Water and Carbon: The Chemical Basis of Life (Chapter 2 Study Notes)

Atoms, Ions, and Molecules: The Building Blocks of Life

Overview of Atomic Composition in Living Organisms

Living organisms are primarily composed of a small subset of elements, which form the foundation for biological molecules and processes.

• Key Elements: Hydrogen (H), Carbon (C), Nitrogen (N), and Oxygen (O) make up approximately 96% of the matter in organisms.

• Complexity from Simplicity: These elements combine to form the complex molecules necessary for life through chemical evolution.

• Building Blocks: Simple molecules such as water (H2O), carbon dioxide (CO2), and ammonia (NH3) serve as the starting points for more complex biological structures.

Structure of Atoms

Parts of an Atom

Atoms are the fundamental units of matter, each consisting of a nucleus surrounded by electrons.

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

• Atomic Structure:

  • Nucleus: Contains protons (positive charge, +1) and neutrons (neutral charge).

  • Electrons: Negatively charged particles (-1) that orbit the nucleus.

• Atoms are electrically neutral when the number of protons equals the number of electrons.

Atomic Number and Mass Number

Atoms are identified and classified by their atomic number and mass number.

• Atomic Number (Z): The number of protons in the nucleus of an atom. Determines the element's identity.

• Mass Number (A): The sum of protons and neutrons in the nucleus.

Example: Carbon has an atomic number of 6 (6 protons) and commonly a mass number of 12 (6 protons + 6 neutrons).

Isotopes

Isotopes are forms of the same element that differ in the number of neutrons.

• Definition: Atoms of an element with the same number of protons but different numbers of neutrons.

• Mass Difference: Isotopes have different mass numbers.

• Stability: Some isotopes are stable, while others are radioactive and decay over time.

Example: Carbon-12 (6 protons, 6 neutrons) and Carbon-14 (6 protons, 8 neutrons).

The Periodic Table and Atomic Structure

Understanding the Periodic Table

The periodic table organizes elements by increasing atomic number and reveals patterns in their chemical properties.

• Key Elements in Biology: H, C, N, O, P, and S make up over 99% of atoms in most organisms.

• Valence Electrons: The number of unpaired electrons in the outermost shell (valence shell) determines the number of bonds an atom can form.

Element	Atomic Number	Common Mass Number	Valence Electrons
Hydrogen (H)	1	1	1
Carbon (C)	6	12	4
Nitrogen (N)	7	14	3
Oxygen (O)	8	16	2
Phosphorus (P)	15	31	5
Sulfur (S)	16	32	2

Additional info: Table entries for valence electrons are inferred based on standard electron configurations.