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# Chapter 2: The Chemistry of Life ## 2.1 The Elements of Life ### Concept 2.1 Living matter is composed of about 20-25 chemical elements * **Matter** consists of chemical elements in pure form and in combinations called **compounds**. * **Elements** cannot be broken down to other substances...

# Chapter 2: The Chemistry of Life ## 2.1 The Elements of Life ### Concept 2.1 Living matter is composed of about 20-25 chemical elements * **Matter** consists of chemical elements in pure form and in combinations called **compounds**. * **Elements** cannot be broken down to other substances by chemical reactions. * A **compound** is a substance consisting of two or more elements in a fixed ratio. ### Essential Elements * About 20-25% of the 92 natural elements are **essential elements** that an organism needs to live a healthy life and reproduce. * **Trace elements** are required by an organism in only minute quantities. ### Figure 2.2: Examples of Essential Trace Elements The table shows examples of the elements Iron (Fe) and Iodine (I). * **Iron (Fe):** Is used by all organisms for cellular respiration. In humans, iron is needed for the transport of oxygen in red blood cells. * **Iodine (I):** Is needed to make thyroid hormones, which regulate development and metabolism. ### Case Study: Evolution of Tolerance to Toxic Elements * Some plants are able to survive in soils with toxic elements because they have evolved mechanisms to tolerate these elements. ## 2.2 Atoms and Molecules ### Concept 2.2 An atom’s structure determines its properties * Each element consists of unique atoms. ### Subatomic Particles * Atoms are composed of: * **Neutrons** (no electrical charge) * **Protons** (positive charge) * **Electrons** (negative charge) * Neutrons and protons are packed together tightly in the **atomic nucleus**. * Electrons form a “cloud” of negative charge around the nucleus. ### Atomic Number and Atomic Mass * **Atomic number:** The number of protons in the nucleus. * **Mass number:** The sum of protons plus neutrons in the nucleus. * **Atomic mass:** The atom's total mass, which can be approximated by the mass number. ### Isotopes * **Isotopes:** Two atoms of the same element that differ in number of neutrons. * **Radioactive isotope:** One in which the nucleus decays spontaneously, giving off particles and energy. ### The Energy Levels of Electrons * **Energy levels:** Electrons exist only at certain discrete levels of potential energy. * An electron can move from one energy level to another only if it absorbs or loses an amount of energy equal to the difference between the energy levels. ### Electron Orbitals * **Orbital:** The three-dimensional space where an electron is found 90% of the time. ### Electron Configuration and Chemical Properties * The chemical behavior of an atom depends mostly on the number of electrons in its outermost shell. * **Valence electrons:** The outer electrons * **Valence shell:** The outermost electron shell. * Atoms with the same number of valence electrons have similar chemical behavior. * **Inert:** Atoms with a completed valence shell are unreactive. ### Figure 2.6 Electron Distribution Diagrams and the Periodic Table The table shows the electron distribution diagrams for the first 18 elements in the periodic table. The elements are arranged by increasing atomic number, with hydrogen at the top left and argon at the bottom right. The number of electrons in each shell is indicated by the number of dots around the element symbol. ## 2.3 Chemical Bonds ### Concept 2.3 The formation and function of molecules depend on chemical bonding between atoms ### Covalent Bonds * **Covalent bond:** The sharing of a pair of valence electrons by two atoms. * **Molecule:** Two or more atoms held together by covalent bonds. * **Single bond:** A single covalent bond; the sharing of one pair of valence electrons. * **Double bond:** A double covalent bond; the sharing of two pairs of valence electrons. * **Electronegativity:** The attraction of a particular atom for the electrons of a covalent bond. * **Nonpolar covalent bond:** A covalent bond in which the electrons are shared equally. * **Polar covalent bond:** A covalent bond in which one atom is more electronegative, and the electrons are not shared equally. ### Ionic Bonds * **Ion:** A charged atom or molecule. * **Cation:** A positively charged ion. * **Anion:** A negatively charged ion. * **Ionic bond:** Attraction between an anion and a cation. * **Ionic compounds (or salts):** Compounds formed by ionic bonds. ### Weak Chemical Bonds *Hydrogen bonds, and van der Waals interactions* * **Hydrogen bond:** Forms when a hydrogen atom covalently bonded to one electronegative atom is also attracted to another electronegative atom. * **van der Waals interactions:** Individually weak and occur only when atoms and molecules are very close together. ### Molecular Shape and Function * A molecule’s shape is usually very important to its function. * Molecular shape determines how biological molecules recognize and respond to each other with specificity. ## 2.4 Chemical Reactions ### Concept 2.4 Chemical reactions make and break chemical bonds * **Chemical reactions:** The making and breaking of chemical bonds, leading to changes in the composition of matter. * **Reactants:** The starting molecules of a chemical reaction. * **Products:** The resulting molecules of a chemical reaction. * **Chemical equilibrium:** The point at which the forward and reverse reactions offset one another exactly. ### Figure 2.13: Photosynthesis: A Chemical Reaction The image depicts a plant performing photosynthesis. Carbon dioxide and water are on the left in blue as reactants. Light energy fuels the process, and the products glucose and oxygen are on the right in green. The chemical equation is: $6CO_2 + 6H_2O \longrightarrow C_6H_{12}O_6 + 6O_2$