Chapter Summary

• Matter is composed of atoms. Each atom consists of a positively charged nucleus made up of protons and neutrons, surrounded by electrons bearing negative charges. Review Figure 2.1

• The number of protons in the nucleus defines an element. There are many elements in the universe, but only a few of them make up the bulk of living organisms: C, H, O, P, N, and S. Review Figure 2.2

• Isotopes of an element differ in their numbers of neutrons. Radioisotopes are radioactive, emitting radiation as they break down.

• Electrons are distributed in electron shells, which are volumes of space defined by specific numbers of orbitals. Each orbital contains a maximum of two electrons. Review Figure 2.4, Activity 2.1

• In losing, gaining, or sharing electrons to become more stable, an atom can combine with other atoms to form a molecule.

• A chemical bond is an attractive force that links two atoms together in a molecule. Review Table 2.1, Animation 2.1

• A compound is a substance made up of molecules with two or more different atoms bonded together in a fixed ratio, such as water (H₂O).

• Covalent bonds are strong bonds formed when two atoms share one or more pairs of electrons. Review Figure 2.5, Focus: Key Figure 2.6

• When two atoms of unequal electronegativity bond with each other, a polar covalent bond is formed. The two ends, or poles, of the bond have partial charges (δ⁺ or δ^–). Review Figure 2.7

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• An ion is an electrically charged particle that forms when an atom gains or loses one or more electrons in order to form a more stable electron configuration. Anions and cations are negatively and positively charged ions, respectively. Different charges attract each other, and like charges repel each other.

• Ionic attractions occur between oppositely charged ions. Ionic attractions are strong in solids (salts) but weaken when the ions are separated from one another in solution. Review Figure 2.9

• A hydrogen bond is a weak electrical attraction that forms between a δ⁺ hydrogen atom in one molecule and a δ^– atom in another molecule (or in another part of the same, large molecule). Hydrogen bonds are abundant in water. Review Figure 2.10

• Nonpolar molecules interact very little with polar molecules, including water. Nonpolar molecules are attracted to one another by very weak bonds called van der Waals forces.

• In chemical reactions, atoms combine or change their bonding partners. Reactants are converted into products.

• Some chemical reactions release energy as one of their products; other reactions can occur only if energy is provided to the reactants.

• Neither matter nor energy is created or destroyed in a chemical reaction, but both change form. Review Figure 2.12

• In organisms, chemical reactions take place in multiple steps so that released energy can be harvested for cellular activities.

• Water’s molecular structure and its capacity to form hydrogen bonds give it unique properties that are significant for life. Review Figure 2.13

• The high specific heat of water means that water gains or loses a great deal of heat when it changes state. Water’s high heat of vaporization ensures effective cooling when water evaporates.

• The cohesion of water molecules refers to their capacity to resist coming apart from one another. Hydrogen bonding between the water molecules plays an essential role in this property.

• A solution is produced when a solid substance (the solute) dissolves in a liquid (the solvent). Water is the critically important solvent for life. The ability of molecules to lose or accept protons (H⁺) is important in biological structure and function.