2.1 Atoms are nature's building material.
Atoms
• All substances are composed of matter, and all matter is composed of atoms. (p. 20)
• Negatively charged electrons circle the nucleus of the atom, while the nucleus is composed of positively charged protons and neutral neutrons. (p. 20)
• An atom's atomic number refers to its number of protons, while its atomic mass refers to the number of protons and neutrons. (p. 20)
• Isotopes are atoms of an element that possess different numbers of neutrons. (p. 21)
• Radioactive isotopes contain nuclei that spontaneously break up into elements with lower atomic numbers. The decay rate is expressed in terms of a half-life. (p. 21)
Electrons Determine the Chemical Behavior of Atoms
• Electrons circle the nucleus of an atom in orbitals. Because the orbitals are so large and are mostly composed of empty space, the nuclei of two atoms never come close enough in nature to interact with each other. (p. 22)
• Because electrons are attracted to the positively charged nucleus, work is necessary so that they stay in their orbits, and thus they have potential energy of position. Moving an electron to a more distant orbital requires energy, while moving an electron closer to the nucleus releases energy. (p. 22)
• Oxidation refers to the loss of an electron; reduction refers to the gain of an electron. (p. 23)
2.2 The atoms of living things are among the smallest.
Kinds of Atoms
• There are 92 naturally occurring elements, arranged in a periodic table based on the interactions of their valence electrons. (p. 24)
2.3 Chemical bonds hold molecules together.
Ionic Bonds Form Crystals
• A molecule is a group of atoms held together by energy in a stable association and joined by chemical bonds. (Compounds are composed of atoms of more than one element.) (p. 25)
• Ionic bonds are formed as attractions between ions of opposite charge, such as those in sodium chloride. (p. 25)
Covalent Bonds Build Stable Molecules
• Covalent bonds form when two atoms share one or more pairs of valence electrons and give rise to true, discrete molecules. (p. 26)
• Covalent bonds are relatively strong, and the strength increases with the number of shared electron pairs. (p. 26)
• A chemical reaction is formed during the formation or breaking of chemical bonds. A reaction may be influenced by several factors, including temperature, concentration of reactants and products, and the presence of catalysts. (p. 26)
2.4 Water is the cradle of life.
Chemistry of Water
• The most outstanding chemical property of water is its ability to form weak chemical associations. (p. 27)
Water Atoms Act Like Tiny Magnets
• A water molecule exhibits electronegativity as both the oxygen and the hydrogen atoms attract the electrons they share in covalent bonds. However, the oxygen atom is more electronegative than the hydrogen atoms. (p. 28)
• Water molecules are polar and exhibit distinct ends with partial charges. (p. 28)
• Hydrogen bonds are formed as opposite charges of bridging hydrogen atoms are attracted, and although each hydrogen bond is relatively weak, the cumulative effect of large numbers of them can be very strong. (p. 28)
Water Clings to Polar Molecules
• Cohesion refers to the attraction of water molecules to other water molecules; adhesion refers to the attraction of other molecules to water molecules. (p. 29)
• The cohesion of water is responsible for its surface tension. (p. 29)
• Water moderates temperatures through its high specific heat and its high heat of vaporization. (p. 29)
• Water is an effective solvent because of its ability to form hydrogen bonds. (p. 30)
Water Ionizes
• A solution's pH is defined as the negative logarithm of its H+ ion concentration. (p. 31)
• Acids are solutions with high H+ concentrations (pH < 7), while bases are solutions with low H+ concentrations (pH > 7). (p. 31)
• Buffers are hydrogen ion reservoirs that either accept or donate H+ as needed. (p. 32)