Upon completion of this lesson, you should be able to:
These items are intended to get you to think about ideas related to those in the lesson. Take a few minutes to respond to them. Use a word processor or an html editor to record your responses and submit them to your teacher.
Throughout this section and indeed this unit, you will rely on your formula writing skills. These skills will be especially useful in the section on solution stoichiometry. You may want to book mark them if you anticipate using them frequently.
Writing chemical formulas for binary ionic compounds relies heavily on your ability to predict ion charges using the periodic table. For example, given the name sodium chloride, you would need to identify the ions in the compound and their individual charges.
Sodium is a Group 1 element, so its ions have a 1+ charge and chlorine is a Group 17 element, so its ions have a 1- charge. The chemical formula for sodium chloride is NaCl because the 1+ charge of a sodium ion is balanced completely by the 1- charge of a chloride ion and vice versa.
Now let's consider a crystal that consists of Group 2 and Group 17 ions like magnesium chloride:
Magnesium ions have a 2+ charge and chloride ions have a 1- charge. The lowest whole number ratio that produces a neutral or balanced formula is 1 Mg2+ : 2 Cl-; therefore, the chemical formula for the compound is MgCl2.
Your Nelson Chemistry Periodic Table provides ion charges on the left side of each cell for the transition elements. What do you notice about ion charges for elements such as titanium, chromium, iron, cobalt, and copper?
Ions of a certain elements can have more than one possible charge. Such elements are called multivalent species. For example, copper is multivalent - its ions can have either a 1+ or a 2+ ion charge (Cu+ or Cu2+). The more common ion charge is list first (on top). In the case of copper it is 2+.
The names of multivalent species contain a Roman numeral to indicate the charge. Copper(II) means a copper ion with a 2+ charge while copper(I) means a copper ion with a 1+ charge.
Writing a chemical formula for a compound that includes a multivalent species requires the same approach used for other binary ionic compounds. Consider the example of iron(II) chloride:
The iron(II) ion is represented by the symbol Fe2+. The Roman numeral II indicates the charge on the iron ion. The chloride ion is Cl-. Two chloride ions are needed to balance the 2+ charge on each iron ion, so the chemical formula is FeCl2.
Some ionic compounds contain polyatomic ions. A polyatomic ion consists of several atoms which together have gained or lost electrons to become stable. For example, the ammonium ion is a group of bonded atoms (one nitrogen and four hydrogen atoms) that have lost one electron. A nitrate ion is one nitrogen atom and three oxygen atoms that together have gained an electron.
The data sheet on the reverse side of your periodic table (or the back inside cover of your Nelson Chemistry text) lists most of the polyatomic ions which you will encounter in this course. You do not have to memorize the names, formulas, and charges of polyatomic ions, but through sheer use you will become familiar with many of them. Refer to these tables consistently and avoid guessing the information associated with these ions.
The formula writing procedure involves writing ion symbols and balancing their charges using numerical subscripts as needed.
The main difference in writing formulas for compounds that contain polyatomic ions is that you may need to place parentheses around a polyatomic ion symbol if its numerical subscript is greater than one. Some examples are:
sodium chlorite
- Sodium is in Group 1 and forms 1+ ions.
- sodium is Na+
- The -ite suffix in chlorite suggests that it may be a polyatomic ion. Find the name chlorite in the data table and list its formula and charge.
- chlorite is ClO2-
- Since the 1+ charge on the sodium ion is balanced by the 1- charge on chlorite, one of each ion is needed in the formula.
- The chemical formula is NaClO2.
iron(III) sulfate
- The Roman numeral III indicates that the iron ion has a 3+ charge.
- iron(III) is Fe3+
- Sulfate has the -ate ending so it is probably a polyatomic ion. A quick look at the data table confirms this
- sulfate is SO42-
- Since the charges are odd and even, you can apply the LCM method to balance the charges.
- Multiply charge values: 2 x 3 = 6
- Divide product by charge of iron: 6 ÷ 3 = 2
- The iron symbol gets the subscript 2: Fe2
- Divide product by charge of sulfate: 6 ÷ 2 = 3
- The sulfate symbol gets the subscript 3: (SO4)3. Note the use of parentheses to separate the 4 from the 3. Without them the polyatomic ion formula would show 43 oxygen atoms: SO43!
- The chemical formula for iron(III) sulfate is Fe2(SO4)3.
ammonium permanganate
- Ammonium is a polyatomic ion. A IUPAC convention is that cation names should end in -ium.
- The polyatomic ions table lists ammonium as a 1+ ion: NH4+.
- The -ate suffix in permanganate suggests that it may be a polyatomic ion. Locate the name permanganate in the data table and list its formula and charge:
- permanganate is MnO4-.
- Since the 1+ charge on the ammonium ion is balanced by the 1- charge on permanganate, one of each ion is needed in the formula.
- The chemical formula is NH4MnO4.
- Note that parentheses are not required because just one of each ion is needed to produce a neutral unit.
An ionic hydrate is a compound that has water as part of its crystalline structure. Bluestone contains five water molecules per copper(II) sulfate unit in the crystal. Its IUPAC name is copper(II) sulfate pentahydrate (CuSO4·5H2O). Anhydrous means without water, thus the name for the anhydrous form of this compound is simply copper(II) sulfate (CuSO4).
A hydrate like rock salt, sodium chloride monohydrate (NaCl·1H2O), can be formed when a salt water lakes dries up leaving behind solid salt. However, not all of the water evaporates. Some water molecules became part of the salt crystals giving rise to hydrated crystals.
In order to convert IUPAC names for ionic hydrates into chemical formulas, you will need to use a prefix system. A prefix changes the meaning of a term or a name. Memorize these prefixes and the numbers associated with them:
Example:
sodium thiosulfate pentahydrate
- Break the name down into three parts:
- the -ate ending in thiosulfate suggests that the anion is polyatomic: S2O32-
- the cation - sodium - is a member of Group 1 and has a 1+ charge: Na+
- the label pentahydrate indicates the presence of five water molecules per formula unit of sodium thiosulfate: ·5H2O.
- Balance the ion charges:
- two Na+ ions are needed for each S2O32- ion.
- Put all this information together to get:
- Na2S2O3·5 H2O
Notice that a dot separates water from the rest of of the formula.
A binary molecular compound consists of atoms from two different elements. Carbon dioxide is a good example. It consists of carbon atoms and oxygen atoms.
To write a chemical formula for a binary molecular compound:
Example: diboron hexahydride.
Diboron means that the molecule contains two boron atoms (B2) and hexahydride means the molecule contains six hydrogen atoms (H6) . Bringing the two symbols together in the order they appear in the name gives: B2H6.
Before the establishment and widespread use of IUPAC rules, a compound could have been known by several different names. In many cases, the names revealed nothing about the composition of the compound. As you can imagine, this had the potential to create confusion and miscommunication among chemists.
Today, most compounds are known by their IUPAC names. Nonetheless, you will still see chemicals like NH3, H2O2, and O3 labelled with their trivial names ammonia, peroxide, and ozone instead of nitrogen trihydride, dihydrogen dioxide, and trioxygen. The same thing happens in real life - few people know the real names of recording artists like Shaggy (Orville Richard Burrell) and Sting (Gordon Sumner).
Since some compounds are also better known by their trivial names, you will need to do some memorizing. Here are the ones you should memorize:
Certain compounds (mainly those that contain carbon) are named using different sets of IUPAC rules. You will learn about these in the organic chemistry unit. For now it is helpful to memorize these names and formulas:
Due to the special nature of hydrogen atoms, the IUPAC rules are not strictly followed for certain classes of hydrogen compounds. Group 16 and Group 17 hydrogen compounds tend to be named as if they are ionic compounds. For example, H2S is hydrogen sulfide and HCl is hydrogen chloride.
If the word acid appears in the name of a compound, you should automatically think about applying a special set of formula writing rules. For example, given the name hydroiodic acid, you should immediately realize the need to apply a different set of rules.
In order to convert an acid name to a chemical formula:
Examples:
Some acid names deviate from the rules because of historical usage. The four cases you should memorize are:
The syllables in italics are added because of the way these names have been pronounced in the past.