Chapter 2: The Chemical Context of Life
Key Terms: element, atom, proton, neutron, electron, atomic number, atomic mass, isotope, valence
electrons, covalent bond, molecule, polar vs. non-polar, anion, cation, hydrogen bond
THE ATOMS OF LIFE
Distribution of Elements in Living Organisms
Only four elements found in greater than trace amounts
Elements are generally light, atomic mass less than 21
Most Abundant Elements: N, O, C, H
All form covalently bonded molecules
Possess breakable chemical bonds to make a variety of molecules
Reflect predominance of water (H2O) in organisms
Many form gaseous molecules that are soluble in water
ATOMS: THE STUFF OF LIFE
Universe Composed of Matter
All matter made of atoms
Very small size, resembling solar system
Composed of smaller subatomic particles
Protons (+) and neutrons (0) in central nucleus
Electrons (-) in circular orbits around nucleus
Same number as protons to balance charge
Dictates chemical activity
Atomic number = number of protons
Neutrons and protons have the same mass
Only protons have electrical charge
Mass versus weight
Mass is the amount of a substance
Weight is the force of gravity exerted on that mass
Atomic mass = mass of protons + mass of neutrons
Mass measured in Daltons
Proton or neutron is roughly 1 Dalton
Electron is 1/1840 Dalton (no mass for our purposes)
Isotopes
All atoms of an element have the same atomic number (proton number)
An element cannot be broken into other elements by chemical means
Isotopes of an element have:
Same number of protons, different number of neutrons
Same number of electrons, thus same chemical properties
Example: carbon-12 versus carbon-13 and carbon-14
Unstable forms, like carbon-14, decay
Emit radioactive energy
Half-life = time for half of a sample's atoms to decay
Electrons
Electrically neutral atom has same number of electrons and protons
Only electrons are involved in chemical reactions
Element that possesses a net electrical charge
Positive charge if electron lost (0-(-1))= +1, a cation
Negative charge if electron gained (0+(-1))= -1, an anion
Energy within the Atom (Fig. 2.7)
(-) electrons are attracted to (+) protons
Energy required to keep electrons in orbit
Electron energy of position is potential energy
Moving electron away from nucleus
Requires energy
Electron then has more potential energy
Moving electron toward nucleus
Releases energy
Electron then has less potential energy
Electrons Determine the Chemical Behavior of Atoms
Arrangement determines chemical properties of element
Orbital describes probable, not actual location
Shapes differ (Fig. 2.8)
Inner s orbitals are spherical
More distant p orbitals are dumbbell-shaped, there are three
Maximum number of two electrons per orbital
Orbitals extremely far away from nucleus, atom mostly empty space
If nucleus were a golf ball, electrons would be 1 km. away
Nuclei of different atoms do not contact one another
Outer electrons ONLY (Valence electrons)interact which gives atom its
chemical behavior
Exchange of electrons between molecules
Oxidation is a loss of electrons
Reduction is a gain of electrons
Chemical energy stored in electrons by oxidation-reduction reactions
Energy level schematics
Electrons represented as concentric rings called energy levels
Electrons in outer most rings hold more energy
Don't confuse energy levels and electron orbitals
The Periodic Table
Eight groups of repeating chemical properties
Based on interactions of valence electrons in outer shell
Maximum of eight electrons in outer shell of elements important to life
Elements at maximum are inert, not reactive
Elements with one less than maximum are highly reactive
Octet rule (rule of eight) states that atoms want their outer shell full
CHEMICAL BONDS HOLD MOLECULES TOGETHER
Molecule Is a Stable Group of Atoms
Compounds are molecules containing more than one kind of element
Covalent Bonds Build Stable Molecules
Two atoms share one or more pairs of valence electrons
Example: single bonded diatomic hydrogen (H2)
Hydrogen has unpaired electron and unfilled outer level
Two atoms combine, each nucleus shares two electrons
Bond requires close proximity of atoms to one another
Covalent bonds are very strong
Double bond shares two pairs of electrons, stronger than a single bond
Structural formulas: H - H or O = O
Molecular formulas: H2 or O2
Molecules with Several Covalent Bonds
Atoms can share electrons with more than one other atom
Example: carbon, has six electrons, four in the outer level
To satisfy octet rule must gain four electrons
Thus can form four chemical bonds
Covalent bonds can be electrically charged or neutral
Nonpolar molecules are composed of elements that have the same strength
Ex. CH4 where C and H are of equal strength so electrons are shared equally
Polar molecules are composed of elements that have different strengths (Fig. 2.11)
Ex. H20 where oxygen is stronger than hydrogen pulling electrons more to one end
Ionic Bonds Form Crystals
Atoms attracted by opposite electrical charges
Atoms donate or receive electrons from other atoms
Example: sodium chloride, common table salt
Sodium atom, loses electron = Na+
Chlorine atom, accepts electron = ClResulting atoms become charged ions, an ionic compound
Bond forms by attraction of ions of opposite charges
Not between two individual atoms
Between one ion and all oppositely charged ions in vicinity
Hydrogen bonds are weak electrostatic bonds between two electronegative atoms (Fig. 2.14)
Ex. Water- slightly negative O attracted to slightly positive H of another molecule
Ex. Ammonia- slightly negative N attracted to slightly positive H of another molecule
Chemical reactions Make and Break Chemical Bonds
Involve shifting atoms without change in number or identity
Reactants: original, pre-reaction molecules
Products: molecules resulting from a reaction
Influenced by several factors
Temperature: heat increases rate
Concentration: reactant versus product have opposite effect
Catalyst: special substance increases rate