Atoms, Ions, and Molecules: Foundations of Anatomy & Physiology

2.1 Atoms, Ions, and Molecules

Introduction

This section introduces the basic chemical principles essential for understanding the structure and function of the human body. It covers the elements present in the body, the nature of atoms and ions, and the types of chemical bonds that form the basis of biological molecules.

The Chemical Elements

• Element: The simplest form of matter with unique chemical properties. Each element is defined by its atomic number, which is the number of protons in its nucleus.

• Periodic Table: Elements are arranged by atomic number and represented by one- or two-letter symbols.

• Biological Elements: 24 elements have a biological role. Six elements (oxygen, carbon, hydrogen, nitrogen, calcium, and phosphorus) make up 98.5% of body weight. Trace elements are present in minute amounts but are vital for health.

Minerals and Electrolytes

• Minerals: Inorganic elements extracted from soil by plants and passed up the food chain. Examples include Ca, P, Cl, Mg, K, Na, and S. They constitute about 4% of body weight and are important for body structure (e.g., calcium in bones and teeth) and enzyme function.

• Electrolytes: Mineral salts necessary for nerve and muscle function. They dissociate in water to form ions that conduct electricity.

Atomic Structure

• Planetary Model: Proposed by Niels Bohr, this model describes the atom as a nucleus (center) containing protons and neutrons, with electrons in concentric clouds (energy levels) around the nucleus.

• Protons: Single positive charge; mass = 1 atomic mass unit (amu).

• Neutrons: No charge; mass = 1 amu.

• Electrons: Single negative charge; very low mass. The number of electrons equals the number of protons in a neutral atom.

• Valence Electrons: Electrons in the outermost shell, determining chemical bonding properties.

Bohr Planetary Models of Elements

• Visual representations show the arrangement of protons, neutrons, and electrons for elements such as carbon, nitrogen, and sodium.

• Key: Protons (red), Neutrons (gray), Electrons (blue).

• Example: Carbon (C) has 6 protons, 6 neutrons, and 6 electrons.

Isotopes and Radioactivity

• Isotopes: Varieties of an element that differ only in the number of neutrons. Extra neutrons increase atomic weight but do not affect chemical properties because the number of valence electrons remains the same.

• Atomic Weight (Relative Atomic Mass): The average mass of all isotopes of an element, accounting for their relative abundance.

• Radioisotopes: Unstable isotopes that decay and emit radiation. Every element has at least one radioisotope.

• Ionizing Radiation: High-energy radiation (e.g., UV, X-rays, alpha, beta, gamma rays) that can eject electrons, destroy molecules, create free radicals, and cause genetic mutations or cancer.

• Physical Half-life: Time required for 50% of a radioisotope to decay to a stable state.

• Biological Half-life: Time required for 50% of a radioisotope to disappear from the body.

Ions, Electrolytes, and Free Radicals

• Ion: A charged particle (atom or molecule) with an unequal number of protons and electrons.

• Ionization: The process of transferring electrons from one atom to another.

• Anion: A particle that gains electron(s), resulting in a net negative charge.

• Cation: A particle that loses electron(s), resulting in a net positive charge.

• Ions with opposite charges are attracted to each other, forming ionic bonds.

Additional info: Free radicals are highly reactive atoms or molecules with unpaired electrons. They can cause cellular damage and are implicated in aging and diseases.