Elements, Atoms, and Compounds

Key Definitions and Concepts

This section introduces the fundamental chemical concepts essential for understanding biological systems, including the nature of matter, elements, atoms, and compounds.

• Matter: Anything that has mass and takes up space. All living and nonliving things are composed of matter.

• Element: A substance that cannot be chemically broken down into simpler substances. Examples include oxygen, carbon, and hydrogen.

• Trace Element: An element required by organisms in very small quantities, such as iron or zinc.

• Compound: A substance consisting of two or more elements combined in a fixed ratio. For example, water (H2O) is a compound of hydrogen and oxygen.

• Emergent Properties: New characteristics that arise at a higher level of organization, which are not present at the preceding level. For example, table salt (NaCl) has properties different from its constituent elements sodium and chlorine.

Major Elements in the Human Body

The human body is primarily composed of a few key elements. Understanding their relative abundance is crucial for studying physiology and biochemistry.

• Oxygen (O)

• Carbon (C)

• Hydrogen (H)

• Nitrogen (N)

These four elements make up the largest contribution to the human body by percentage of body weight.

Atomic Structure and Subatomic Particles

Subatomic Particles

Atoms are composed of three main types of subatomic particles: protons, neutrons, and electrons. Their properties are summarized below.

Atomic Number and Isotopes

• Atomic Number: The number of protons in the nucleus of an atom, which determines the element's identity.

• Isotopes: Atoms of the same element that have different numbers of neutrons, and thus different mass numbers.

• Radioactive Isotope: An unstable isotope that emits radiation as it decays.

Example Table: Isotopes of Selected Elements

Electron Arrangement and Chemical Bonds

Electron Orbitals

Electrons are found in orbitals that surround the nucleus at distinct energy levels. Each orbital can hold a maximum of two electrons.

• Electrons fill the lowest energy orbitals first.

• The arrangement of electrons determines how atoms interact and bond with each other.

Types of Chemical Bonds

Atoms form chemical bonds to achieve stable electron configurations. The main types of bonds are:

• Ionic Bond: Formed when one atom transfers electrons to another, resulting in oppositely charged ions that attract each other.

• Covalent Bond: Formed when two atoms share one or more pairs of electrons.

• Polar Covalent Bond: A type of covalent bond where electrons are shared unequally, leading to partial charges on the atoms.

• Nonpolar Covalent Bond: Electrons are shared equally between atoms.

• Hydrogen Bond: A weak attraction between a slightly positive hydrogen atom and a slightly negative atom (often oxygen or nitrogen).

Comparison Table: Types of Bonds

Chemical Equations

Chemical reactions are represented by equations showing reactants and products. For example, the balanced equation for cellular respiration is:

Reactants: , Products: ,

Properties of Water

Unique Properties of Water

Water is essential for life due to its unique chemical and physical properties:

• Cohesion: The ability of water molecules to stick together due to hydrogen bonding.

• Adhesion: The ability of water molecules to stick to other substances.

• Surface Tension: The measure of how difficult it is to stretch or break the surface of a liquid.

• Evaporative Cooling: The process by which the surface of an object becomes cooler during evaporation, due to the loss of highly energetic molecules.

• Thermal Energy: The total kinetic energy of molecules in a body of matter.

• Temperature: The average speed of molecules moving within matter.

Water as a Solvent

Water is known as the "universal solvent" because it can dissolve many substances, facilitating chemical reactions in living organisms.

• Solvent: The dissolving agent in a solution.

• Substances that dissolve in water are typically ionic or polar.

Acids, Bases, and pH

pH Scale and Hydrogen Ion Concentration

The pH scale measures the concentration of hydrogen ions () in a solution. It ranges from 0 (most acidic) to 14 (most basic).

• As increases, pH decreases (solution becomes more acidic).

• As decreases, pH increases (solution becomes more basic).

Table: Effects of Acids and Bases

Ocean Acidification

When dissolves in ocean water, it forms carbonic acid, lowering the pH and causing ocean acidification. This process can negatively impact marine life, especially organisms that build shells or skeletons from calcium carbonate.

Relationship Between Carbonate Ion Concentration and Calcification

There is a positive correlation between carbonate ion concentration and the rate of calcification by reef organisms. As carbonate concentration increases, so does the calcification rate.

The Search for Life

Importance of Water

The search for life on other planets often focuses on finding water, as it is essential for all known forms of life.