BackChapter 2: The Chemical Context of Life – Essential Elements and Atomic Structure
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Chapter 2: The Chemical Context of Life
Essential Elements in Biology
Living organisms are composed primarily of a small subset of elements, which are crucial for biological structure and function. These elements are often referred to as the SPONCH elements (Sulfur, Phosphorus, Oxygen, Nitrogen, Carbon, Hydrogen), and together they account for approximately 98% of the body weight in most living things.
Major Elements: Oxygen (O), Carbon (C), Hydrogen (H), Nitrogen (N)
Minor Elements: Calcium (Ca), Phosphorus (P), Potassium (K), Sulfur (S), Sodium (Na), Chlorine (Cl), Magnesium (Mg)
Trace Elements: Elements required in very small amounts (less than 0.01% of body mass), such as Boron (B), Chromium (Cr), Copper (Cu), Fluorine (F), Iodine (I), Iron (Fe), Manganese (Mn), Zinc (Zn), etc.
Element | Symbol | Percentage of Body Mass (including water) |
|---|---|---|
Oxygen | O | 65.0% |
Carbon | C | 18.5% |
Hydrogen | H | 9.5% |
Nitrogen | N | 3.3% |
Calcium | Ca | 1.5% |
Phosphorus | P | 1.0% |
Potassium | K | 0.4% |
Sulfur | S | 0.3% |
Sodium | Na | 0.2% |
Chlorine | Cl | 0.2% |
Magnesium | Mg | 0.1% |
Additional info: Trace elements are essential for specific biochemical functions, such as enzyme activity and hormone production.
Atomic Structure
Atoms are the fundamental units of matter, composed of a nucleus containing protons and neutrons, surrounded by electrons in defined energy levels or shells.
Atomic Number: The number of protons in the nucleus; determines the identity of the element.
Mass Number: The total number of protons and neutrons; gives the atomic mass in Daltons ().
Electrons: Negatively charged particles, equal in number to protons in a neutral atom. Their mass is negligible compared to protons and neutrons.
Example: A helium atom has 2 protons, 2 neutrons, and 2 electrons.
Electron Shells and Energy Levels
Electrons occupy shells (energy levels) around the nucleus. Each shell has a specific energy and can hold a certain number of electrons. The arrangement of electrons determines the chemical properties and reactivity of the atom.
First Shell: Closest to the nucleus, lowest energy, holds up to 2 electrons.
Second Shell: Next highest energy, holds up to 8 electrons.
Third Shell: Higher energy, holds up to 18 electrons.
Potential Energy: Electrons in higher shells have greater potential energy due to their position further from the nucleus.
Electron Transitions: Electrons can move between shells by absorbing or releasing energy (e.g., during photosynthesis).
Analogy: A ball bouncing down stairs represents electrons losing energy as they move to lower shells.
Valence Electrons and Chemical Reactivity
The outermost shell, or valence shell, determines an atom's chemical behavior. Atoms with incomplete valence shells are reactive, while those with full shells are inert.
Octet Rule: Most atoms 'prefer' to have 8 electrons in their valence shell.
Orbitals: Regions of space where electrons are likely to be found; each orbital holds up to 2 electrons.
Example: Neon has a full valence shell and is chemically inert; oxygen has an incomplete valence shell and is reactive.
Additional info: The arrangement of electrons in shells and orbitals is fundamental to understanding chemical bonding and molecular structure.
