Chapter 2

Matter is composed of chemical elements; about $25\%$ of elements are essential for human life; the four elements make up about $96\%$ of the weight of most living organisms.
A compound is a substance consisting of two or more different elements in a fixed ratio.
Emergent properties: table salt (NaCl) illustrates properties that arise from the combination of elements and are not predicted from the elements alone; NaCl is an ionic compound formed from Na and Cl.
Major elements by weight in organisms include:
- $\mathrm{O}=65\%\quad \mathrm{C}=18.5\%\quad \mathrm{H}=9.5\%\quad \mathrm{N}=3.3\%.$

Some trace elements are required to prevent disease.
Fluoride is added to municipal water and dental products to reduce tooth decay.
Chemicals are added to food to preserve, enhance nutrition, or improve appearance.
Checkpoint concept: upper intake levels set to avoid adverse effects (example from cereal iron fortification).

Each element consists of one kind of atom; an atom is the smallest unit of matter that retains the element’s properties.
Subatomic particles:
- Protons and neutrons in the nucleus
- Electrons orbit the nucleus
Atomic number $Z$ equals the number of protons.
Mass number $A=Z+N{n}$ (where $N{n}$ is the number of neutrons).
Atomic mass ~ mass number.
Isotopes: same $Z$ , different numbers of neutrons.
Example relationships:
- Nitrogen, common isotope: $^{14}_{7}N$ (7 protons, 7 neutrons)

Radioactive isotopes are useful as tracers in monitoring chemical fates; detected by imaging instruments.
They also pose safety concerns in handling.
Checkpoint concept: tracers exploit radioactive decay signals.

Electrons are arranged in electron shells at characteristic distances from the nucleus.
Outer shell not full → atoms interact with other atoms to form bonds (attractions).
Ionic bond: transfer of an electron between atoms, creating ions that attract.
Covalent bond: atoms share electrons rather than transfer them.
Checkpoint concept: covalent bonds and electron sharing/transfer underlie bond formation.

Nonpolar covalent bond: electrons shared equally.
Polar covalent bond: electrons pulled toward more electronegative atom (as in water).
Common molecules show different bonding: H2, O2, CH4, H2O.
Valence (typical maximum bonds per element):
- $\text{Valence}(H)=1\quad \text{Valence}(O)=2\quad \text{Valence}(N)=3\quad \text{Valence}(C)=4.$
Water’s polarity arises from polar covalent bonds between H and O.

An ion is an atom or molecule with a net electrical charge due to gain/loss of electrons.
Oppositely charged ions attract to form an ionic bond; table salt is NaCl.

Hydrogen bonds are a key weak interaction.
Water is a polar molecule with hydrogen bonds between molecules.
Hydrogen bonds contribute to water’s properties and structure.
Checkpoint concept: neighboring water molecules hydrogen-bond via polar covalent bonds.

Chemical reactions rearrange atoms by breaking and forming bonds.
Matter is conserved; reactions transform reactants to products.
Example: formation of water from hydrogen and oxygen:
$2\,\mathrm{H2} + \mathrm{O2} \rightarrow 2\,\mathrm{H_2O}.$

Water exists as gas, liquid, solid.
Ice is less dense than liquid water due to hydrogen-bonding patterns in the solid.
As ice forms, molecules are less densely packed, causing ice to float.
Checkpoint concept: freezing water can crack rocks via ice expansion.

A solution is a uniform mixture of substances.
Water’s polarity makes it an excellent solvent; many solutes dissolve to form aqueous solutions.
Checkpoint concept: blood and most body fluids are aqueous solutions.

In water, a small fraction dissociates into ions.
The pH scale describes acidity/basicity.
A buffer minimizes pH changes.
Relationship: a solution with lower pH is more acidic (more H+).
Example concept: 10^x relationships between H+ concentration and pH (e.g., each unit change is a 10-fold change).
Basic reference: neutral is pH 7.

CO2 from fossil fuels is rising in the atmosphere and ocean.
About $25\%$ of human-generated CO2 is absorbed by oceans, lowering seawater pH (ocean acidification).
Acidification: extra H+ combines with carbonate ions to form bicarbonate, reducing carbonate available for calcification.
In controlled experiments, lower carbonate ion concentrations reduce calcification rates in reef organisms.
Reaction: $\mathrm{CO3^{2-}} + \mathrm{H^+} \rightarrow \mathrm{HCO3^-}.$

Emergent properties of water support life on Earth and may influence life elsewhere.
Checkpoint concept: presence of water is a key factor in assessing potential for life.

Describe the importance of chemical elements to living organisms.
Explain the formation of compounds.
Describe the structure of an atom.
Distinguish between ionic, hydrogen, and covalent bonds.
Define a chemical reaction and explain how it changes the composition of matter.
List and define the life-supporting properties of water.
Explain the pH scale and the formation of acid and base solutions.
Explain how rising CO2 levels affect coral reefs.

Major elements by weight (example):
$\mathrm{O}=65\%\quad \mathrm{C}=18.5\%\quad \mathrm{H}=9.5\%\quad \mathrm{N}=3.3\%.$
Water formation: $2\,\mathrm{H2} + \mathrm{O2} \rightarrow 2\,\mathrm{H_2O}.$
Ocean acidification reaction: $\mathrm{CO3^{2-}} + \mathrm{H^+} \rightarrow \mathrm{HCO3^-}.$
pH neutral point: $\mathrm{pH}=7 \quad \text{(neutral)}.$
Valence (typical bonding capacity): $\text{H}=1,\; \text{O}=2,\; \text{N}=3,\; \text{C}=4.$