Chapter 2: The Chemical Context of Life

Learning Objectives

• Describe matter, chemical elements, and chemical compounds.

• Explain how atomic structure determines element properties.

• Identify and describe types of chemical bonds and their effects on molecular shape.

• Interpret chemical reactions and their significance in biology.

Concept 2.1: Matter and Elements

Definition of Matter and Elements

Matter is anything that has mass and occupies space. All organisms are composed of matter, which is made up of chemical elements. An element is a substance that cannot be broken down into other substances by chemical reactions.

Chemical Composition of the Human Body

The human body is primarily composed of a few key elements. The most abundant are oxygen, carbon, hydrogen, and nitrogen, which together with calcium and phosphorus, make up 99% of living matter.

Concept 2.2: Atomic Structure and Properties

Structure of Atoms

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

• Protons: Positively charged particles found in the nucleus; determine the element.

• Neutrons: Electrically neutral particles found in the nucleus; determine isotopes.

• Electrons: Negatively charged particles that form a cloud around the nucleus; determine chemical behavior.

Atomic Number, Mass Number, and Atomic Mass

The atomic number is the number of protons in an atom's nucleus. The mass number is the sum of protons and neutrons. Atomic mass is the average mass of all isotopes of an element, weighted by their natural abundance.

Isotopes and Radioactivity

Isotopes are atoms of the same element with different numbers of neutrons. Some isotopes are radioactive and decay spontaneously, emitting particles and energy.

Energy Levels of Electrons

Electrons occupy energy levels or shells around the nucleus. The further an electron is from the nucleus, the higher its potential energy. Electrons can move between shells by absorbing or releasing energy.

Electron Shells and Valence Electrons

The chemical behavior of an atom is largely determined by the distribution of electrons in its shells, especially the valence electrons in the outermost shell. Atoms with full valence shells are chemically inert.

Electron Orbitals

Orbitals are three-dimensional regions where electrons are likely to be found. Each shell contains a specific number of orbitals, and each orbital can hold up to two electrons.

The Periodic Table

The periodic table organizes elements by increasing atomic number and similar chemical properties. Elements in the same column have similar valence electron configurations and chemical behaviors.

Concept 2.3: Chemical Bonds and Molecular Structure

Chemical Bonds

Atoms with incomplete valence shells can share or transfer electrons, forming chemical bonds. The main types of bonds are:

• Covalent bonds: Sharing of electron pairs between atoms.

• Ionic bonds: Transfer of electrons from one atom to another, resulting in oppositely charged ions.

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

• Van der Waals interactions: Weak attractions due to transient local charges.

Covalent Bonds

A covalent bond involves the sharing of valence electrons. A single bond shares one pair, while a double bond shares two pairs. The structural formula shows the arrangement of atoms and bonds.

Electronegativity and Bond Polarity

Electronegativity is an atom's ability to attract electrons in a bond. If atoms have similar electronegativities, the bond is nonpolar covalent (equal sharing). If they differ, the bond is polar covalent (unequal sharing), resulting in partial charges.

Ionic Bonds and Ionic Compounds

Ionic bonds form when electrons are transferred from one atom to another, creating ions. A cation is positively charged; an anion is negatively charged. The resulting compounds are called ionic compounds or salts.

Weak Chemical Interactions

Weak bonds, such as hydrogen bonds and van der Waals interactions, are crucial for the structure and function of large biological molecules. Their reversibility allows for dynamic biological processes.

Hydrogen Bonds

A hydrogen bond forms when a hydrogen atom covalently bonded to one electronegative atom is attracted to another electronegative atom, commonly oxygen or nitrogen.

Van der Waals Interactions

These are weak attractions that occur when transient local partial charges attract molecules that are close together. They are significant in large molecules and biological surfaces.

Molecular Shape and Function

The shape of a molecule is determined by the positions of its atoms' orbitals and is critical for its biological function. Molecular shape determines how molecules recognize and interact with each other, such as hormones binding to receptors.

Concept 2.4: Chemical Reactions

Making and Breaking Bonds

Chemical reactions involve the making and breaking of chemical bonds. The starting substances are reactants, and the resulting substances are products.

Photosynthesis: An Example of a Biological Chemical Reaction

Photosynthesis is a key chemical reaction in biology, converting carbon dioxide and water into glucose and oxygen using sunlight:

Chemical Equilibrium

All chemical reactions are reversible. Chemical equilibrium is reached when the forward and reverse reactions occur at the same rate, and the concentrations of reactants and products remain constant.

Summary Table: Types of Chemical Bonds

Additional info: These notes provide foundational chemistry concepts essential for understanding biological molecules and processes, as covered in General Biology Chapter 2.