Chapter 2 - bio 1201

Chapter 2: The Chemical Context of Life

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

This section introduces the basic building blocks of matter, focusing on elements and compounds as foundational concepts in biology.

• Element: A substance that cannot be broken down into other substances by chemical means. Examples include oxygen, carbon, and hydrogen.

• Compound: A substance consisting of two or more elements combined in a fixed ratio. Example: Water (H2O) is a compound of hydrogen and oxygen.

• Distinguishing Elements and Compounds: Elements are pure substances; compounds are chemical combinations of elements.

Example: Table salt (NaCl) is a compound made from the elements sodium (Na) and chlorine (Cl).

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

The properties of each element are determined by the structure of its atoms, including the number and arrangement of subatomic particles.

• Atomic Structure: Atoms consist of a nucleus (containing protons and neutrons) and electrons orbiting the nucleus.

• Key Terms:

  • Proton: Positively charged particle in the nucleus.

  • Neutron: Neutral particle in the nucleus.

  • Electron: Negatively charged particle orbiting the nucleus.

  • Atomic Number: Number of protons in an atom; determines the element.

  • Mass Number: Sum of protons and neutrons in the nucleus.

  • Atomic Weight: Average mass of atoms of an element, accounting for isotopes.

• Isotopes: Atoms of the same element with different numbers of neutrons. Example: Carbon-12 and Carbon-14.

• Applications:

  • Radioactive isotopes are used in medical imaging and dating fossils.

  • Atomic number and mass number are used to identify and classify elements.

Example: The atomic number of carbon is 6, meaning every carbon atom has 6 protons.

Additional info: The arrangement of electrons in shells determines chemical reactivity.

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

Chemical bonds are forces that hold atoms together in molecules and compounds, influencing their structure and function.

• Covalent Bond: A chemical bond formed by the sharing of electron pairs between atoms. Example: The bond between two hydrogen atoms in H2.

• Ionic Bond: A bond formed when one atom transfers electrons to another, resulting in oppositely charged ions. Example: The bond in NaCl.

• Polar Covalent Bond: Electrons are shared unequally, creating partial charges. Example: The bonds in water (H2O).

• Nonpolar Covalent Bond: Electrons are shared equally between atoms.

• Hydrogen Bond: A weak bond between a hydrogen atom and an electronegative atom (such as oxygen or nitrogen).

• Van der Waals Interactions: Weak attractions between molecules due to temporary charge differences.

Bond Type	Strength	Example
Covalent	Strong	H2O, O2
Ionic	Strong (in dry conditions)	NaCl
Hydrogen	Weak	Between water molecules
Van der Waals	Very Weak	Between nonpolar molecules

Distinguishing Bonds: Covalent bonds involve electron sharing; ionic bonds involve electron transfer; hydrogen bonds are weak attractions involving hydrogen; van der Waals interactions are transient and weak.

Additional info: Chemical bonding determines molecular shape, which affects biological function.