Chapter 2: The Chemical Context of Life

Concept 2.1: Matter and Elements

Matter is anything that has mass and occupies space. All living organisms are composed of matter, which is made up of chemical elements. Elements are substances that cannot be broken down into other substances by chemical reactions. Compounds are substances consisting of two or more elements combined in a fixed ratio.

• Element: A pure substance consisting of only one type of atom.

• Compound: A substance formed from two or more elements in a fixed ratio (e.g., H2O).

• Major elements in living matter: Carbon (C), Hydrogen (H), Oxygen (O), Nitrogen (N), Calcium (Ca), and Phosphorus (P) make up 99% of living matter.

• Trace elements: Elements required by an organism in only minute quantities.

Concept 2.2: Atomic Structure and Properties

An atom is the smallest unit of matter that retains the properties of an element. Atoms are composed of subatomic particles: protons, neutrons, and electrons.

• Proton: Positively charged particle found in the nucleus; determines the element.

• Neutron: Neutral particle found in the nucleus; determines isotope.

• Electron: Negatively charged particle found in electron shells around the nucleus; determines chemical behavior.

Atomic Number, Mass Number, and Atomic Mass

• Atomic number: Number of protons in the nucleus (unique to each element).

• Mass number: Sum of protons and neutrons in the nucleus (always a whole number).

• Atomic mass: Average mass of all isotopes of an element, weighted by their natural abundance (may have decimals).

Isotopes and Radioactivity

• Isotopes: Atoms of the same element with different numbers of neutrons.

• Radioactive isotopes: Unstable isotopes that decay spontaneously, emitting particles and energy.

• Half-life: The time required for half of the radioactive atoms in a sample to decay.

Energy Levels and Electron Shells

Electrons have potential energy due to their position relative to the nucleus. Electrons are arranged in shells around the nucleus, with each shell holding a specific maximum number of electrons.

• First shell: up to 2 electrons

• Second and third shells: up to 8 electrons each

• Fourth shell: up to 18 electrons

Electron Distribution and Chemical Properties

• Valence electrons: Electrons in the outermost shell; determine chemical behavior.

• Atoms with full valence shells are chemically inert (e.g., noble gases).

The Periodic Table

The periodic table organizes elements by increasing atomic number and groups elements with similar chemical properties together.

Concept 2.3: Chemical Bonds and Molecular Structure

Atoms with incomplete valence shells can share or transfer electrons, resulting in chemical bonds. The main types of chemical bonds are covalent, ionic, and hydrogen bonds.

Covalent Bonds

• Covalent bond: Sharing of a pair of valence electrons between two atoms.

• Single bond: Sharing of one pair of electrons (e.g., H—H).

• Double bond: Sharing of two pairs of electrons (e.g., O═O).

• Molecule: Two or more atoms held together by covalent bonds.

• Valence: Bonding capacity of an atom (number of covalent bonds it can form).

Electronegativity and Bond Polarity

• Electronegativity: Atom's ability to attract electrons in a covalent bond.

• Electronegativity increases from left to right and bottom to top on the periodic table.

• Nonpolar covalent bond: Electrons are shared equally.

• Polar covalent bond: Electrons are shared unequally, resulting in partial charges (e.g., H2O).

Ionic Bonds

• Ionic bond: Attraction between oppositely charged ions formed by the transfer of electrons.

• Cation: Positively charged ion (lost electrons).

• Anion: Negatively charged ion (gained electrons).

• Ionic compound (salt): Compound formed by ionic bonds, often forming crystalline structures (e.g., NaCl).

Weak Chemical Interactions

• Hydrogen bond: Weak attraction between a hydrogen atom covalently bonded to an electronegative atom and another electronegative atom (commonly O or N).

• Van der Waals interactions: Weak attractions due to transient local partial charges; important in large biological molecules.

Molecular Shape and Function

The shape of a molecule is determined by the positions of its atoms' orbitals and is crucial for its function. Molecular shape determines how biological molecules recognize and interact with each other (e.g., hormone-receptor binding).

Concept 2.4: Chemical Reactions

Chemical reactions involve the making and breaking of chemical bonds. The starting substances are called reactants, and the resulting substances are called products.

• Chemical equation: Symbolic representation of a chemical reaction (e.g., ).

• Photosynthesis: Example of a chemical reaction powered by sunlight, converting carbon dioxide and water into glucose and oxygen.

• Reversibility: All chemical reactions are reversible; equilibrium is reached when forward and reverse reactions occur at the same rate.

Chemical Equilibrium

• Chemical equilibrium: State in which the rate of the forward reaction equals the rate of the reverse reaction; concentrations of reactants and products remain constant.

• Example: