Elements, Atoms, and Compounds

Overview of Chapter 2

This chapter introduces the chemical basis of life, focusing on elements, atoms, compounds, and the properties of water that support life. Understanding these concepts is essential for studying biological processes at the molecular level.

Elements and Matter in Living Organisms

Composition of Living Organisms

All living organisms are composed of matter, which itself is made up 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.

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

• Trace elements (<4%): Elements required in minute quantities, but essential for health (e.g., iron, iodine, fluoride).

Major and Trace Elements in the Human Body

The human body contains both major and trace elements, each with specific biological roles.

Role of Trace Elements

• Some trace elements are required to prevent disease (e.g., iodine for thyroid function, fluoride for dental health).

• Chemicals may be added to food to preserve it, enhance nutrition, or improve appearance.

Atoms: Structure and Properties

Subatomic Particles

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

• Protons: Positively charged particles located in the nucleus.

• Neutrons: Electrically neutral particles also found in the nucleus.

• Electrons: Negatively charged particles that orbit the nucleus.

The number of protons in the nucleus defines the element's atomic number.

Atomic Number, Mass Number, and Isotopes

• Atomic number: Number of protons in an atom's nucleus.

• Mass number: Sum of protons and neutrons in the nucleus.

• Atomic mass: Approximately equal to the mass number (measured in daltons).

• Isotopes: Atoms of the same element with different numbers of neutrons, thus different mass numbers.

Example: Nitrogen Isotopes

• A nitrogen atom has 7 protons. The most common isotope has 7 neutrons (mass number = 14). A radioactive isotope may have 9 neutrons (mass number = 16).

Table: Isotopes of Carbon

Radioactive Isotopes

• Radioactive isotopes are unstable and decay over time, emitting radiation.

• They are useful as tracers in biological research and medical imaging, but can also pose health risks.

Chemical Bonds and Compounds

Electron Arrangement and Chemical Properties

The distribution of electrons in an atom determines its chemical behavior. Electrons occupy shells around the nucleus, and atoms with incomplete outer shells tend to form chemical bonds.

• Chemical bond: An attraction between atoms that enables the formation of chemical compounds.

Types of Chemical Bonds

• Ionic bonds: Formed by the transfer of electrons from one atom to another, resulting in oppositely charged ions that attract each other.

• Covalent bonds: Formed when atoms share electrons. Can be single, double, or triple bonds depending on the number of shared electron pairs.

• Hydrogen bonds: Weak attractions between a hydrogen atom in one molecule and an electronegative atom (like oxygen or nitrogen) in another molecule.

Example: Water Molecule

• Water (H2O) is formed by covalent bonds between hydrogen and oxygen.

• The bonds are polar covalent, with electrons shared unequally, making water a polar molecule.

Polarity and Hydrogen Bonds

• Polarity: Separation of electrical charges within a molecule.

• In water, oxygen is more electronegative than hydrogen, resulting in partial negative and positive charges.

• Hydrogen bonds form between the slightly positive hydrogen of one water molecule and the slightly negative oxygen of another.

Ionic Compounds

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

• Oppositely charged ions attract to form ionic compounds (e.g., sodium chloride, NaCl).

Chemical Reactions

Making and Breaking Bonds

Chemical reactions involve the breaking and forming of chemical bonds, resulting in the transformation of reactants into products.

• Chemical reactions do not create or destroy matter; they rearrange it.

Example Equation

Formation of water:

Properties of Water Essential for Life

Cohesion, Adhesion, and Surface Tension

• Cohesion: Tendency of molecules of the same kind to stick together (important for water transport in plants).

• Adhesion: Tendency of different substances to cling to each other.

• Surface tension: Measure of how difficult it is to break the surface of a liquid.

Temperature Moderation

• Water absorbs and releases heat slowly due to hydrogen bonding, helping moderate Earth's climate and organismal temperature.

• Evaporative cooling: As water evaporates, the surface cools down.

Density of Ice

• Water is less dense as a solid (ice) than as a liquid, so ice floats.

• Hydrogen bonds stabilize and keep water molecules apart in ice.

Water as a Solvent

• Solution: Uniform mixture of two or more substances.

• Solvent: Dissolving agent (water is the universal solvent).

• Polar or charged solutes dissolve easily in water, forming aqueous solutions (e.g., blood, cytoplasm).

Acids, Bases, and pH

pH Scale and Buffers

• Water can dissociate into hydrogen ions (H+) and hydroxide ions (OH-).

• pH scale: Measures how acidic or basic a solution is (ranges from 0 to 14).

• Buffer: Substance that minimizes changes in pH.

pH Table (Selected Examples)

Water and the Search for Life

Importance of Water in Biology and Astrobiology

• The unique properties of water support life on Earth and are considered essential in the search for extraterrestrial life.

Summary: Key Concepts to Master

• Importance of chemical elements to living organisms

• Formation and properties of compounds

• Structure of atoms and subatomic particles

• Types of chemical bonds: ionic, covalent, hydrogen

• Nature of chemical reactions

• Life-supporting properties of water

• Understanding the pH scale, acids, bases, and buffers

Additional info: Expanded explanations and context have been added to ensure completeness and clarity for exam preparation.