The Chemical Basis of Life: Matter, Elements, and Atomic Structure

The Chemical Basis of Life

Introduction

This section introduces the foundational chemical principles underlying biological systems, focusing on matter, elements, atoms, and their interactions. Understanding these basics is essential for grasping more complex biological processes.

Matter, Elements, and Atoms

Matter

• Matter is anything that possesses mass and occupies space.

• Matter is tangible and can exist in three primary states:

  • Solid

  • Liquid

  • Gas

• All matter is composed of different elements.

Elements

• An element is a substance that cannot be broken down into simpler substances by ordinary chemical means.

• There are about 100 known elements; 92 occur naturally, and others are synthesized.

• Each element is represented by a unique symbol (e.g., C for carbon, H for hydrogen, Cu for copper).

• Elements can combine to form molecules or compounds:

  • A compound contains atoms of two or more different elements in a fixed ratio (e.g., NaCl).

  • A molecule consists of two or more atoms held together by covalent bonds (e.g., H2, glucose).

Essential Elements and Trace Elements

• About 25 elements are essential for life.

• Four elements—carbon (C), hydrogen (H), nitrogen (N), and oxygen (O)—make up about 96% of living matter.

• Other elements are required in smaller amounts and are called trace elements (e.g., Fe, F, I).

Element	Symbol	Percentage of Body Weight
Oxygen	O	65.0%
Carbon	C	18.5%
Hydrogen	H	9.5%
Nitrogen	N	3.3%
Calcium	Ca	1.5%
Phosphorus	P	1.0%
Other elements	-	1.2%

Additional info: Table values are representative and may vary slightly by source.

Atoms

• An atom is the smallest unit of an element that retains its chemical properties.

• Atoms are extremely small (0.1–0.5 nanometers in diameter).

• Atoms of different elements differ in their structure.

Atomic Structure

Subatomic Particles

• Atoms are composed of three main subatomic particles:

  • Protons: +1 charge, mass ≈ 1 amu

  • Neutrons: no charge, mass ≈ 1 amu

  • Electrons: -1 charge, mass ≪ 1 amu (about 1/2000 amu)

• Protons and neutrons are located in the atomic nucleus.

• Electrons move outside the nucleus in an electron cloud.

Atomic Number and Mass Number

• The atomic number is the number of protons in an atom and defines the element.

• All atoms of a given element have the same atomic number.

• The mass number is the sum of protons and neutrons in the nucleus.

• Electrons contribute negligibly to atomic mass.

Example: Carbon has 6 protons (atomic number 6) and typically 6 neutrons (mass number 12).

Isotopes

• Isotopes are atoms of the same element with different numbers of neutrons (and thus different mass numbers).

• Some isotopes are stable, while others are radioactive (radioisotopes).

• Radioisotopes decay at a constant rate, emitting particles and energy, and can be used in biological research and medicine.

Summary Table: Subatomic Particles

Particle	Charge	Location	Approximate Mass (amu)
Proton	+1	Nucleus	1
Neutron	0	Nucleus	1
Electron	-1	Electron cloud	~0 (1/2000)

Key Concepts and Applications

• Understanding the structure and properties of matter is foundational for all biological sciences.

• Elements and their combinations (molecules and compounds) form the basis of biological molecules.

• Atomic structure determines the chemical behavior of elements, which in turn influences biological processes.