covalent Ex. Carbon-oxygen bond C O ΔEn = or > 1.7 ⇒ Ionic Ex. Sodium chloride (NaCl) Na Cl Predict the polarity of bonds between the following pairs of atoms: HW - read pages 8-14 - answer p. 18 #1-3, 5, 8, 9 Attachments HowStuffWorks How Carbon­14 Dating Works.flv