CONCEPT 2.1: Matter consists of chemical elements in pure form and in combinations called compounds

Organisms are composed of matter

Matter is anything that takes up space and has mass. All living and nonliving things are composed of matter, which can exist in various forms such as solids, liquids, and gases.

• Element: A substance that cannot be broken down to other substances by chemical reactions.

• Compound: A substance consisting of two or more elements in a fixed ratio. Compounds have emergent properties different from their constituent elements.

• Example: Sodium (Na) and chlorine (Cl) combine to form sodium chloride (NaCl), which has properties distinct from either element alone.

The Elements of Life

Of the 92 naturally occurring elements, about 20–25% are essential for life. Four elements—carbon, hydrogen, oxygen, and nitrogen—make up 96% of living matter. The remaining 4% consists mainly of calcium, phosphorus, potassium, and sulfur. Trace elements are required in minute quantities but are vital for proper biological function.

Case Study: Evolution of Tolerance to Toxic Elements

Some elements can be toxic to organisms. Certain species adapt to environments containing toxic elements, such as plant communities thriving on serpentine soils, which contain high levels of heavy metals.

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CONCEPT 2.2: An element’s properties depend on the structure of its atoms

Atoms and Subatomic Particles

An atom is the smallest unit of matter that retains the properties of an element. Atoms are composed of three types of subatomic particles:

• Protons: Positively charged; determine the element.

• Neutrons: No charge; determine isotope.

• Electrons: Negatively charged; determine chemical behavior.

Neutrons and protons form the atomic nucleus, while electrons form a cloud around the nucleus. The mass of protons and neutrons is nearly identical and measured in daltons; electrons are much lighter and typically ignored in mass calculations.

Atomic Number and Atomic Mass

• Atomic number: Number of protons in the nucleus.

• Mass number: Sum of protons and neutrons.

• Atomic mass: Total mass of the atom, approximated by the mass number.

Isotopes

Isotopes are atoms of the same element with different numbers of neutrons. Some isotopes are radioactive and decay spontaneously, emitting particles and energy.

Radioactive Tracers

Radioactive isotopes are used in medicine as diagnostic tools. Radioactive tracers can track atoms through metabolism and are used with imaging instruments such as PET scanners to monitor cancer growth and metabolism.

Radiometric Dating

Radiometric dating uses the decay of radioactive isotopes to estimate the age of fossils and rocks. The rate of decay is expressed as the half-life, which varies among isotopes.

The Energy Levels of Electrons

Energy is the capacity to cause change. Potential energy is the energy matter possesses due to its location or structure. Electrons have different potential energies based on their distance from the nucleus and are found in electron shells with characteristic energy levels.

Electron Distribution and Chemical Properties

The chemical behavior of an atom is determined by the distribution of electrons in its shells, especially the valence electrons in the outermost shell. Elements with a full valence shell are chemically inert.

Electron Orbitals

An orbital is a three-dimensional space where an electron is found 90% of the time. Each shell contains a specific number of orbitals, and no more than two electrons can occupy a single orbital. Atoms interact to complete their valence shells.

CONCEPT 2.3: The formation and function of molecules and ionic compounds depend on chemical bonding between atoms

Chemical Bonds

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

Covalent Bonds

A covalent bond involves the sharing of a pair of valence electrons between two atoms. Covalent bonds can be single (one pair shared) or double (two pairs shared). The structural formula represents the arrangement of atoms and bonds.

• Single bond: H—H

• Double bond: O═O

Bonding capacity is called valence. Covalent bonds can form between atoms of the same or different elements. Electronegativity describes an atom’s attraction for electrons in a covalent bond.

Polar and Nonpolar Covalent Bonds

In nonpolar covalent bonds, electrons are shared equally. In polar covalent bonds, one atom is more electronegative, resulting in unequal sharing and partial charges.

Ionic Bonds

Ionic bonds form when atoms transfer electrons, creating oppositely charged ions (cations and anions) that attract each other. Ionic compounds, or salts, are stable when dry but dissociate easily in water.

Weak Chemical Interactions

Weak bonds, such as hydrogen bonds and van der Waals interactions, are important for maintaining the structure and function of biological molecules. Their reversibility can be advantageous.

Hydrogen Bonds

A hydrogen bond forms when a hydrogen atom covalently bonded to an electronegative atom is attracted to another electronegative atom, usually oxygen or nitrogen.

Van der Waals Interactions

Van der Waals interactions are weak attractions between molecules due to transient local partial charges. Collectively, these interactions can be strong, as seen in gecko toe hairs adhering to surfaces.

Molecular Shape and Function

The shape of a molecule is determined by the positions of its atoms’ orbitals and is crucial for its function. Molecules with similar shapes can bind the same biological receptors, as seen with morphine and endorphins.

CONCEPT 2.4: Chemical reactions make and break chemical bonds

Chemical Reactions

Chemical reactions involve the making and breaking of chemical bonds. Reactants are the starting molecules, and products are the resulting molecules. All chemical reactions are reversible, and equilibrium is reached when the forward and reverse reactions occur at the same rate.

Photosynthesis

Photosynthesis is a key chemical reaction in biology, converting carbon dioxide and water into glucose and oxygen using sunlight. The balanced equation is:

Chemical Equilibrium

At equilibrium, the concentrations of reactants and products remain constant because the forward and reverse reactions occur at the same rate. The equation for a reversible reaction is: