9-1. Chemical Change
A. The medieval search for a way to change ordinary metals into gold is called alchemy.
B. A chemical reaction results in the formation of a new substance whose properties are different from those of the individual substances that participate in the reaction.
C. A heterogeneous substance is composed of separate materials of different kinds that are not chemically combined and retain their individual properties.
9-2. Three Classes of Matter
A. Three classes of matter are elements, compounds, and mixtures.
1. An element cannot be broken down or changed into another element by chemical means. All other substances are combinations of elements.
2. Compounds are formed when two or more elements combine chemically to produce a new substance. The properties of a compound are different from those of the elements it contains.
3. Mixtures consist of elements or compounds (or both) that are not chemically combined and retain their characteristic properties.
a. Heterogenous mixtures are those whose constituents are not uniformly mixed and are easy to separate.
b. Homogeneous mixtures, or solutions, are those whose constituents are uniformly mixed.
B. Compounds can be distinguished from solutions by two tests:
1. Compounds boil and freeze at definite temperatures and are not altered by a change of state. Mixtures lack definite boiling and freezing temperatures and can be altered by a change of state.
2. In a compound, the elements are present in a specific ratio by mass according to the law of definite proportions. In a mixture, the components are not present in a specific ratio by mass.
9-3. The Atomic Theory
A. The English schoolteacher John Dalton (1766-1844) proposed an atomic theory which provided the basis for our modern understanding of matter.
1. Dalton stated that all matter was composed of basic particles called atoms.
2. Dalton was the first to establish the relative masses of the atoms of different elements.
B. According to the modern atomic theory of matter:
1. Atoms are the ultimate particles of any element.
2. Molecules are the ultimate particles of a gaseous compound.
3. The ultimate particles of liquid and solid compounds may be atoms, molecules, or ions.
C. A chemical formula indicates the number of atoms and types of elements present in a compound.
1. The kinds of atoms in a compound are represented by their chemical symbols.
2. The numbers of each kind of atom are represented by subscript numbers.
9-4. Metals and Nonmetals
A. Most elements are metals.
B. All metals, except mercury, are solid at room temperature. Nonmetals may be solid, liquid, or gaseous.
C. Metals have a characteristic metallic luster; nonmetals do not.
D. All metals are opaque; most nonmetals are transparent in thin sheets.
E. Metals can be shaped by bending or hammering. Solid nonmetals are brittle.
F. Metals are good conductors of heat and electricity. Nonmetals are insulators.
9-5. Chemical Activity
A. Active elements combine readily to form compounds. Inactive elements have little tendency to react chemically.
B. Active elements liberate more heat when they react than do inactive elements.
C. Active elements usually form stable compounds.
9-6. Families of Elements
A. Elements which resemble one another in the way they react chemically are said to belong to the same family of elements.
B. Some examples of chemical families include:
1. The halogens, or "salt formers," are active nonmetals.
2. The alkali metals are active metals and have low melting points.
3. The inert gases are inactive nonmetals.
9-7. The Periodic Table
A. The Russian chemist Dmitri Mendeleev formulated the periodic law about 1869 which states that when elements are listed in order of atomic number, elements with similar chemical and physical properties appear at regular intervals.
B. The periodic table is a listing of the elements according to atomic number in a series of rows such that elements with similar properties form vertical columns.
9-8. Groups and Periods
A. The periodic table arranges chemical families of elements in vertical columns called groups.
B. The horizontal rows of elements of the periodic table are called periods.
C. The transition elements are placed between groups 2 and 3 and include:
1. The rare-earth metals (atomic numbers 57-71).
2. The actinides (atomic numbers 89-105).
D. Mendeleev's periodic table, although arranged by increasing atomic mass rather than atomic number, helped him predict the existence and properties of a number of elements.
9-9. Shells and Subshells
A. The electrons in an atom that have the same principal quantum number n occupy the same shell.
B. The electrons in an atom that have the same orbital quantum number l occupy the same subshell. The larger the value of l, the more electrons the subshell can hold.
C. A shell or subshell that contains its full quota of electrons is said to be closed.
9-10. Explaining the Periodic Table
A. The electrons in a closed shell or subshell are tightly bound to the atom and not easily removed.
B. The inert gases all have closed subshells and are thus chemically inactive.
C. Metal atoms have one or several electrons outside closed shells or subshells and combine chemically by losing these electrons to nonmetal atoms.
D. Nonmetal atoms need one or several electrons to achieve closed shells or subshells and combine chemically by taking electrons from metal atoms or by sharing electrons with nonmetal atoms.
E. Across any period of the periodic table, metallic activity (losing electrons) decreases from left to right and nonmetallic activity (gaining electrons) increases from left to right.
9-11. Types of Bond
A. A covalent bond is formed when one or more pairs of electrons are shared by two or more atoms.
B. An ionic bond is formed when electrons are transferred between two or more atoms and the resulting ions of opposite charge attract each other.
C. Covalent bonds form molecules. Ionic bonds usually do not form molecules butaggregates of ions attracted by charge.
9-12. Covalent Bonding
A. Covalent compounds are substances whose atoms are joined by one or more pairs of electrons in a covalent bond.
B. Polar covalent compounds are those in which the shared electron pairs are closer to one atom than to the other, making one part of the molecule relatively negative and another part relatively positive.
C. Organic compounds are covalent compounds that contain the element carbon.
9-13. Ionic Bonding
A. Ionic compounds are formed by electron transfer and are usually crystalline solids with high melting points.
B. Some ionic compounds contain a metal and a single nonmetal; others contain a metal and an atom group consisting of two or more nonmetals.
9-14. Ionic Compounds
A. When a metal atom combines with a nonmetal atom to form an ionic compound, the chemical formula of the ionic compound formed can be determined by knowing how many electrons the metal atom loses and how many electrons the nonmetal atom gains.
B. To write the formula of an ionic compound consisting of a metal and a nonmetal:
1. Determine the ionic charge of the metal and of the nonmetal.
2. Write the value of the ionic charge of the nonmetal as a subscript following the metal.
3. Write the value of the ionic charge of the metal as a subscript following the nonmetal.
4. If the value of the ionic charge for either the metal or the nonmetal is 1, no subscript is written.
9-15. Atom Groups
A. Atom groups appear as units in many compounds and remain together during chemical reactions.
B. The sulfate group SO4 is an example of an atom group.
C. A precipitate is an insoluble solid that results from a chemical reaction in solution.
D. When two or more atom groups of the same kind are present in the formula of a compound, parentheses are placed around the group.
Example: Ca(NO3)2
9-16. Naming Compounds
A. A compound ending in -ide usually is composed of only two elements. Hydroxides which contain the OH- ion are an exception.
B. A compound ending in -ate contains oxygen and two or more other elements.
C. When the same pair of elements occurs in two or more compounds, a prefix (mono = 1, di = 2, tri = 3, tetra = 4, penta = 5, hexa =6, and so on) may be used to indicate the number of one or both kinds of atoms in the molecule.
D. When one of the elements in a compound is a metal that can form different ions, the ionic charge of the metal is given by a roman numeral.
9-17. Chemical Equations
A. A chemical equation expresses the results of a chemical change.
B. In a chemical equation the formulas of the reacting substances (reactants) appear on the left-hand side and the formulas of the products appear on the right-hand side.
C. Chemical equations must be balanced, meaning that the number of atoms of each kind of element must be the same on both sides of the equation. Unbalanced chemical equations have unequal numbers of at least one kind of atom on both sides of the equation.
D. Unbalanced chemical equations are balanced by placing numbers, or coefficients, in front of the reactants and products to adjust their amounts.
1. The numerical coefficient multiplies the number of each kind of atom in the formula.
2. The subscripts of the formulas of the reactants and products are never changed when balancing a chemical equation since this changes the identity of the substances. |
