In this chapter, you will learn how to deduce and write chemical formulas and how to use the information incorporated into chemical formulas. The chapter ends with an overview of the relationship between names and formulas—chemical nomenclature.
Scanning electron microscope image of sodium chloride crystals.
Chemical compounds, their formulas, and their names are topics discussed in this chapter.
A molecular compound is made up of discrete units called molecules, which
typically consist of a small number of nonmetal atoms held together by covalent
bonds. Molecular compounds are represented by chemical formulas,
symbolic representations that, at minimum, indicate
• the elements present
• the relative number of atoms of each element
Chemical formula – relative numbers of atoms of each element present
Empirical formula – the simplest whole number formula
Structural formula – the order and type of attachements
– shows multiple bonds
- may show lone pairs
- hard to show 3-d
In the molecular model, the black spheres are carbon, the red are oxygen, and the
white are hydrogen. To show that one H atom in the molecule is fundamentally
different from the other three, the formula of acetic acid is often written as HC2H3O2 or (see Section 5-3). To show that this H atom is bonded to an O atom, the formulas CH3CO2H and CH3COOH are also used. For a few chemical compounds, you may find
different versions of chemical formulas in different sources.
The sizes of atoms, reflected in the various sizes of the colored spheres, are related to the locations of the elements in the periodic table, as discussed in Section 9-3.
Positive and negaive ions joined together by electrostatic forces
Metals tend to lose electrons to form cations
Non-metals tend to gain electrons to form anions
Ionic solids formulae are reported as the formula unit – inappropriate to call it a molecular formula
Na loses one electron to form the sodium ion
Cl gains one electron to form the chloride ion
Centers of ions are shown in the ball and stick model for clarity
Space filling model shows how the ions are actually in contact with one another.
We will discuss face centered cubic and other types of packing in chapter 13
In a sample of solid sulfur, there are eight
sulfur atoms in a sulfur molecule. In solid
white phosphorus, there are four phosphorus
atoms per molecule.
They come in various forms called allotropes – these are one allotrope of each
Molecular formula tells us there are TWO moles of C per mole of halothane.
We also know about the MASSES of the compound and its elemental components.
Therefore we can talk about PERCENT COMPOSITION BY MASS
If you know the molecular wt it is beneficial to choose that number, then only first three steps are required.
Water vapour absorbed by magnesium perchlorate
Carbon dioxide absorbed by sodium hydroxide.
The differences in mass of the absorbers before and after yiled the masses of water and CO2 produced in the reaction
Combustion takes place in an excess of oxygen so you cannot measure oxygen. Oxygen CAN be analyzed separately but is usually determined by difference.
(a) Oxygen gas passes through the combustion tube containing the sample being
analyzed. This portion of the apparatus is enclosed in a high-temperature furnace.
Products of the combustion are absorbed as they leave the furnace—water vapor by
magnesium perchlorate, and carbon dioxide gas by sodium hydroxide (producing
sodium carbonate). The differences in mass of the absorbers, after and before the
combustion, yield the masses of H2O and CO2 produced in the combustion reaction.
(b) A molecular picture of the combustion of ethanol. Each molecule of ethanol
produces two molecules CO2 and three H2O molecules. Combustion takes place in
an excess of oxygen, so that oxygen molecules are present at the end of the reaction.
Note the conservation of mass.
Metals are electron sources
Non-metals are electron sinks
Sodium goes to the +1 oxidation state
Chlorine goes tot eh –1 oxidation state
These two compounds contain the same elements— lead and oxygen—but in different proportions. Their names and formulas must
convey this fact: lead(IV) oxide = PbO2 (red-brown); lead(II) oxide = PbO (yellow).
Write the unmodified name of the metal
Then write the name of the nonmetal, modifed to end in ide.
Ionic compounds must be electrically neutral
In naming binary acids we use the prefix hydro- followed by the name of the other nonmetal modified with an -ic ending. The most important binary acids are listed below.
1. Polyatomic anions are more common than polyatomic cations. The most
familiar polyatomic cation is the ammonium ion NH4
+.
2. Very few polyatomic anions carry the -ide ending in their names. Of those
listed, only OH- (hydroxide ion) and CN- (cyanide ion) do. The common
endings are -ite and -ate, and some names carry prefixes, hypo- or per-.
3. An element common to many polyatomic anions is oxygen, usually in
combination with another nonmetal. Such anions are called oxoanions.
4. Certain nonmetals (such as Cl, N, P, and S) form a series of oxoanions containing
different numbers of oxygen atoms. Their names are related to the
oxidation state of the nonmetal atom to which the O atoms are bonded,
ranging from hypo- (lowest) to per- (highest) according to the following
scheme.
Learn the most common ions first. When you understand the scheme on the previous slide, the names of several others will become obvious. Over time, the rest will become more familiar to you
Most oxoacids are ternary compounds composed of hydrogen, oxygen and one other nonmental.
Oxoacids are molecular compounds, salts are ionic compounds
Ic and ate names are assigned to compounds (rather than ite and ate as in the oxoanions) in which the central nonmetal atom has an oxidation state equal to the periodic group number – 10
For halogens ic and ate names are assigned to compounds in which the halogen has an oxidation state of +5.
The piece of filter paper was soaked in a water solution of cobalt(II) chloride and then allowed to dry. When kept in dry air, the paper is blue in color (anhydrous CoCl2). In humid air, the paper changes to pink (CoCl2 • 6 H2O).
These are structural isomers. The structures are different
these molecules do not have the same formula, they are different
c) Now these molecules have the same formula and ALSO the same connectivity. These are geometric isomers.