Composition of Matter

Elements: Fundamental Units of Matter

Elements are pure substances that cannot be broken down into simpler substances by ordinary chemical means. They are the building blocks of all matter, including the human body.

• 96% of the human body is composed of four main elements:

  • Oxygen (O): Most abundant; constitutes about 65% of body mass.

  • Carbon (C): Essential for organic molecules.

  • Hydrogen (H): Found in water and most organic compounds.

  • Nitrogen (N): Key component of proteins and nucleic acids.

• The periodic table lists all known elements.

Basic Atomic Subparticles

Atoms: The Building Blocks of Elements

An atom is the smallest unit of an element that retains its chemical properties. Atoms are composed of three types of subatomic particles:

• The mass of an electron is so small it is often considered negligible.

Atomic Neutrality and Ions

• Atoms are electrically neutral when the number of protons equals the number of electrons.

• Positive and negative charges cancel each other out.

• Ions are atoms that have lost or gained electrons, resulting in a net charge.

• Electrons determine an atom’s chemical behavior and bonding properties.

Atomic Models

Planetary and Orbital Models

The structure of the atom can be visualized using different models:

• Planetary Model: Portrays the atom as a miniature solar system with protons and neutrons in the nucleus and electrons orbiting in shells.

• Electron Shells: Electrons are arranged in energy levels (shells) around the nucleus. The arrangement of electrons in these shells determines how atoms interact in chemical reactions.

Molecules and Compounds

Definitions and Examples

• Molecule: Two or more atoms of the same element chemically combined.

• Compound: Two or more atoms of different elements chemically combined.

Example of molecule formation:

• H (atom) + H (atom) → H2 (molecule)

• The reactants are the atoms on the left; the product is the molecule on the right, represented by a molecular formula.

Example of compound formation:

• 4H + C → CH4 (methane)

• Properties of a compound differ from those of its constituent atoms.

Summary Table: Key Atomic Subparticles

Key Terms

• Element: A substance that cannot be broken down into simpler substances by chemical means.

• Atom: The smallest unit of an element, consisting of protons, neutrons, and electrons.

• Molecule: Two or more atoms bonded together.

• Compound: A molecule containing atoms of different elements.

• Ion: An atom or molecule with a net electric charge due to the loss or gain of electrons.

Example: Water Molecule Formation

• 2H2 + O2 → 2H2O

• This reaction shows the formation of water from hydrogen and oxygen gases.

Additional info: Understanding the basic structure of matter is essential for comprehending physiological processes, as all biological functions are rooted in chemical interactions at the atomic and molecular levels.