The Chemical Context of Life

Vocabulary

This section introduces essential terms for understanding the chemical basis of biological systems.

• Element: A substance that cannot be broken down into other substances by chemical means.

• Atom: The smallest unit of an element, retaining its chemical properties.

• Atomic nucleus: The central part of an atom, containing protons and neutrons.

• Proton: A positively charged subatomic particle found in the nucleus.

• Neutron: A neutral subatomic particle found in the nucleus.

• Electron: A negatively charged subatomic particle orbiting the nucleus.

• Atomic number: The number of protons in an atom, unique to each element.

• Mass number: The sum of protons and neutrons in an atom.

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

• Radioisotope: An isotope with an unstable nucleus that emits radiation.

• Energy: The capacity to do work or cause change.

• Electron shell: Regions around the nucleus where electrons are likely to be found.

• Valence electrons: Electrons in the outermost shell, involved in chemical bonding.

• Covalent bond: A chemical bond formed by the sharing of electron pairs between atoms.

• Ionic bond: A bond formed by the transfer of electrons from one atom to another, resulting in oppositely charged ions.

• Hydrogen bond: A weak bond between a hydrogen atom and an electronegative atom.

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

• Molecular formula: Notation showing the types and numbers of atoms in a molecule (e.g., H2O).

• Structural formula: A representation showing the arrangement of atoms in a molecule.

• Single bond, double bond, triple bond: Covalent bonds involving one, two, or three shared pairs of electrons, respectively.

• Electronegativity: The tendency of an atom to attract electrons in a covalent bond.

• Polarity: Distribution of electrical charge over the atoms in a molecule.

• Polar covalent bond: A covalent bond in which electrons are shared unequally.

• Nonpolar covalent bond: A covalent bond in which electrons are shared equally.

• Cation: A positively charged ion.

• Anion: A negatively charged ion.

• Salt: An ionic compound formed from the neutralization of an acid and a base.

• Van der Waals interactions: Weak attractions between molecules or parts of molecules.

• Chemical reaction: The process of making and breaking chemical bonds, leading to changes in the composition of matter.

• Reactants: Substances that start a chemical reaction.

• Products: Substances formed as a result of a chemical reaction.

Objectives

After studying this chapter, students should be able to:

Atoms and Elements

Atoms are the fundamental units of matter, and elements are pure substances composed of only one type of atom.

• Define matter: Anything that has mass and occupies space.

• Define atom and element: An atom is the smallest unit of an element; an element is a substance made of only one kind of atom.

• Use the periodic table: The periodic table organizes elements by atomic number and properties. Atomic number equals the number of protons; atomic mass (mass number) equals protons plus neutrons.

• Major elements in living organisms: Four elements—carbon (C), hydrogen (H), oxygen (O), and nitrogen (N)—make up about 96% of living matter.

• Distinguish between atomic number and atomic mass: Atomic number is the number of protons; atomic mass is the sum of protons and neutrons.

• Isotopes: Atoms of the same element with different numbers of neutrons. Some isotopes are radioactive.

• Distinguish between atoms, ions, and isotopes: Ions are atoms with a net charge due to loss or gain of electrons; isotopes differ in neutron number.

Atomic Structure and Electron Configuration

The arrangement of electrons in shells determines an atom's chemical properties and reactivity.

• Electron shells: Electrons occupy energy levels (shells) around the nucleus. The first shell holds up to 2 electrons; the second and third hold up to 8 each.

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

• Electron configuration: The distribution of electrons among the atom's shells. Atoms are most stable when their valence shell is full.

• Bohr model: A diagram showing electrons in discrete shells around the nucleus.

• Lewis dot method: A representation of valence electrons as dots around the element symbol.

Atoms, Ions, and Molecules

Atoms can gain or lose electrons to form ions, and combine to form molecules through chemical bonds.

• Charged atoms: Atoms that lose electrons become cations (+); those that gain electrons become anions (−).

• Neutral atoms: Atoms with equal numbers of protons and electrons.

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

• Compounds: Substances composed of two or more different elements combined in fixed ratios.

• Molecular formula vs. structural formula: Molecular formula shows the number and type of atoms (e.g., H2O); structural formula shows the arrangement of atoms.

Chemical Bonds

Chemical bonds hold atoms together in molecules and compounds. The main types are covalent, ionic, and hydrogen bonds.

• Covalent bond: Atoms share electron pairs. Can be single, double, or triple bonds depending on the number of shared pairs.

• Single, double, triple bonds: Single bond shares one pair (), double shares two pairs (), triple shares three pairs ().

• Ionic bond: Formed when one atom transfers electrons to another, creating oppositely charged ions (e.g., Na+ and Cl− form NaCl).

• Hydrogen bond: Weak attraction between a hydrogen atom and an electronegative atom (e.g., between water molecules).

• Van der Waals interactions: Weak attractions due to transient local charges.

Electronegativity and Polarity

Electronegativity determines how strongly an atom attracts electrons in a bond, affecting molecular polarity.

• Electronegativity: The tendency of an atom to attract electrons. Higher electronegativity means stronger attraction.

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

• Nonpolar covalent bond: Electrons are shared equally (e.g., O2).

• Polar molecules: Molecules with uneven charge distribution.

• Nonpolar molecules: Molecules with even charge distribution.

Chemical Reactions and Equilibrium

Chemical reactions involve the making and breaking of bonds, transforming reactants into products. Some reactions are reversible and reach equilibrium.

• Chemical reaction: The process of rearranging atoms to form new substances.

• Reactants and products: Reactants are starting materials; products are formed substances.

• Molecular weight: The sum of atomic masses of all atoms in a molecule.

• Chemical equilibrium: The point at which the rate of the forward reaction equals the rate of the reverse reaction.

• Reversible reaction: A reaction that can proceed in both directions.

Summary Table: Types of Chemical Bonds

This table compares the main types of chemical bonds found in biological molecules.

Key Equations

• Atomic mass:

• Molecular weight:

Examples and Applications

• Water molecule (H2O): Has polar covalent bonds and forms hydrogen bonds with other water molecules, leading to unique properties like cohesion and high specific heat.

• Salt (NaCl): Formed by ionic bonding between sodium and chlorine atoms.

• Oxygen molecule (O2): Nonpolar covalent bond, electrons shared equally.

Additional info: This guide expands on the objectives and vocabulary by providing definitions, examples, and context for each concept, ensuring a comprehensive understanding suitable for General Biology students.