Life, Chemistry, and Water — Practice Flashcards

Why It Matters

Living organisms are collections of atoms and molecules linked together by chemical bonds.
The laws of chemistry govern both living and nonliving things.
Understanding how the structure of chemical substances relates to their behavior is the first step in understanding biology.

Elements in Life

Life is built from about 25 key elements.
CHON (Carbon, Hydrogen, Oxygen, Nitrogen) make up about 96\% of the weight of living organisms.
Ca, P, K, Na, Cl, Mg, S make up about 4\%.
Trace elements: each is present at less than 0.01\% of organisms.

Electrons and Chemical Activity

Valence electrons determine chemical activity.
Outer energy level filled = nonreactive; Not filled = chemically reactive.
Atoms gain, lose, or share valence electrons to achieve stability.
Ionic processes involve losing or gaining electrons (formation of ions).
Covalent processes involve sharing electrons (formation of covalent bonds).
A single missing electron leads to loss or gain of an electron (ion formation); missing more than one or two electrons leads to sharing electrons to complete octets.

Chemical Bonds and Chemical Reactions

Reactive elements tend to form molecules by creating chemical bonds.
The four most important chemical linkages in biological molecules are ionic bonds, covalent bonds, hydrogen bonds, and van der Waals forces.
Chemical reactions occur when atoms or molecules interact to form new bonds or break existing ones.

Ionic Bonds: NaCl Crystal

Ionic bonds form between oppositely charged ions in a crystal lattice.
Cation = positively charged (lost an electron).
Anion = negatively charged (gained an electron).
Key features:
- Strong attractive forces across large distances and in all directions.
- Vary in strength depending on the environment.
- Occur between metal and nonmetal elements.
Example: NaCl forms a giant ionic lattice.

Covalent Bonds

Covalent bonds result from the sharing of valence electrons.
Represented by a pair of dots or a single line (one bond).
Occur between nonmetal elements.
Lead to the formation of molecules and compounds.

Methane and Covalent Bond Models

Methane (CH₄) features a tetrahedral arrangement with a bond angle of 109.5^{\circ}.
Visual models include:
- Ball-and-stick model.
- Space-filling model.
A carbon atom serves as a building block for molecular models (as shown with methane and cholesterol in common textbooks).

Types of Covalent Bonds

Nonpolar covalent bonds: electrons shared equally.
Polar covalent bonds: electrons shared unequally.
Electrons are drawn as shared pairs in covalent bonds.

Electronegativity and Bond Character

Electronegativity (denoted as (\chi)) measures an atom’s attraction for shared electrons.
Higher electronegativity means a stronger pull on shared electrons.
Covalent bond polarity arises from differences in electronegativity between bonded atoms.

Relative Electronegativity (General Idea)

Electronegativity tends to increase across a period and decrease down a group.
Some common reference values (approximate): F > O > N > Cl > C > H (the exact numbers vary by table, but the trend is consistent).

Relationship Between Electronegativity and Bond Type

Bond Type vs. Electronegativity Difference (Δχ):
- Nonpolar covalent: \Delta\chi = 0.00 \text{ to } 0.4
- Polar covalent: \Delta\chi = 0.5 \text{ to } 1.7
- Ionic: \Delta\chi > 1.7
Charge on bonds:
- Nonpolar covalent: uncharged share electrons equally.
- Polar covalent: partially charged due to unequal sharing.
- Ionic: charges present on ions.

Polar vs Nonpolar Molecules

Polar molecules attract other polar and charged molecules; they are hydrophilic (water-loving).
Nonpolar molecules tend to cluster in nonpolar environments and are hydrophobic (water-fearing).

Hydrogen Bonds

Hydrogen bonds form between a partially positive hydrogen atom (δ+) and a partially negative atom (δ−) such as O or N.
They are individually weak but collectively very strong.
Hydrogen bonds help stabilize the 3D structure of biological molecules.
They account for many properties of water.
Example patterns include O–H···O and N–H···O interactions.

Van der Waals Forces

Weak electrostatic forces between neutral molecules.
arise at very short distances.
Collectively, they help stabilize biological molecules and structures.

Water and Life: Why Water Matters

Water is central to life and is the main component of organisms.
In humans: about 75\% of body weight is water at birth, about 60\% in adulthood.
Living systems can survive only a few days without water.
Water’s unique properties underpin life processes.

Water as a Polar Molecule

A water molecule is polar: the oxygen atom carries a partial negative charge, while the hydrogen atoms carry partial positive charges due to unequal sharing of electrons.
Structural sketch shows a bent shape with two lone pairs on oxygen contributing to polarity.

Hydrogen Bonding Across Water Molecules

Hydrogen bonds create a lattice-like network among water molecules.
These bonds influence several key properties of water: density, specific heat, heat of vaporization, cohesion, adhesion, and surface tension.

Water Density and Ice

Water becomes more structured as it freezes.
Ice has a crystalline lattice with fewer molecules per unit volume than liquid water.
Consequently, ice is less dense than liquid water and can float on it.
This is why ice floats in liquid water.

Specific Heat and Heat of Vaporization

Water has a high specific heat and a high heat of vaporization because hydrogen bonds must be broken before molecules can move freely.
This allows water to absorb or release heat with only moderate changes in temperature.
Water helps stabilize temperatures on Earth and in cells.
Temperature range for liquid water is typically from 0^{\circ}C to 100^{\circ}C under standard conditions: 0^{\circ}C \le T \le 100^{\circ}C.

Cohesion, Surface Tension, and Related Properties

Cohesion: attraction between water molecules of the same kind, largely due to hydrogen bonding.
Surface tension: the surface layer resists rupture under tension because of cohesive forces.

Adhesion and Capillary Action

Adhesion: attraction of water molecules to other substances.
This polarity enables water to climb against gravity in narrow tubes, a phenomenon known as capillary action.

Terminology to Be Familiar With

Atom, Element, Molecule, Compound
Electronegativity, Covalent bond, Ionic bond, Hydrogen bond
Water, Density, Cohesion, Adhesion, Surface tension, Capillary action
Specific heat, Heat of vaporization, Solvent, Solute, pH, Acid, Base, Buffer

After This Lecture, You Should Be Able To (Learning Objectives)

Distinguish between an element and an atom, and between a molecule and a compound.
Answer: What determines the chemical reactivity of an atom?
Explain how an ionic bond forms.
Explain how a covalent bond forms.
Define electronegativity and relate it to nonpolar covalent vs polar covalent bonds.
Explain how the polarity of water contributes to some of its properties.
Explain how hydrogen bonds between water molecules contribute to water’s properties.

For Next Lecture

Reading: Chapter 3 sections 3.1–3.3.
Begin to connect Chapter 2 concepts to Chapter 3 content.

Visual Summary Reference (from Chapter Material)

Protons, Neutrons, and Electrons form Atoms.
Chemistry of life leads to Organic carbon compounds and Water as central themes.
Polar molecules dissociate into ions in water.
Bonds: Ionic vs Covalent with shared or transferred electrons.
Polarity (polar vs nonpolar) affects bonding and properties.
Hydrogen bonds and water’s properties (density, heat capacity, vaporization, cohesion, adhesion, capillary action).
pH: \text{pH} = -\log[H^+]
Acid–Base–Buffer concepts and their relationship to pH.

Notes on Key Equations and Values

Water’s pH relationship: \mathrm{pH} = -\log [H^+]
Water’s temperature range for liquid state: 0^{\circ}C \le T \le 100^{\circ}C
Methane bond angle: 109.5^{\circ}
Water composition examples: 75\% body weight at birth; 60\% in adults
Ion and bond descriptions, including cation/anion concepts and lattice formation in salts.