The Chemical Basis of Life: Elements, Atoms, and Water

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The Chemical Basis of Life

Introduction to Matter and Elements

All living organisms are composed of matter, which is made up of chemical elements. Understanding the structure and properties of elements and compounds is fundamental to biology.

Matter is composed of chemical elements.
- About 25 elements are essential for human life.
- Four elements (carbon, hydrogen, oxygen, nitrogen) make up about 96% of the weight of most living organisms.
An element is a substance that cannot be broken down into other substances by chemical means.
A compound is a substance consisting of two or more different elements in a fixed ratio.
Emergent properties: Compounds have properties different from those of their constituent elements.

Atoms: Structure and Properties

An atom is the smallest unit of matter that retains the properties of an element. Atoms are composed of subatomic particles:

Protons (positively charged) and neutrons (neutral) are packed into the nucleus.
Electrons (negatively charged) orbit the nucleus.

The unique number of protons in an atom is its atomic number.
An atom's mass number is the sum of protons and neutrons in the nucleus.
The atomic mass is approximately equal to the mass number.
Isotopes are atoms of the same element with different numbers of neutrons.

Electron Shells and Chemical Bonds

Electrons occupy different energy levels called electron shells. Atoms with incomplete outer shells tend to interact with other atoms, forming chemical bonds:

Chemical bonds are attractions that hold atoms together.
Nonpolar covalent bonds: Electrons are shared equally between atoms.
Polar covalent bonds: Electrons are shared unequally, as in water, where electrons are pulled closer to the more electronegative atom.
Ionic bonds: Formed when one atom transfers electrons to another, resulting in oppositely charged ions that attract each other.
Hydrogen bonds: Weak bonds important in biology, such as those between water molecules.

Chemical Reactions

Chemical reactions involve the breaking and forming of bonds, converting reactants to products. These reactions do not create or destroy matter but rearrange it.

Chemical reaction example: The formation of water from hydrogen and oxygen.

General equation:

Properties of Water

Water's structure and hydrogen bonding give it unique properties essential for life:

Cohesion: Tendency of molecules of the same kind to stick together.
Adhesion: Tendency of different kinds of molecules to stick together.
Surface tension: Measure of how difficult it is to break the surface of a liquid.
Heat capacity: Water absorbs heat when hydrogen bonds break and releases heat when they form, helping to stabilize temperatures.
Evaporative cooling: As water evaporates, the surface cools because the most energetic molecules leave.
Water can exist as a gas, liquid, or solid.
Ice is less dense than liquid water due to stable hydrogen bonds, causing ice to float.

Water as a Solvent

Water's polarity makes it an excellent solvent for polar and charged substances, forming aqueous solutions when these solutes dissolve.

Solution: A liquid consisting of a uniform mixture of two or more substances.
Solvent: The dissolving agent (water in biological systems).
Solute: The substance dissolved.

Summary Table: Types of Chemical Bonds

Bond Type	Description	Example
Nonpolar Covalent	Electrons shared equally	,
Polar Covalent	Electrons shared unequally
Ionic	Electron transfer creates ions
Hydrogen Bond	Weak attraction between polar molecules	Between water molecules

Key Terms and Definitions

Atom: Smallest unit of an element retaining its properties.
Element: Substance that cannot be broken down by chemical means.
Compound: Substance of two or more elements in a fixed ratio.
Isotope: Atoms of the same element with different numbers of neutrons.
Cohesion: Attraction between molecules of the same substance.
Adhesion: Attraction between different substances.
Surface tension: Difficulty of breaking the surface of a liquid.
Evaporative cooling: Cooling of a liquid's surface as molecules with highest energy evaporate.

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