Covalent Bonds, Polarity, and Hydrogen Bonding

Covalent Bonds and Double Bonds

• Definition of Covalent Bonds: Interactions where atoms achieve stability by sharing outer-shell electrons.

• Carbon Outer Shell Structure: Carbon possesses 4 electrons in its outermost electron shell, leaving 4 spots available for bonding.

• Double Bond Characteristics:

  • A double bond occurs when two atoms share two pairs of electrons (4 electrons total shared between the two atoms).
  • Notation: Represented visually in chemical drawings by two parallel lines between the atoms (e.g., $C=C$) rather than a single line.
  • Relative Strength: Double bonds are stronger than single bonds because there are two distinct linkages holding the atoms together (analogous to using two pieces of tape instead of one).

• Example - Ethylene ($C_2H_4$):

  • Consists of two carbon atoms connected to each other via a double bond.
  • Each carbon atom shares its remaining two outer-shell electrons with two individual hydrogen atoms via single bonds on the sides.

Molecular Polarity and Electron Distribution

• Electrical Neutrality vs. Inertness: Covalently bound molecules can be overall electrically neutral (containing equal total numbers of protons and electrons), yet remain non-inert due to local charge imbalances.

• Definition of Polarity: An unequal distribution of shared electrons across a molecule, creating localized regions of partial positive and negative charge.

• Analogy for Unequal Sharing:

  • Two siblings born 21 months apart display competitive behavior over toys.
  • The younger sibling sits in the middle of the living room floor, using a rich imagination to converse with surrounding stuffed animals.
  • When the older sibling enters the room, the younger sibling subtly shifts all the toys slightly closer to their own body while continuing to talk, knowing the older sibling intends to take them.
  • Similarly, certain "selfish" atoms in polar molecules share electrons with neighboring atoms but pull those shared electrons subtly closer to their own nuclei.

• Electronegativity and Electropositivity in Water ($H_2O$):

  • A water molecule consists of 1 oxygen atom and 2 hydrogen atoms.
  • Electronegative Atom (Oxygen): Oxygen acts as a selfish atom. It shares electrons with hydrogen but holds those shared electrons closer to its own nucleus than to the hydrogen nuclei. This concentrated electron density makes the oxygen region slightly negatively charged.
  • Electropositive Atoms (Hydrogen): The shared electrons are pulled away from the hydrogen nuclei. As a result, the hydrogen region carries a slight positive charge.
  • Overall Charge: The complete water molecule remains overall electrically neutral, but exhibits a distinct localized charge distribution.

• Key Electronegative Atoms in Biology:

  • The primary atoms responsible for polar behavior in biological macromolecules are:
  • Oxygen ($O$)
  • Nitrogen ($N$)
  • Phosphorus ($P$)
  • The presence of oxygen, nitrogen, or phosphorus in a macromolecule indicates that the molecule is likely polar and possesses an unequal electron distribution.

Hydrogen Bonding and Physical States of Matter

• Principle of Electrostatic Attraction: Opposite electrical charges attract each other; positive charges are strongly attracted to negative charges.

• Mechanism of Hydrogen Bonding:

  • Intermolecular interactions occur when the electropositive portion (hydrogen) of one polar molecule is attracted to the electronegative portion (e.g., oxygen) of an adjacent polar molecule.
  • These non-covalent interactions hold adjacent polar molecules close together.

• Color Conventions in Water Models:

  • Oxygen is depicted as red (electronegative/partially negative).
  • Hydrogen is depicted as white (electropositive/partially positive).

• Impact on Physical States and Cohesion:

  • Non-Polar/Weakly Interactive Small Molecules:
  • Molecular oxygen ($O_2$) and carbon dioxide ($CO_2$) are small molecules (e.g., $CO_2$ consists of 3 atoms).
  • Lacking these strong cohesive polar interactions, their individual molecules remain far apart, causing them to exist as gases at room temperature.
  • Water ($H_2O$):
  • Despite being a small molecule, the cohesive hydrogen bonds hold individual $H_2O$ molecules tightly together.
  • This cohesion dramatically alters the properties of the small molecule, converting it into a liquid state at room temperature rather than a gas.
  • These cohesive properties are essential for enabling biological processes and solution chemistry.

Questions & Discussion

• Question: When water is heated to a boil and turns into steam, which bonds are breaking: the covalent bonds holding the oxygen and hydrogen atoms together within each water molecule, or the hydrogen bonds between separate water molecules?

• Answer: The hydrogen bonds between separate water molecules are breaking.

• Evidence and Reasoning:

  • Relative Bond Strength: Covalent bonds involve direct electron sharing, whereas hydrogen bonds do not share electrons and rely solely on weaker electrostatic attractions. Because hydrogen bonds are significantly weaker than covalent bonds, thermal energy disrupts hydrogen bonds long before breaking covalent bonds.
  • Chemical Identity of Steam: Steam remains moist and retains the chemical properties of water ($H_2O$). Because the oxygen and hydrogen atoms remain linked together, the underlying covalent bonds are demonstrably intact.
  • Mechanism of Phase Change: Liquid water remains liquid because hydrogen bonds hold the molecules in close proximity. Boiling adds heat energy, breaking these intermolecular hydrogen bonds and allowing the intact water molecules to disperse farther apart into a gas (steam).

• Session Schedule Note: The discussion paused for a break at 12:15 p.m.