02 BONDS AND POLARITY

key words

  • ionic bond: a bond between two ions
    i. note: ions occur because between two atoms, one is so electronegative that it strips the other’s electrons away completely. the electron transfer itself is NOT the ionic bond; it only allows an ionic bond to occur. as such, any two ions of opposite charge can form an ionic bond. the ions didn’t need to have acquired their respective charges via electron transfer with each other

  • covalent bond: two atoms sharing a pair of valence electrons

  • hydrogen bond: fragile bond occurring when slightly pos. hydrogen of one water molecule attracted to slightly neg. hydrogen of a nearby molecule

  • bonding capacity: the max number of bonds an atom can form in order to complete its electron shells

  • electronegativity: measure of atomic ability to attract electrons in a chemical bond

  • polar: overall charge unevenly distributed, with net partial charges

  • nonpolar: overall charge evenly distributed, with no net charge

  • hydrophilic: substance with an affinity for water

  • hydrophobic: subtance repelled by water

  • solute: substance being dissolved

  • solvent: substance doing the dissolving

  • dissolution: process of a solute dissolves in a solvent to form a solution (homogenous mixture)

  • solubility: max amount of solute that can dissolve in a given amount of solvent

class notes

  • ionic bonds: ions attracted to each other, but their respective electrons still only circle their own nuclei
    i. remember, electron transfer is not the ionic bond
    ii. cation: positive ion
    iii. anion: negative ion
    iv. form ionic compounds (not called molecules)
        a. salt

  • covalent bonds: shared electrons attracted to both individual nucleus and other atom nucleus
    i. stable: sharing electrons tends to create full shells
    ii. form molecules (bonds based on bonding capacity)
    iii. may be polar or nonpolar
        a. polar: electrons are shared unequally due to electronegativity inequality (i.e. electrons spend more time by highly-electronegative than little-electronegative) → creates partial charges on atoms (not as charged as ions)
        b. nonpolar: electrons shared equally (e.g. between two atoms of same element)

  • bonding capacity: # of unpaired valence electrons
    i. “holes” in the shell

  • electronegativity: measure of an atom’s tendency to attract electrons to itself

  • hydrogen bonding: weak interaction between hydrogen of polar molecule and electronegative atom of another polar molecule
    i. not as strong as covalent/ionic, but water is so plentiful that H-bonds affect bio systems strongly
    ii. not always between H and O

  • cohesion: water hydrogen-bonding with itself

  • water as a solvent: able to dissolve solids (especially polar compounds) well
    i. hydration shells around individual ions when dissolving ionic compounds

hydrophilic

  • ions

  • polar molecules

hydrophobic

  • oil

  • nonpolar molecules

  • pH scale: lower → acidic, higher → basic
    i. acid: proton (H+) donor
        a. has hydrogens weakly bound that often get pulled away
    ii. base: proton (H+) acceptor
    iii. changing pH (acidity) of solution has to do with adding/removing protons (hydrogen ions)
    iv. ionized form: when the acid/base has lost/gained electron
        a. always an acid/base, regardless of whether it has already lost/gained electron or not

textbook notes

  • bonding capacity = valence; equals number of unpaired electrons required to complete valence shell

  • molecules like H2 and O2 are pure elements, not compounds: compounds have multiple different elements

  • polar covalent bonds vary in polarity depending on atoms’ electronegativity differences

  • hydrogen bonds break and reform quickly as water molecules move randomly

  • ionic compound doesn’t consist of molecules (like a covalent compound): ionic compound formula (e.g. NaCl) only indicates elemental ratio, as NaCl is not a molecule
    i. “ion” also applies to whole molecules that are charged (e.g. NH4+)

  • solution: completely homogenous mixture of multiple substances
    i. aqueous solution: the solute is dissolved in water (solvent)

study questions

  1. list the types of chemical bonds
    ionic, covalent, polar covalent, nonpolar covalent, hydrogen bonds

  2. list the elements with higher and lower electronegativity
    C/H have lower, approx. equal electronegativities. O/N are both highly electronegative

  3. explain how bonding capacity/electronegativity affect covalent bonding
    when two atoms form a covalent bond, this indicates that both atoms have approximately similar electronegativity values, meaning they share electrons. by sharing electrons, this often allows both atoms to gain a full set of electrons in their respective shells.

  4. explain why ions or polar molecules are hydrophilic
    due to their charges/partial charges, water molecules are attracted to ions/polar molecules, meaning they have an affinity for water

  5. define a biological acid or base
    a biological acid, like a carboxyl group, has a loosely-bound hydrogen ion that it often loses, or “donates.” a biological base, like an amino group, accepts these lost hydrogen ions.

  1. explain how water acts as a solvent
    water helps ionic compounds and polar molecules dissolve. with ionic compounds, water molecules form intermolecular attractions with the ions; the partially neg. oxygen atom is attracted to the cation, and the partially pos. hydrogen atoms are attractd to the anions. these attractions cause water molecules to form a hydration shell around individual ions of an ionic compound, eventually breaking apart the compound to form an ionic solution. with polar compounds, water molecules can similarly break them apart by forming hydrogen bonds with them.

  2. compare and contrast the types of chemical bonds
    covalent bonds form between atoms that share electrons. covalent bonds can be either polar or nonpolar; polar covalent bonds form between atoms with vastly different electronegativity values, and the electrons are shared unequally between the two atoms. nonpolar covalent bonds form between atoms with similar electronegativity atoms, meaning the electrons are shared equally between the two atoms. on the other hand, ionic bonds are formed between two ions, one cation and one anion.

  3. predict whether a molecule is polar

  4. predict whether a molecule is hydrophilic/hydrophobic

  5. explain the relationship between protons and pH
    pH indicates the acidity of a solution. the acidity is based on the proton (H+) concentration; if the proton concentration is high, the solution is acidic, and the pH level drops. conversely, if the proton concentration is low, the solution is basic, and the pH level rises.