CHEMISTRY THE CENTRAL SCIENCE

2 ATOMS, MOLECULES, AND IONS

CHAPTER SUMMARY AND KEY TERMS

SECTIONS 2.1 AND 2.2 Atoms are the basic building blocks of matter. They are the smallest units of an element that can combine with other elements. Atoms are composed of even smaller particles, called subatomic particles. Some of these subatomic particles are charged and follow the usual behavior of charged particles: Particles with the same charge repel one another, whereas particles with unlike charges are attracted to one another. We considered some of the important experiments that led to the discovery and characterization of subatomic particles. Thomson's experiments on the behavior of cathode rays in magnetic and electric fields led to the discovery of the electron and allowed its charge-to-mass ratio to be measured. Mil-likan's oil-drop experiment determined the charge of the electron. Bec-querel's discovery of radioactivity, the spontaneous emission of radiation by atoms, gave further evidence that the atom has a substructure. Rutherford's studies of how thin metal foils scatter α particles led to the nuclear model of the atom, showing that the atom has a dense, positively charged nucleus.

SECTION 2.3 Atoms have a nucleus that contains protons and neutronselectrons move in the space around the nucleus. The magnitude of the charge of the electron, 1.602 × 10−19 C, is called the electronic charge. The charges of particles are usually represented as multiples of this charge—an electron has a 1- charge, and a proton has a 1+ charge. The masses of atoms are usually expressed in terms of atomic mass units (1 amu = 1.66054 × 10?24 g). The dimensions of atoms are often expressed in units of angstroms (1 Å = 10−10 m).

Elements can be classified by atomic number, the number of protons in the nucleus of an atom. All atoms of a given element have the same atomic number. The mass number of an atom is the sum of the numbers of protons and neutrons. Atoms of the same element that differ in mass number are known as isotopes.

SECTION 2.4 The atomic mass scale is defined by assigning a mass of exactly 12 amu to a 12C atom. The atomic weight (average atomic mass) of an element can be calculated from the relative abundances and masses of that element's isotopes. The mass spectrometer provides the most direct and accurate means of experimentally measuring atomic (and molecular) weights.

SECTION 2.5 The periodic table is an arrangement of the elements in order of increasing atomic number. Elements with similar properties are placed in vertical columns. The elements in a column are known as a group. The elements in a horizontal row are known as a period. Themetallic elements (metals), which comprise the majority of the elements, dominate the left side and the middle of the table; the nonmetallic elements (nonmetals) are located on the upper right side. Many of the elements that lie along the line that separates metals from nonmetals aremetalloids.

SECTION 2.6 Atoms can combine to form molecules. Compounds composed of molecules (molecular compounds) usually contain only nonmetallic elements. A molecule that contains two atoms is called a diatomic molecule. The composition of a substance is given by its chemical formula. A molecular substance can be represented by its empirical formula, which gives the relative numbers of atoms of each kind. It is usually represented by its molecular formula, however, which gives the actual numbers of each type of atom in a molecule. Structural formulasshow the order in which the atoms in a molecule are connected. Ball-and-stick models and space-filling models are often used to represent molecules.

SECTION 2.7 Atoms can either gain or lose electrons, forming charged particles called ions. Metals tend to lose electrons, becoming positively charged ions (cations). Nonmetals tend to gain electrons, forming negatively charged ions (anions). Because ionic compounds are electrically neutral, containing both cations and anions, they usually contain both metallic and nonmetallic elements. Atoms that are joined together, as in a molecule, but carry a net charge are called polyatomic ions. The chemical formulas used for ionic compounds are empirical formulas, which can be written readily if the charges of the ions are known. The total positive charge of the cations in an ionic compound equals the total negative charge of the anions.

SECTION 2.8 The set of rules for naming chemical compounds is called chemical nomenclature. We studied the systematic rules used for naming three classes of inorganic substances: ionic compounds, acids, and binary molecular compounds. In naming an ionic compound, the cation is named first and then the anion. Cations formed from metal atoms have the same name as the metal. If the metal can form cations of differing charges, the charge is given using Roman numerals. Monatomic anions have names ending in -ide. Polyatomic an-ions containing oxygen and another element (oxyanions) have names ending in -ate or -ite.

SECTION 2.9 Organic chemistry is the study of compounds that contain carbon. The simplest class of organic molecules is the hydrocarbons, which contain only carbon and hydrogen. Hydrocarbons in which each carbon atom is attached to four other atoms are called alkanes. Alkanes have names that end in -ane, such as methane and ethane. Other organic compounds are formed when an H atom of a hydrocarbon is replaced with a functional group. An alcohol, for example, is a compound in which an H atom of a hydrocarbon is replaced by an OH functional group. Alcohols have names that end in -ol, such as methanol and ethanol. Compounds with the same molecular formula but a different bonding arrangement of their constituent atoms are called isomers.

KEY SKILLS

• Describe the basic postulates of Dalton's atomic theory. (Section 2.1)

• Describe the key experiments that led to the discovery of electrons and to the nuclear model of the atom. (Section 2.2)

• Describe the structure of the atom in terms of protons, neutrons, and electrons. (Section 2.3)

• Describe the electrical charge and relative masses of protons, neutrons, and electrons. (Section 2.3)

• Use chemical symbols together with atomic number and mass number to express the subatomic composition of isotopes. (Section 2.3)

• Understand how atomic weights relate to the masses of individual atoms and to their natural abundances. (Section 2.4)

• Describe how elements are organized in the periodic table by atomic number and by similarities in chemical behavior, giving rise to periods and groups. (Section 2.5)

• Describe the locations of metals and nonmetals in the periodic table. (Section 2.5)

• Distinguish between molecular substances and ionic substances in terms of their composition. (Sections 2.6 and 2.7)

• Distinguish between empirical formulas and molecular formulas. (Section 2.6)

• Describe how molecular formulas and structural formulas are used to represent the compositions of molecules. (Section 2.6)

• Explain how ions are formed by the gain or loss of electrons and be able to use the periodic table to predict the charges of common ions. (Section 2.7)

• Write the empirical formulas of ionic compounds, given the charges of their component ions. (Section 2.7)

• Write the name of an ionic compound given its chemical formula, or write the chemical formula given its name. (Section 2.8)

• Name or write chemical formulas for binary inorganic compounds and for acids. (Section 2.8)

• Identify organic compounds and name simple alkanes and alcohols. (Section 2.9)