Chapter 2: The Chemical Context of Life – Study Notes

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Chapter 2: The Chemical Context of Life

Concept 2.1: Matter Consists of Chemical Elements in Pure Form and in Combinations Called Compounds

All living organisms are composed of matter, which is anything that takes up space and has mass. Matter is made up of elements and compounds, each with unique properties essential for life.

Element: A substance that cannot be broken down to other substances by chemical reactions (e.g., oxygen, carbon).
Compound: A substance consisting of two or more elements in a fixed ratio (e.g., water, H2O; sodium chloride, NaCl).
Compounds have characteristics different from those of their constituent elements.

Example: Sodium (Na, a reactive metal) and chlorine (Cl, a poisonous gas) combine to form sodium chloride (NaCl, table salt), which is edible and safe.

Table 2.1: Elements in the Human Body

Element	Symbol	Percentage of Body Mass (including water)
Oxygen	O	65.0%
Carbon	C	18.5%
Hydrogen	H	9.5%
Nitrogen	N	3.3%
Calcium	Ca	1.5%
Phosphorus	P	1.0%
Potassium	K	0.4%
Sulfur	S	0.3%
Sodium	Na	0.2%
Chlorine	Cl	0.2%
Magnesium	Mg	0.1%

Trace elements (less than 0.01% of mass) are also essential for life, including iron (Fe), zinc (Zn), and others.

Concept 2.2: An Element's Properties Depend on the Structure of Its Atoms

Each element consists of unique atoms, which are the smallest units of matter that retain the properties of the element.

Atom: Composed of subatomic particles: neutrons (no charge), protons (positive charge), and electrons (negative charge).
Neutrons and protons form the atomic nucleus.
Electrons form a "cloud" of negative charge around the nucleus.
Neutron and proton mass are nearly identical and measured in daltons.

Atomic Number and Atomic Mass

Atomic number: Number of protons in the nucleus (defines the element).
Mass number: Sum of protons and neutrons in the nucleus.
Atomic mass: Total mass of the atom, approximately equal to the mass number.

Example: Nitrogen has 7 protons (atomic number 7) and typically 7 neutrons (mass number 14).

Isotopes

Atoms of the same element with different numbers of neutrons are called isotopes.
Radioactive isotopes decay spontaneously, emitting particles and energy (used in dating fossils, medical imaging).

Example: Carbon-12, Carbon-13, and Carbon-14 are isotopes of carbon, differing in neutron number.

Electron Distribution and Chemical Properties

The chemical behavior of an atom is determined by the distribution of electrons in electron shells.
The valence electrons (in the outermost shell) are most important for chemical reactivity.
Elements with a full valence shell are chemically inert (e.g., noble gases).

Example: Helium (He) has a full valence shell and is inert; oxygen (O) has two unpaired electrons and can form two bonds.

Table: Subatomic Particles in Selected Elements

Element	Protons (P)	Neutrons (N)	Electrons (E)	Atomic # (A.#)	Atomic Mass (A.M.)
Hydrogen	1	0	1	1	1
Helium	2	2	2	2	4
Carbon	6	6	6	6	12
Carbon-14	6	8	6	6	14
Nitrogen	7	7	7	7	14
Calcium 2+	20	20	18	20	40

Concept 2.3: The Formation and Function of Molecules Depend on Chemical Bonding Between Atoms

Atoms with incomplete valence shells can share or transfer valence electrons, resulting in chemical bonds that hold atoms together in molecules and compounds.

Covalent Bonds

Covalent bond: The sharing of a pair of valence electrons by two atoms.
A single bond is the sharing of one pair of electrons; a double bond is the sharing of two pairs.
Molecule: Two or more atoms held together by covalent bonds (e.g., H2, O2, H2O).
Structural formula: Shows how atoms are bonded (e.g., H—H).
Molecular formula: Shows the number and type of atoms (e.g., H2).

Example: Methane (CH4) has four single covalent bonds between carbon and hydrogen.

Electronegativity and Bond Polarity

Electronegativity: An atom’s attraction for electrons in a covalent bond.
In a nonpolar covalent bond, electrons are shared equally.
In a polar covalent bond, one atom is more electronegative, causing unequal sharing and partial charges (e.g., H2O).

Example: In water, oxygen is more electronegative than hydrogen, resulting in a partial negative charge on oxygen and partial positive charges on hydrogens.

Ionic Bonds

Atoms may transfer electrons, resulting in charged particles called ions.
Cation: Positively charged ion (e.g., Na+).
Anion: Negatively charged ion (e.g., Cl−).
Ionic bond: Attraction between a cation and an anion (e.g., NaCl).
Ionic compounds (salts): Compounds formed by ionic bonds, often found as crystals in nature.

Example: Sodium donates an electron to chlorine, forming Na+ and Cl−, which combine to form sodium chloride (NaCl).

Hydrogen Bonds

A hydrogen bond forms when a hydrogen atom covalently bonded to one electronegative atom is attracted to another electronegative atom.
Common in water (H2O) and ammonia (NH3), and important for the structure of DNA and proteins.

Example: Hydrogen bonds between water molecules contribute to water’s unique properties, such as high surface tension and boiling point.

Additional info: These foundational chemical principles are essential for understanding the molecular basis of life, including the structure and function of biological macromolecules, cellular processes, and metabolism.