Chapter 5 Flashcards
A mixture
This balloon is filled with a mixture of hydrogen and oxygen gas. The relative amounts of hydrogen and oxygen are variable.
A chemical compound
This balloon is filled with water, composed of molecules that have a fixed ratio of hydrogen to oxygen.
Joseph Proust
formally stated the idea that elements combine in fixed proportions to form compounds.
The law of constant composition states:
All samples of a given compound have the same proportions of their constituent elements.
chemical formula
indicates the elements present in a compound and the relative number of atoms of each.
H2O is the chemical formula for water:
it indicates that water consists of hydrogen and oxygen atoms in a 2:1 ratio.
NaCl for table salt
indicating sodium and chlorine atoms in a 1:1 ratio.
C12H22O11 for table sugar (sucrose),
indicating carbon, hydrogen, and oxygen atoms in a 12:22:11 ratio.
How to List the Elements in Order in Compounds
- Chemical formulas list the most metallic elements first. The formula for table salt is NaCl, not ClNa.
- In compounds that do not include a metal, the more metal-like element is listed first.
- Metals are found on the left side of the periodic table and nonmetals on the upper right side.
- Among nonmetals, those to the left in the periodic table are more metal-like than those to the right and are normally listed first. We write NO2 and NO, not O2N and ON.
- Within a single column in the periodic table, elements toward the bottom are more metal-like than elements toward the top. We write SO2, not O2S.
polyatomic ions
Some chemical formulas contain groups of atoms that act as a unit. When several groups of the same kind are present, their formula is set off in parentheses with a subscript to indicate the number of that group. Many of these groups of atoms have a charge associated with them
How to determine the total number of each type of atom in a compound containing a group within parentheses
multiply the subscript outside the parentheses by the subscript for each atom inside the parentheses.
empirical formula
gives the relative number of atoms of each element in a compound.
molecular formula
gives the actual number of atoms of each element in a molecule of the compound.
structural formula
uses lines to represent chemical bonds and shows how the atoms in a molecule are connected to each other.
Pure substances may be
either elements or compounds.
Elements may be
either atomic or molecular.
Compounds may be
either molecular or ionic.
Atomic elements
are those that exist in nature with single atoms as their basic units. Most elements fall into this category.
Molecular elements
do not normally exist in nature with single atoms as their basic units. Instead, these elements exist as diatomic molecules—two atoms of that element bonded together—as their basic units.
Molecular compounds
are compounds formed from two or more nonmetals. The basic units of molecular compounds are molecules composed of the constituent atoms.
Ionic compounds
- When a metal, which has a tendency to lose electrons, combines with a nonmetal, which has a tendency to gain electrons, one or more electrons transfer from the metal to the nonmetal, creating positive and negative ions that are attracted to each other.
- A compound composed of a metal and a nonmetal is considered ionic.
- Unlike molecular compounds, ionic compounds do not contain individual molecules but rather cations and anions in an alternating three-dimensional array.
formula unit
The basic unit of ionic compounds
Write formulas for ionic compounds that form from aluminum and oxygen and magnesium and oxygen.
Al3+ O2- Mg2+ O2-
Al2O3 Mg2O2
The first step in naming an ionic compound is identifying it as one.
Remember, any time you have a metal and one or more nonmetals together in a chemical formula, you can assume the compound is ionic.
