AP Biology: The Chemistry of Life — Electronegativity and Bond Types

Water and Phospholipid Membranes

• Water interacts with biological molecules due to its polarity; it is a polar solvent.
• Phospholipids have hydrophilic (water-loving) heads and hydrophobic (water-fearing) tails.
• In aqueous environments, phospholipids arrange into a phospholipid membrane with a bilayer:
  • Hydrophilic phospholipid heads face outward toward water on both sides.
  • Hydrophobic phospholipid tails face inward away from water.
• This arrangement creates a selective barrier essential for cellular compartments and membrane dynamics.

The World of Elements

• Elements are the different kinds of atoms.
• Common life-related elements include: H, C, N, O, P, S, Na, K, Mg, Ca, etc.
• Elements combine in biological molecules to form compounds with varied properties.

Essential Elements in Life

• About 25 chemical elements are essential for life.
• Four elements make up 96% of living matter:
  • Carbon, Hydrogen, Oxygen, Nitrogen
  • $ext{CHON} ext{ (major four)}$
• Four elements make up most of the remaining 4%:
  • Phosphorus, Calcium, Sulfur, Potassium
  • $ext{P}, ext{Ca}, ext{S}, ext{K}$
• The remaining elements are trace elements essential in smaller amounts.

Distribution of Elements in a Typical Human

• Major groupings by presence in biomolecules:
  • $ext{CHO}$ (Carbon, Hydrogen, Oxygen)
  • $ext{CHONS}$ (Carbon, Hydrogen, Oxygen, Nitrogen, Sulfur)
  • $ext{CHONP}$ (Carbon, Hydrogen, Oxygen, Nitrogen, Phosphorus)
• Other organisms can differ in exact composition and amounts.

By Bonds: Types of Bonds and Attractions

• Strong bonds between atoms:
  • Nonpolar Covalent
  • Polar Covalent
  • Ionic
• Weaker attractions between molecules/parts of molecules:
  • Hydrogen bonds (strongest of the weak category)
  • van der Waals forces (not the focus here)
• Determining bond type involves electronegativity differences between bonded atoms.
• How are atoms connected? By comparing their electronegativities.

Electronegativity

• Definition: a measure of how strongly an element pulls on shared electrons in a bond.
• Main determinants:
  • Number of protons in the nucleus: more protons → greater pull → higher electronegativity.
  • Atomic size: smaller atom → outer electrons are closer to the nucleus → higher pull → higher electronegativity.
• In short: electronegativity increases with more protons and decreases with larger atomic radius.

Electronegativity Across the Periodic Table

• Key values (Pauling scale) for representative elements:
  • Hydrogen: $EN(H) = 2.1$
  • Carbon: $EN(C) = 2.5$
  • Nitrogen: $EN(N) = 3.0$
  • Oxygen: $EN(O) = 3.5$
  • Fluorine: $EN(F) = 4.0$
• In life contexts, the most relevant trend is that O > N > C > H in electronegativity, with F being the most electronegative.
• Note: the periodic table pattern shows increasing EN across a period and higher EN for nonmetals (especially halogens) relative to metals.

How Electronegativity Difference Determines Bond Type

• Let $riangle EN = |EN<em>A - EN</em>B|$ between two bonded atoms A and B.
• Bond type categories (as used in this material):
  • $riangle EN \le 0.4 \Rightarrow \text{Nonpolar Covalent}$
  • 0.4 < \triangle EN < 1.7 \Rightarrow \text{Polar Covalent}
  • $\triangle EN \ge 1.7 \Rightarrow \text{Ionic}$
• This framework helps predict how electrons are shared or transferred in bonds.

Bonds in CHON (the 96%): What to Expect

• CHON electronegativity order (from most to least):
  • EN(O) = 3.5 > EN(N) = 3.0 > EN(C) = 2.5 > EN(H) = 2.1
• Consequences for bond types among CHON pairs:
  • Polar covalent bonds are common when there is a significant difference, e.g., O–H, C–O, C–N, H–N, O–N.
  • Nonpolar covalent bonds are common when electronegativities are similar, e.g., C–H, C–C, O–O (though O–O is less common as a typical single-bond arrangement in organic molecules).
  • Ionic bonds are unlikely among CHON nonmetals because there are no metal atoms to donate electrons and form full ions; typically, ionic bonds involve metal–nonmetal combinations.

Bond Classifications and Examples (from the CHON context)

• Polar bonds (typical CHON):
  • C–O
  • C–N
  • H–O
  • H–N
  • O–N
• Ionic bonds: none among CHON pairs in typical biomolecules (no metal donors in this set).
• Nonpolar Covalent bonds (often associated with CH-only or symmetry-delimited pairs):
  • C–H
  • C–C
  • O–O (uncommon in typical biomolecules but possible in some peroxide-like species)
• None (rare or non-existent in standard biomolecules):
  • N–N (uncommon in stable biomolecules outside of specific functional groups)
  • H–H (gas-phase diatomic molecule, not a primary covalent bond in biomolecules)
• Practical reminder: watch for C–H and C–C bonds as the most frequent nonpolar covalent bonds in organic molecules.

Putting It All Together: Practice Connections

• The chemical nature of life relies on a balance of bond types to determine molecule shape, reactivity, and function (membranes, proteins, nucleic acids).
• Water’s polarity and hydrogen bonding drive interactions with polar covalent bonds in biomolecules and influence membrane properties.
• Membranes rely on amphipathic phospholipids: hydrophilic heads interact with water; hydrophobic tails cluster away from water to form a barrier.
• The CHONP framework explains why life chemistry prioritizes certain bonds and functional groups (e.g., alcohols, carbonyls, amines, phosphates).

Quick Reference: Thresholds and Notation

• Bond-type decision rule:
  • Let $\triangle EN = |EN<em>A - EN</em>B|$.
  • $\triangle EN \le 0.4 \Rightarrow \text{Nonpolar Covalent}$
  • 0.4 < \triangle EN < 1.7 \Rightarrow \text{Polar Covalent}
  • $\triangle EN \ge 1.7 \Rightarrow \text{Ionic}$
• Key CHON bonds to remember as common: ${\text{C–O}, \text{C–N}, \text{H–O}, \text{H–N}, \text{O–N}}$
• Common nonpolar CH bonds: ${\text{C–H}, \text{C–C}}$
• Less common or context-specific: $\text{O–O (uncommon)}, \text{N–N (uncommon)}, \text{H–H (gas)}$