Concept 2.1: Matter Consists of Chemical Elements in Pure Form and in Combinations Called Compounds

Definitions of Matter, Element, and Compound

Understanding the basic building blocks of matter is essential in biology. Matter is anything that has mass and occupies space. Elements are pure substances consisting of only one type of atom, while compounds are substances formed from two or more elements chemically combined in fixed ratios.

Major Elements in Living Matter

Four elements make up about 96% of living matter: carbon, hydrogen, oxygen, and nitrogen. These elements are fundamental to biological molecules.

Additional info: The remaining 4% consists of elements such as calcium, phosphorus, potassium, sulfur, and trace elements.

Essential vs. Trace Elements

Essential elements are required for an organism to survive, grow, and reproduce. Trace elements are required in minute amounts but are still vital for health.

• Essential element: Needed in large amounts (e.g., oxygen, carbon).

• Trace element: Needed in small amounts (e.g., iron, iodine).

• Example: Iodine is a trace element; its deficiency can cause goiter.

Concept 2.2: An Element’s Properties Depend on the Structure of Its Atoms

Subatomic Particles

Atoms are composed of subatomic particles: protons, neutrons, and electrons. Their arrangement determines the atom’s properties.

Atomic Number and Mass Number

The atomic number is the number of protons in an atom, which defines the element. The mass number is the sum of protons and neutrons.

• Atomic number: Number of protons

• Mass number: Number of protons + number of neutrons

Example: Carbon has atomic number 6 and mass number 12.

Isotopes

Isotopes are atoms of the same element with different numbers of neutrons. Some isotopes are unstable and radioactive.

• Stable isotopes: Do not change over time

• Radioactive isotopes: Decay spontaneously, emitting radiation

• Applications: Used in medical imaging, cancer treatment, and research

Electron Shells and Chemical Behavior

Electrons are arranged in shells around the nucleus. The chemical behavior of an atom is determined by the number of electrons in its outermost shell (valence electrons).

• Valence electrons: Electrons in the outermost shell

• Valence: The bonding capacity of an atom, usually equal to the number of unpaired electrons in the valence shell

Concept 2.3: The Formation and Function of Molecules Depend on Chemical Bonding Between Atoms

Types of Chemical Bonds

Chemical bonds form between atoms due to interactions among subatomic particles, especially electrons. There are three major types of chemical bonds:

• Covalent bonds: Atoms share pairs of electrons. Can be non-polar (equal sharing) or polar (unequal sharing).

• Ionic bonds: Atoms transfer electrons, resulting in positively charged ions (cations) and negatively charged ions (anions), which attract each other.

• Hydrogen bonds: Weak attractions between a hydrogen atom covalently bonded to an electronegative atom and another electronegative atom.

Example: Water molecules are held together by hydrogen bonds.

Comparison of Molecules and Compounds

A molecule is two or more atoms held together by covalent bonds. A compound is a substance containing two or more different elements combined in a fixed ratio.

Common Biological Molecules

Additional info: Molecular oxygen (O2) is not a compound because it contains only one element.

Electronegativity

Electronegativity is the tendency of an atom to attract electrons in a chemical bond. Differences in electronegativity lead to polar covalent bonds.

Concept 2.4: Chemical Reactions Make and Break Chemical Bonds

Chemical Reactions

Chemical reactions involve the making and breaking of chemical bonds, resulting in the formation of new substances.

• Reactants: Substances that start a reaction

• Products: Substances formed by the reaction

Example: Cellular Respiration

The chemical equation for cellular respiration:

Atoms must be balanced on both sides of the equation.

Concept 2.5: Hydrogen Bonding Gives Water Properties That Help Make Life Possible on Earth

Properties of Water

Water’s unique properties are due to hydrogen bonding between molecules, which results from water being a polar molecule.

• Water is an excellent solvent

• Liquid at body temperature

• Can absorb and hold heat to stabilize temperature

• Allows for evaporative cooling

Hydrophilic vs. Hydrophobic

• Hydrophilic: Substances that dissolve in water (e.g., salts, sugars)

• Hydrophobic: Substances that do not dissolve in water (e.g., oils, fats)

Surface Tension and Density

Water’s surface tension allows insects to walk on water. Water is most dense at 4°C, less dense when frozen, which is why ice floats.

Acids, Bases, and the pH Scale

pH Scale

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

• pH > 7: Basic

• pH = 7: Neutral

• pH < 7: Acidic

Example: Blood pH is approximately 7.35–7.40.

Acids and Bases

• Acid: Substance that increases H+ concentration in solution

• Base: Substance that decreases H+ concentration (often by accepting H+ or releasing OH-)

Water Dissociation

When water dissociates, it forms hydronium ions (, often represented as ) and hydroxide ions ().

Logarithmic Nature of pH

Each unit change in pH represents a tenfold change in H+ concentration.

Example: A solution with pH 3 is 1,000 times more acidic than one with pH 6.

Buffers

Buffers are substances that minimize changes in pH by accepting or donating H+ ions. They are made up of weak acids and their corresponding weak bases.

• Function: Help maintain stable pH in biological systems

Key Terms and Definitions

Summary Table: pH Values of Common Substances

Additional Info

• Water’s polarity and hydrogen bonding are crucial for its role as a solvent and for supporting life.

• Biological systems rely on buffers to maintain homeostasis in pH.