Structure of Water, Hydrogen Bonding, and Properties of Water

Study Guide - Smart Notes

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Matter: Anything that takes up space and has mass (e.g., rocks, metals, oils, gases, organisms).
Element: A substance that cannot be broken down into other substances by chemical reactions. There are 92 naturally occurring elements, organized in the periodic table.
Compound: A substance consisting of two or more different elements combined in a fixed ratio (e.g., H2O, NaCl).
Essential elements: 20-25% of the 92 elements are essential for survival and reproduction. CHOPN (Carbon, Hydrogen, Oxygen, Phosphorus, Nitrogen) make up 96% of living matter.
Trace elements: Required in very small quantities but are vital for organism function (e.g., iron, iodine).

Atomic number: Number of protons in an atom.
Atomic mass: Number of protons plus neutrons, averaged over all isotopes.
Elements in the same vertical column (group) have the same number of valence electrons.
Elements in the same horizontal row (period) have the same number of electron shells.

Atoms form chemical bonds to achieve stability, often by completing their valence shell (octet rule).
Valence shell: Outermost electron shell of an atom.

Chemical bond: Attraction between two atoms due to sharing or transferring valence electrons.
Electronegativity: Measure of an atom's ability to attract electrons to itself. Increases across a period and decreases down a group.

Covalent bond: Two or more atoms share electrons (usually between nonmetals).
Forms molecules and compounds.
Types:
- Single bond: 1 pair of shared electrons
- Double bond: 2 pairs of shared electrons
- Triple bond: 3 pairs of shared electrons
Two types of covalent bonds:
- Nonpolar covalent: Electrons shared equally (e.g., O2).
- Polar covalent: Electrons shared unequally, resulting in partial charges (e.g., H2O).

Ionic bond: Attraction between oppositely charged ions (usually between a metal and a nonmetal).
Forms ionic compounds and salts (e.g., NaCl, LiF).
Involves transfer of electrons:
- Cation: Positively charged ion
- Anion: Negatively charged ion

Hydrogen bond: The partially positive hydrogen atom in one polar covalent molecule is attracted to an electronegative atom (usually O, N, or F) in another molecule.
Intermolecular bond: Forms between molecules, not within them.
Hydrogen bonds are responsible for many unique properties of water.

Polarity
Cohesion
Adhesion
Capillary action
Temperature control (high specific heat, high heat of vaporization, evaporative cooling)
Density (floating ice)
Solvent properties
pH and buffering

Water molecules have polar covalent bonds due to unequal sharing of electrons between oxygen and hydrogen.
This results in partial negative charge on oxygen and partial positive charges on hydrogens.

Cohesion: Attraction of water molecules to each other due to hydrogen bonding.
Allows for transport of water and nutrients against gravity in plants.
Responsible for surface tension—the ability of water to resist external force.

Adhesion: Attraction of water molecules to other polar or charged substances.
Helps water cling to cell walls in plants, resisting the downward pull of gravity.

Upward movement of water due to cohesion, adhesion, and surface tension.
Occurs when adhesion is greater than cohesion; essential for water and nutrient transport in plants.

High specific heat: Water resists changes in temperature due to hydrogen bonding.
Heat must be absorbed to break hydrogen bonds; heat is released when bonds form.
Moderates air and ocean temperatures, benefiting marine life and stabilizing environments.
High heat of vaporization: Water requires a large amount of energy to evaporate, leading to evaporative cooling (e.g., sweating in humans).

As water freezes, it expands and becomes less dense due to hydrogen bonds forming a crystalline structure.
Allows ice to float, insulating aquatic life in winter.
Each water molecule in ice forms up to four hydrogen bonds with neighbors.

Water is a versatile solvent due to its polarity.
Can dissolve ionic compounds (e.g., NaCl) and polar molecules (e.g., sugars, proteins) by forming hydrogen bonds.
"Like dissolves like": polar solvents dissolve polar solutes.
Solution: Homogenous mixture of two or more substances.
Solvent: Dissolving agent (water).
Solute: Substance dissolved in the solvent.

pH: Measure of how acidic or basic a solution is.
Acid: Releases hydrogen ions (H+) in water.
Base: Accepts H+ or releases hydroxide ions (OH-).
Water can dissociate into H+ and OH-.
Buffer: Solution that resists changes in pH when acids or bases are added; crucial for maintaining pH stability in biological systems.

Bond Type	Mechanism	Example	Relative Strength
Covalent (Nonpolar)	Equal sharing of electrons	O2	Strong
Covalent (Polar)	Unequal sharing of electrons	H2O	Strong
Ionic	Transfer of electrons	NaCl	Strong (in dry state)
Hydrogen Bond	Attraction between partial charges	Between H2O molecules	Weak (individually), strong collectively

Mixing KCl (an ionic compound) with water results in dissociation into K+ and Cl- ions, which are stabilized by water's polarity.
Water's high specific heat and evaporative cooling help regulate climate and organism temperature.
If ice were denser than liquid water, aquatic life would be threatened as bodies of water would freeze from the bottom up.