5-3: Compound Names and Formulas
Tuesday, August 18, 2015
5:51 PM
Key Terms:
1. Empirical formula – the composition of a compound in terms of the relative number and kinds of atoms in the
simplest ratio.
2. Molecular formula – a chemical formula that shows the number and kinds of atoms in a molecule, but not the
arrangement of atoms.
I. Naming Ionic Compounds
a. Names of cations include the elements of which they are composed
i. Examples: sodium -> sodium ion, calcium -> calcium ion
b. Names of anions are altered names of elements
i. The ending of the element's name usually is changed to -ide.
c. Some cation names must show their charge
i. Fe2O3 and FeO are both forms of iron oxide, but they are quite different. So, we need to differentiate
between the two by giving them distinct names.
1) Fe2O3 includes an iron ion with a 3+ charge, thus it is called iron (III) oxide
2) FeO includes an iron ion with a 2+ charge, so it is called iron (II) oxide
d. Determining the charge of a transition metal
i. Ionic compounds have a total charge of zero, so the total positive charges must equal the total
negative charges.
ii. In the case of Fe2O3:
1) An oxide ion has a 2- charge. Three of them would have a total charge of 6-.
2) For two iron ions to have a total charge of 6+, each ion must have a charge of 3+.
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II. Writing Formulas for Ionic Compounds
III. Naming Covalent Compounds
We use different rules to name covalent compounds, than we do for ionic compounds.
a. Numerical prefixes are used to name covalent compounds of two elements
i. For two-element covalent compounds, numerical prefixes tell
how many atoms of each element are in the molecule.
ii. Whichever element is farther to the right in the periodic table is
named second and ends in -ide.
IV. Chemical Formulas for Covalent Compounds
a. A compound's simplest formula is its empirical formula
i. An empirical formula tells us the smallest whole-number ratio of atoms that are in a compound.
Formulas for most ionic compounds are empirical formulas.
ii. Covalent compounds have empirical formulas, too.
iii. Scientists have to analyze unknown compounds and the masses of elements they contain to
determine their empirical formulas.
b. Determining empirical formulas
i. Once you determine the mass of each element in a compound, you can calculate the amount of each
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i. Once you determine the mass of each element in a compound, you can calculate the amount of each
element in moles. The empirical formula for the compound is the ratio of these amounts.
c. Different compounds can have the same empirical formula
i. Multiple compounds can have the same empirical formula, since it is just a ratio.
ii. These compounds are usually very different, having their own distinct properties.
iii. To distinguish between compounds in these instances, we use molecular formulas.
d. Molecular formulas are determined from empirical formulas
i. A compound's molecular formula tells you how many atoms are in one molecule of a compound.
ii. This is much more exact than the empirical formulas discussed above.
iii. In some cases, a compound's molecular formula is the same as its empirical formula
iv. A compound's molecular formula is a small whole-number multiple of its empirical formula
Quiz:
1. Which part of an ionic compound should be written first?
2. What does a Roman numeral in parentheses after a cation indicate?
3. What kind of formula tells the actual numbers of atoms in one molecule of a compound?
4. What kind of formula gives the relative numbers of atoms of each element in a compound?
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