Chapter 2 The Chemical Context of Life

1.What is an atom? –Smallest unit of matter that retains the physical & chemical properties of its element –Element – a substance that cannot be broken down into other substances by chemical rxn 2.What is an atom made of? –Proton – (+1) charge, found in nucleus, 1 amu/1 dalton –Neutron – no charge, found in nucleus, 1 amu/1 dalton –Electron – (-1) charge, orbit nucleus, mass negligible Chapter 2 The Chemical Context of Life

Figure 2.4 Simplified models of a helium (He) atom Nucleus (a) (b) Cloud of negative charge (2 electrons) Electrons

1.What is an atom? 2.What is an atom made of? 3.What do these numbers mean? 23 Na 12 C 11 Na 6 C 4.What is an isotope? Radioisotope? –Atoms of an element that have the same atomic # but different atomic mass (due to different #s of neutrons) –Unstable isotope where the nucleus spontaneously decays emitting subatomic particles &/or energy as radioactivity. 5. What are radioisotopes used for? Chapter 2 The Chemical Context of Life Atomic mass = protons + neutrons Atomic # = # of protons

1.What is an atom? 2.What is an atom made of? 3.What do these numbers mean? 4.What is an isotope? Radioisotope? 5.What are radioisotopes used for?  Radiation cancer therapy (“nuclear medicine”) -  Cell research (trace path of protein from ribosome-to-final destination) 6.How are atoms held together?  Chemical bonds!!! 7.What are the different types of bonds? –Covalent – sharing of electrons Polar covalent – UNequal sharing Non-polar covalent – equal sharing –Ionic – complete transfer of electrons Chapter 2 The Chemical Context of Life

Figure 2.10 Formation of a covalent bond Hydrogen atoms (2 H) Hydrogen molecule (H 2 ) In each hydrogen atom, the single electron is held in its orbital by its attraction to the proton in the nucleus. 1 When two hydrogen atoms approach each other, the electron of each atom is also attracted to the proton in the other nucleus. 2 The two electrons become shared in a covalent bond, forming an H 2 molecule. 3

(a) (b) Name (molecular formula) Electron- shell diagram Structural formula Space- filling model Hydrogen (H 2 ). Two hydrogen atoms can form a single bond. Oxygen (O 2 ). Two oxygen atoms share two pairs of electrons to form a double bond. HH O O Figure 2.11 A, B Single and double covalent bonds

Name (molecular formula) Electron- shell diagram Structural formula Space- filling model (c) Methane (CH 4 ). Four hydrogen atoms can satisfy the valence of one carbon atom, forming methane. Water (H 2 O). Two hydrogen atoms and one oxygen atom are joined by covalent bonds to produce a molecule of water. (d) H O H HH H H C Figure 2.11 C, D Covalent bonding in compounds

Cl – Chloride ion (an anion) – The lone valence electron of a sodium atom is transferred to join the 7 valence electrons of a chlorine atom. 1 Each resulting ion has a completed valence shell. An ionic bond can form between the oppositely charged ions. 2 Na Cl + Na Sodium atom (an uncharged atom) Cl Chlorine atom (an uncharged atom) Na + Sodium on (a cation) Sodium chloride (NaCl) Figure 2.13 An ionic bond –Is an attraction between anions and cations

1.What is an atom? 2.What is an atom made of? 3.What do these numbers mean? 4.What is an isotope? Radioisotope? 5.What are radioisotopes used for? 6.How are atoms held together? 7.What are the different types of bonds? 8.What is electronegativity? –Atom’s ability to attract & hold electrons –In Biology – most concerned with N & O Chapter 2 The Chemical Context of Life

Figure 2.12 This results in a partial negative charge on the oxygen and a partial positive charge on the hydrogens. H2OH2O –– O H H ++ ++ Because oxygen (O) is more electronegative than hydrogen (H), shared electrons are pulled more toward oxygen. In a polar covalent bond –The atoms have differing electronegativities –Share the electrons unequally ––

Hydrogen Bonds  – –  + +  + + Water (H 2 O) Ammonia (NH 3 ) O H H  + +  – – N H H H A hydrogen bond results from the attraction between the partial positive charge on the hydrogen atom of water and the partial negative charge on the nitrogen atom of ammonia. ++ ++ Figure 2.15 A hydrogen bond –Forms when a hydrogen atom covalently bonded to one electronegative atom is also attracted to another electronegative atom

Molecular shape –Determines how biological molecules recognize and respond to one another with specificity

Morphine Carbon Hydrogen Nitrogen Sulfur Oxygen Natural endorphin (a) Structures of endorphin and morphine. The boxed portion of the endorphin molecule (left) binds to receptor molecules on target cells in the brain. The boxed portion of the morphine molecule is a close match. (b) Binding to endorphin receptors. Endorphin receptors on the surface of a brain cell recognize and can bind to both endorphin and morphine. Natural endorphin Endorphin receptors Morphine Brain cell Figure 2.17

1.What is an atom? 2.What is an atom made of? 3.What do these numbers mean? 4.What is an isotope? Radioisotope? 5.What are radioisotopes used for? 6.How are atoms held together? 7.What are the different types of bonds? 8.What is electronegativity? 9.How are bonds created between atoms? Chapter 2 The Chemical Context of Life

ReactantsReactionProduct 2 H 2 O2O2 2 H 2 O + + Chemical reactions –Convert reactants to products