Chemical Context of Life: Key Concepts and Vocabulary

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Chapter 2: The Chemical Context of Life

Atoms and Elements

The study of biology requires an understanding of the chemical basis of life, beginning with atoms and elements. These fundamental units form the building blocks of all matter.

Atom: The smallest unit of matter that retains the properties of an element.
Element: A substance that cannot be broken down into other substances by chemical means. Examples include carbon, oxygen, and hydrogen.
Atomic number: The number of protons in the nucleus of an atom, which defines the element.
Isotope: Atoms of the same element with different numbers of neutrons, resulting in different mass numbers.
Example: Carbon-12 and Carbon-14 are isotopes of carbon.

Structure of Atoms

Atoms consist of subatomic particles: protons, neutrons, and electrons. Their arrangement determines the atom's properties and behavior.

Proton: Positively charged particle found in the nucleus.
Neutron: Neutral particle found in the nucleus.
Electron: Negatively charged particle orbiting the nucleus.
Electrical charge: Protons (+), electrons (−), neutrons (0).
Orbital shell: Regions around the nucleus where electrons are likely to be found.
Valence electrons: Electrons in the outermost shell, important for chemical bonding.

Elements Essential for Life

Living organisms are composed mainly of a few key elements, with others present in smaller amounts.

Major elements: Carbon (C), hydrogen (H), oxygen (O), nitrogen (N).
Other important elements: Sulfur (S), phosphorus (P), calcium (Ca), iron (Fe), magnesium (Mg).
Example: Proteins contain carbon, hydrogen, oxygen, nitrogen, and sometimes sulfur.

Chemical Bonds and Molecules

Atoms combine to form molecules through chemical bonds, which can be classified by their strength and properties.

Covalent bond: A bond formed when two atoms share one or more pairs of electrons.
Polar covalent bond: Electrons are shared unequally, resulting in partial charges.
Nonpolar covalent bond: Electrons are shared equally between atoms.
Ionic bond: Formed when one atom donates an electron to another, creating ions.
Hydrogen bond: Weak attraction between a hydrogen atom and an electronegative atom (e.g., oxygen or nitrogen).
Double covalent bond: Two pairs of electrons are shared between atoms.
Macromolecule: Large molecules such as proteins, nucleic acids, carbohydrates, and lipids.
Molecular formula: Shows the types and numbers of atoms in a molecule (e.g., for water).
Partial charges: Result from unequal sharing of electrons in polar covalent bonds.
Example: Water () is a polar molecule with partial positive and negative charges.

Ions and Ionic Compounds

Ions are charged particles formed when atoms gain or lose electrons. Ionic compounds are formed from the attraction between oppositely charged ions.

Cation: Positively charged ion (loss of electrons).
Anion: Negatively charged ion (gain of electrons).
Example: Sodium ion () and chloride ion () form sodium chloride ().

Properties of Water

Water is essential for life due to its unique chemical and physical properties, many of which arise from hydrogen bonding.

Cohesion: Attraction between water molecules, leading to surface tension.
Adhesion: Attraction between water molecules and other substances.
Heat of vaporization: Energy required to convert water from liquid to gas.
High specific heat: Water resists changes in temperature.
Density of ice vs. water: Ice is less dense than liquid water due to hydrogen bonding.
Hydrophilic: Substances that interact well with water.
Hydrophobic: Substances that do not interact well with water.
Example: Water droplets on a leaf demonstrate cohesion and adhesion.

Solutions, Solutes, and Electrolytes

Solutions are mixtures of solutes dissolved in solvents. Electrolytes are substances that dissociate into ions in water.

Solute: Substance dissolved in a solution.
Solvent: Substance that dissolves the solute (water is the universal solvent).
Solution: Homogeneous mixture of solute and solvent.
Electrolyte: Substance that produces ions when dissolved in water, conducting electricity.
Example: Table salt () is an electrolyte in water.

pH, Acids, and Bases

The pH scale measures the concentration of hydrogen ions in a solution, indicating its acidity or basicity.

pH: Scale from 0 to 14; lower values are acidic, higher values are basic.
Acid: Substance that increases hydrogen ion concentration () in solution.
Base: Substance that decreases hydrogen ion concentration, often by producing hydroxide ions ().
Formula:
Example: Lemon juice is acidic (low pH), while soap is basic (high pH).

Key Vocabulary Table

The following table summarizes important vocabulary terms for this chapter.

Term	Definition
Atom	Smallest unit of an element retaining its properties
Element	Pure substance consisting of one type of atom
Isotope	Atoms of the same element with different numbers of neutrons
Covalent bond	Bond formed by sharing electrons between atoms
Ionic bond	Bond formed by transfer of electrons, creating ions
Hydrogen bond	Weak attraction between a hydrogen atom and an electronegative atom
Solute	Substance dissolved in a solvent
Solvent	Substance that dissolves the solute
Electrolyte	Substance that dissociates into ions in solution
pH	Measure of hydrogen ion concentration
Valence electron	Electron in the outermost shell of an atom

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