Atoms, molecules, and chemical bonds form the foundation of life's chemistry. These tiny building blocks combine in countless ways, creating the diverse substances that make up our world.

Understanding atomic structure and chemical bonding is key to grasping how living things function. From DNA to proteins, these concepts explain how biological molecules form and interact within organisms.

Atomic Structure

Subatomic Particles

• Atoms are the basic units of matter and the defining structure of elements
• Atoms consist of three subatomic particles: protons, neutrons, and electrons
• Protons have a positive charge, are located in the nucleus, and determine the atomic number of an element
• Neutrons have no charge, are located in the nucleus, and contribute to the mass of an atom
• Electrons have a negative charge, occupy the space outside the nucleus, and are involved in chemical bonding
  • Electrons are arranged in energy levels or shells around the nucleus (1s, 2s, 2p, 3s, etc.)
  • The electron configuration of an atom determines its chemical properties and bonding behavior

Atomic Models

• The Bohr model of the atom depicts electrons orbiting the nucleus in fixed circular paths or shells
  • This model is useful for understanding the basic structure of an atom but has limitations
• The quantum mechanical model describes the probability of finding an electron in a certain region around the nucleus
  • This model uses complex mathematical equations to represent the behavior of electrons as wave functions

Chemical Building Blocks

Elements and Atoms

• An element is a pure substance composed of one type of atom
  • Elements are represented by unique chemical symbols (H for hydrogen, O for oxygen, etc.)
  • The periodic table organizes elements based on their atomic number and chemical properties
• Atoms of the same element have the same number of protons but may vary in the number of neutrons (isotopes)
  • Isotopes of an element have different masses but similar chemical properties (carbon-12 and carbon-14)

Molecules and Compounds

• A molecule is a group of two or more atoms held together by chemical bonds
  • Molecules can be composed of the same type of atom (O₂) or different types of atoms (H₂O)
• A compound is a substance made up of two or more different elements chemically combined in a fixed ratio
  • Compounds have properties that differ from those of their constituent elements (NaCl, table salt)
• The chemical formula of a compound indicates the number and type of atoms present (CH₄, methane)

Chemical Bonds

Covalent Bonds

• A covalent bond is a type of chemical bond formed by the sharing of electrons between atoms
• Covalent bonds form when atoms share one or more pairs of valence electrons to achieve a stable electron configuration
  • Single covalent bonds involve the sharing of one pair of electrons (H₂)
  • Double and triple covalent bonds involve the sharing of two or three pairs of electrons, respectively (O₂, N₂)
• Covalent bonds are common in molecules and compounds composed of nonmetals (CH₄, H₂O)

Ionic Bonds

• An ionic bond is a type of chemical bond formed by the electrostatic attraction between oppositely charged ions
• Ionic bonds form when one atom (typically a metal) transfers one or more electrons to another atom (typically a nonmetal)
  • The atom that loses electrons becomes a positively charged cation (Na⁺)
  • The atom that gains electrons becomes a negatively charged anion (Cl⁻)
• Ionic compounds are held together by the strong electrostatic forces between the cations and anions (NaCl, table salt)

Hydrogen Bonds

• A hydrogen bond is a type of intermolecular force that occurs between a hydrogen atom bonded to a highly electronegative atom (N, O, or F) and another electronegative atom
• Hydrogen bonds are weaker than covalent or ionic bonds but play a crucial role in the properties of many substances
  • Hydrogen bonding is responsible for the unique properties of water (high boiling point, surface tension, and cohesion)
  • Hydrogen bonds are important in the structure and function of biological molecules (DNA, proteins)