Chapter 2: The Chemical Basis of Life – Elements, Atoms, and Compounds

Chapter 2: Big Ideas

• Elements, Atoms, and Compounds: Understanding the basic building blocks of matter and how they combine.

• Chemical Bonds: Exploring how atoms interact and form molecules.

• Water's Life-Supporting Properties: Examining the unique characteristics of water that make life possible.

Elements, Atoms, and Compounds

Organisms Are Composed of Elements, Usually Combined into Compounds

All living organisms are made of matter, which consists of chemical elements. These elements combine in specific ways to form compounds essential for life.

• Matter: Anything that occupies space and has mass.

• Chemical Element: A substance that cannot be broken down into other substances by chemical means. About 25 elements are essential for human life.

• Major Elements: Four elements—oxygen (O), carbon (C), hydrogen (H), and nitrogen (N)—make up about 96% of the weight of most living organisms.

• Compound: A substance consisting of two or more different elements combined in a fixed ratio. Compounds have properties different from their constituent elements.

Examples of Compounds

Compounds are formed when elements react chemically. For example, sodium (Na) and chlorine (Cl) combine to form sodium chloride (NaCl), commonly known as table salt.

• Sodium (Na): A highly reactive metal.

• Chlorine (Cl): A poisonous, greenish gas.

• Sodium Chloride (NaCl): An edible compound formed from sodium and chlorine, essential for life.

Element	Symbol	Properties	Compound Formed
Sodium	Na	Soft, reactive metal	Sodium chloride (NaCl)
Chlorine	Cl	Poisonous, greenish gas

Key Terms and Concepts

• Element: Pure substance consisting of only one kind of atom.

• Compound: Substance formed from two or more elements in a fixed ratio.

• Essential Elements: Elements required for an organism to survive, grow, and reproduce.

Application Example

Table salt (NaCl) is a compound formed from sodium and chlorine. While both elements are hazardous in their pure forms, their combination results in a safe and essential nutrient for humans.

Additional info: The concept of elements and compounds is foundational in biology, as it explains the chemical basis of life and the molecular diversity found in living organisms.