BIOL 150 1st Edition Lecture 3 How Life Works How do cells work Cells are composed of atoms and molecules Structures and interactions among molecules are responsible for cells to work Properties of atoms that form molecules Nucleus protons and neutrons Electrons orbit nucleus Atomic number number of protons in an atom Atomic mass Mass Number protons neutrons Isotope atoms of the same element that have the same number of protons and electrons but differ in the number of neutrons Does this affect atomic mass Yes Protons are positively charged particles Electrons are negatively charged particles Example Hydrogen H has 1 proton 1 electron The first electron shall consists of a single spherical orbital Carbon C has 6 protons 6 electrons The first electron shell consists of a single spherical orbital The second electron shell consists of four orbitals three dumbbell shaped orbitals and the larger spherical orbital Energy levels of hydrogen and carbon In a simplified diagram the electron energy levels are depicted as circles and the electrons that occupy them as dots Energy The electrons of an atom differ in the amounts of potential energy they possess We will see the importance of this when we consider photosynthesis and consider fluorescence microscopy to view the locations of molecules in cells Chemical Bonds Chemical bonds link atoms to form molecules and mediate interactions among molecules Covalent Bonds between two atoms that share 1 or more electrons Example H H Two atoms of hydrogen combine to form hydrogen gas by sharing electrons in a molecular orbital The electrons in an atom s outermost shell are the valence electrons These notes represent a detailed interpretation of the professor s lecture GradeBuddy is best used as a supplement to your own notes not as a substitute The most stable configuration occurs when the outermost electron shell contains 8 electrons Nonpolar Covalent Bonds Electrons shared equally between atoms Polar Covalent Bonds Between two atoms that don t share electrons equally Example H O I O atom is the most electronegative than H atom H The bonds linking the hydrogen and oxygen atoms are polar with a partial positive charge near the hydrogen atoms and a partial negative charge near the oxygen atom Electronegativity an atom s ability to attract and form bonds with electrons Most atoms prefer to have an outer shell of 8 electrons Atoms on the left side of the periodic table have less than a half full valence shell so it requires more energy to gain electrons than to lose them Atoms on the right side of the periodic table tend to attract electrons Electronegativity increases as you move to the right across the periodic table Another way to look at electronegativity the Noble gasses He etc are exceptions Fluorine F is the most electronegative atom Covalent Bonding in Compounds Consider the shape of the water molecule The hydrogens H are asymmetrically positioned on the oxygen atom O This leaves a portion of the oxygen exposed The concentration of electrons in this region makes the oxygen partially negatively charged The average electron density around the oxygen atom is about 10X that around the hydrogen atoms Consider the shape of the water molecule The hydrogen atoms are bent at 104 5 degrees Why 1 The tetrahedral arrangement of electron pairs around the oxygen atom 2 The presence of lone pair electrons on the oxygen They are not involved in the covalent bonds but containing 2e repulse each other This forces the hydrogen s closer together Covalent bonding and Electronegativity In Methane the electronegativity s of Carbon 2 5 and hydrogen 2 1 are similar so electrons are nearly equally shared in their covalent bonds Nonpolar Covalent Bond In water the electronegativity s of Oxygen 3 5 and hydrogen 2 1 differ significantly So the electrons are unequally shared with hydrogen Polar Covalent Bond Hydrogen Bonds Hydrogen bonds form when the partial positive charge of hydrogen atoms are attracted to the partial negative charge of oxygen atoms If you have a nonpolar molecule it probably won t dissolve mix well with H2O