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Chemical Names and Formulas

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Chemistry has its own language. Chemists communicate in this language to describe the millions of known compounds. This communication depends on a standard system of naming and writing the formulas for compounds. Chemists formed a group to standardize the system of naming and called themselves the International Union of Physical and Applied Chemists, or IUPAC. In this lesson we will build on your knowledge of naming compounds. It is very important that we are able to write the chemical formula for and name compounds if we are to understand chemical reactions. Some of this may be a review from earlier science courses. Work out each familiar example and exercise as review.

Chemical Formulas

A chemical formula is a shorthand method to represent compounds that uses the elements' symbols and subscripts. The chemical formula gives the following information:

• The different elements in the compound.

• The number of atoms of each element in the compound.

For example: Water H2O

The compound water contains hydrogen, H, and oxygen, O, atoms. The subscripts that follow each element indicate the number of atoms of that element in the compound. The subscript following the symbol for hydrogen is 2, indicating there are two hydrogen atoms in each water molecule. Notice there is no subscript following the symbol for oxygen; this shows that there is only one atom of oxygen in a water molecule. Chemists do not write the number one as a subscript when only one atom of that kind of element exists in the compound.

Example: Calcium phosphate Ca3(PO4)2

The compound contains calcium, Ca, phosphorous, P, and oxygen, O. The subscript after the symbol for calcium indicates there are 3 atoms of calcium in one molecule. The subscript, 2, outside the bracket indicates that every subscript inside the bracket is to be multiplied by 2. This means there are 2 x 1 = 2 atoms of phosphorous and 2 x 4 = 8 atoms of oxygen in each molecule of Ca3(PO4)2.

Ionic Compounds

An ion is a charged particle. An ion is formed when a neutral atom gains or loses electrons. When an atom loses electrons the number of positively charged protons exceeds the number of negatively charged electrons, resulting in a positively charged particle. Positively charged ions are called cations. A negative ion, called an anion, is formed when an atom gains one or more electrons. Since electrons are negatively charged, the resulting ion has an overall negative charge.

Ionic compounds are formed when two or more oppositely charged ions are attracted to each other. This chemical attraction is called a chemical bond. An ionic bond is formed when a negatively charged ion is attracted to a positively charged ion. Ions combine together so that their charges add to zero.

Ionic compounds are usually made of metal and non-metal ions. For example:

NaCl sodium and chlorine

Fe2O3 iron and oxygen

CuSO4 copper and sulfate

Ca3(PO4)2 calcium and phosphate

We can use the periodic table to predict the charges of many atoms. The alkali metals, group 1, will each tend to lose 1 electron to produce ions with a 1+ charge. The alkaline earth metals, group 2, tend to form ions with a 2+ charge. Many of the transition metals, because of their electron arrangement, tend to form more than one ion charge. The chalcogens, group 6, tend to form 2– ions and the halogens, group 7, tend to form 1– ions.

Naming Binary Ionic Compounds

A binary compound contains two different kinds of elements. There can be more than one atom of each element in a binary compound. Binary ionic compounds usually contain one kind of metal ion combined with one kind of non-metal ion. Metal ions are usually positively charged and non-metal ions are usually negatively charged.

When naming an ionic compound from its formula follow the rules below:

The cation (positive ion) is named first, followed by the anion (negative ion).

• Write the name of the metallic element (positive ion).

• Write the name of the non-metallic element (negative ion) and change the ending to "ide".

Example 1. Write the name of NaCl.

• Step 1: Name the first element.

Na = sodium

• Step 2: Name the second element and change the ending to "-ide".

Cl = chlorine = chloride

The name of the compound is sodium chloride.

Example 2. Write the name of Mg3P2.

• Step 1: Name the first element.

Mg = magnesium

• Step 2: Name the root of the second element and add "ide".

P = phosphorous = phosphide

The name of the compound is magnesium phosphide.

Practice: Write the names for each of these formulas:

1. CaI2

2. Na4C

3. SrBr2

4. Li3N

5. BaS

Writing Binary Ionic Formulas, Given the Compound's Name

Remember the formula contains the symbols of the elements and the subscripts indicating the number of atoms of each element.

There are two methods for determining the formula of a compound, but the following points must hold true:

• The formula must have cation first (positively charged ion, usually the metal) followed by the anion (negatively charged ion, the non-metal).

• The sum of the charges of the ions must be zero. That is, the number of positive charges must equal the number of negative charges.

• You may not change the charge of the ions to make the ion charges equal zero.

Method 1 – Lowest Common Multiple

• Write the symbols for the ions involved.

• Determine the lowest whole number ratio that will give an overall net charge of zero. That is, the number of positive charges must equal the number of negative charges.

Example: Write the formula for aluminum oxide.

• Step 1: Write the ions and their charges from the ion chart.

Al3+ and O2–

• Step 2: Determine the lowest common multiple.

In this case, the lowest common multiple for 3 and 2 is 6. To get 6 positive charges, we would need 2 aluminums (2 x 3+ = 6+). To get 6 negative charges we need 3 oxygens (3 x 2– = 6–). We make these numbers the subscripts for each.

The formula for aluminum oxide is Al2O3.

Practice: Write the formulas for each of these:

1. strontium chloride

2. aluminum iodide

3. calcium sulfide

4. aluminum nitride

5. sodium oxide

Writing Formulas for Ionic Compounds Having Metal Ions With Multiple Charges (Multivalent).

Most of the transition metals have more than one possible ion charge. For example,

Ion Possible Ion Charges

Copper 1+, 2+

Iron 2+, 3+

Cobalt 2+, 3+

Tin 2+, 4+

We use Roman numerals following the metal ion's name to indicate the ion's charge. For example,

Copper (I) = Cu+

Copper (II) = Cu2+

Iron (II) = Fe2+

Iron (III) = Fe3+

Example: Write the formula for iron (III) chloride

• Step 1: Write out the ions.

iron (III) and chloride

Fe3+ and Cl–

• Step 2: Balance the charges.

FeCl3

Example: Write the formula for lead (IV) sulfide.

• Step 1: Write the ions.

lead (IV) and sulphide

Pb4+ and S2–

• Step 2: Balance the charges.

PbS2

Practice: Write the formulas for each of these:

1. iron (II) chloride

2. copper (I) iodide

3. lead (IV) sulfide

Naming Binary Ionic Compounds Having Metal Ions With Multiple Charges

We name these compounds in a very similar manner as those ions with a single ion charge, except we must determine the charge on the metal ion.

To determine the charge on the metal ion,

• Write the name of the ions.

• Multiply the charge of the anion by its subscript.

• Divide this number by the subscript of the metal ion. The result is the charge on the metal ion.

Example: Write the name for CoBr2.

• Step 1: Write the names of the ions. (including the charge of the anion)

cobalt bromide

Co? Br1–

• Step 2: Multiply the charge of the bromide by its subscript then divide by the subscript for cobalt.

The charge on the bromide is 1– so, 1 x 2 = 2 (we will ignore the charge)

The subscript for cobalt is one (CoBr2). This means the charge on the cobalt is 2+ (since the total positive charge and total negative charge must add to zero).

• Step 3: Write the name, indicating the charge of cobalt (2+) using roman numerals.

cobalt (II) bromide

Example: Write the name for Fe2O3.

• Step 1: Write the names of the ions.

iron oxide

Fe? O2–

• Step 2: Multiply the charge of the oxide by its subscript then divide by the subscript for cobalt.

The charge on the oxide is 2– so, 3 x 2 = 6 (we will ignore the charge)

The subscript for iron is 2. This means the charge on the iron is 6 ÷ 2 = 3+.

• Step 3: Write the name(writing the charge of iron (3+) in roman numerals)

Fe3+ O2–

iron (III) oxide

Practice: Write the names for each of these:

1. Ni2O3

2. PbO

3. CuBr2

Writing Formulas for Compounds Containing Polyatomic Ions

Some ions are composed of several atoms joined covalently. These are called polyatomic ions (poly = many). Examples of common polyatomic ions are listed below.

NH4+ ammonium ion 1+

SO42– sulfate ion 2–

PO43– phosphate ion 3–

NO3– nitrate ion 1–

C2H3O2– acetate ion 1–

Although the polyatomic ions have more than one atom, we will name these like binary compounds. The charge for polyatomic ions is for the whole group of atoms not just for the atom written last. DO NOT change the subscripts of polyatomic ions; if you change the subscripts you change the identity of these ions.

When indicating the presence of more than one polyatomic ion in a compound, we use parenthesis around the polyatomic ion, followed by its subscript. For example, the compound Al(C2H3O2)3 has one aluminum ion and 3 acetate ions. Placing the acetate ion in parenthesis and following it with the subscript 3 indicates there are 3 acetate ions.

Example: Write the formula for iron (III) sulfate

• Step 1: Write out the ions.

iron (III) and sulfate

Fe3+ and SO42–

• Step 2: Balance the charges. Use brackets around polyatomic ions when there is more than one present.

Fe2(SO4)3

Example: Write the formula for lead (IV) nitrate

• Step 1: Write the ions.

lead (IV) and nitrate

Pb4+ and NO3–

• Step 2: Balance the charges.

Pb(NO3)4

Practice: Write the formulas for each of the following compounds:

1. sodium sulfate

2. magnesium nitrate

3. ammonium carbonate

4. calcium hydroxide

Writing Names for Compounds Containing Polyatomic Ions

Example: Write the name for KNO3.

• Step 1: Identify the cation.

K+, from group 1, does not have a variable charge, so we don't use a Roman numeral.

The name of the ion is potassium.

• Step 2: Identify the anion.

NO3– is the nitrate ion.

• Step 3: Write the name of the cation first, followed by the anion.

The name of the compound is potassium nitrate.

Example: Write the name of Cu3(PO4)2.

• Step 1: Identify the cation.

Copper, but Copper is one of the ions with a variable charge so we must use a Roman numeral to indicate the charge on the ion. To determine the charge of the copper ion, we must look at the anion.

• Step 2: Identify the anion.

The anion is the phosphate ion (PO43–). The parenthesis followed by the number 2 indicates there are 2 phosphate ions in this compound. From our chart we know each phosphate has a charge of 3– for a total charge of 2 x 3 = 6.

• Step 3: Determine the charge of the cation.

The total charge of the anions is 6, so the charge of the copper is 6 ÷ 3 (subscript on the Cu) = 2.

• Step 4: Write the name of the cation first, followed by the anion.

The name of the compound is copper (II) phosphate.

Example: Write the name of Hg2Cl2.

• Step 1: Identify the cation.

The cation is mercury. Mercury is a special case for a metal ion. The mercury (I) ion is like a polyatomic ion. The two mercury atoms act together for a total 2+ charge. The mercury (I) ion is Hg22+.

• Step 2: Identify the anion.

The anion is the chloride ion.

• Step 3: Write the name of the cation first, followed by the anion.

The name of the compound is mercury (I) chloride.

Example: Write the name of NH4SCN.

• Step 1: Identify the cation.

The cation is NH4+, the ammonium ion.

• Step 2: Identify the anion.

The anion is SCN–, the thiocyanate ion.

• Step 3: Write the name of the cation first, followed by the anion.

The name of the compound is ammonium thiocyanate.

Practice: Write the names for each of these formulas:

1. Ni(NO3)2 6. Ag2CrO4

2. MgSO4 7. PbCO3

3. KMnO4 8. Sn(SO4)2

4. Fe(OH)3 9. Na2HPO4

5. Ca(C2H3O2)2 10. NH4NO2

Naming Binary Molecular (Covalent) Compounds

Non-metals tend to combine chemically by sharing electron pairs. These bonds are known as covalent bonds. Neutral compounds made of atoms joined covalently are called molecular or covalent compounds.

We name covalent compounds differently than ionic compounds. We must indicate the number of each element by adding a prefix in front of the element's name.

The prefixes are:

one = mono six = hexa

two = di seven = hepta

three = tri eight = octa

four = tetra nine = nona

five = penta ten = deca

Naming covalent compounds:

• Step 1: Name the first element in full using a prefix only when there are two or more of that element. That is, omit "mono" if only one of that element is in the compound.

NO is nitrogen monoxide, but N2O is dinitrogen monoxide

• Step 2: Name the second element and end in "ide". Use prefixes to indicate the number of that element (including mono).

• Step 3: Write the name of the compound writing the substance found more to the left on the periodic table first.

Example: Write the name for CO2.

This is a covalent compound since it is made of two non-metal atoms.

• Step 1: Name the first atom with prefixes.

There is only one carbon atom. We omit the "mono-" for the first element, so the first part of the name is carbon.

• Step 2: Name the second element using prefixes and end in "ide".

The second element is oxygen. There are two oxygen atoms, so the second part of the name is dioxide.

• Step 3: Write the name of the compound writing the substance found more to the left on the periodic table first.

The name of CO2 is carbon dioxide.

Example: Write the name for N2O4.

• Step 1: Name the first atom with prefixes.

There are two nitrogens, so the first part of the name is dinitrogen.

• Step 2: Name the second element using prefixes and end in "ide".

The second element is oxygen. There are four oxygen atoms, so the second part of the name is tetraoxide.

• Step 3: Write the name of the compound.

The name of N2O4 is dinitrogen tetraoxide.

Practice: Write the names for each of these formulas:

1. NI3 4. P4O10

2. CO 5. N2O5

3. SF6

Writing Formulas for Binary Molecular Compounds

Writing formulas for binary covalent compounds involves the following steps:

• Step 1: Write the symbol for the first element followed by the subscript indicated by the prefix.

• Step 2: Write the symbol of the second element followed by the prefix indicated by its prefix.

Do not reduce!

Example: Write the formula for dinitrogen monoxide.

• Step 1: The first symbol is N and its subscript is 2.

• Step 2: The second symbol is O and its subscript is 1.

The formula for dinitrogen monoxide is N2O.

Example: Write the formula for sulphur hexafluoride.

• Step 1: The first symbol is S and its subscript is 1.

• Step 2: The second symbol is F and its subscript is 6.

The formula for sulphur hexafluoride is SF6.

Practice: Write the formulas for each of these names:

1. dichlorine heptaoxide

2. carbon disulphide

3. silicon tetrafluoride

4. diphosphorous pentaoxide

5. tetraarsenic decaoxide

Diatomic Molecules

Some elements do not exist as single atoms. These elements exist as pairs of atoms joined covalently, called diatomic molecules. The elements that exist as diatomic molecules are hydrogen gas (H2), oxygen gas (O2), fluorine gas (F2), chlorine gas (Cl2), bromine gas or liquid (Br2), solid iodine (I2), nitrogen (N2) and astatine (At2).

When oxygen gas, hydrogen gas, etc. is used the formula will be O2, H2, etc.

Practice: Mixed Compounds

1. For each of the following, correctly write the formula in the space provided.

a. lead (IV) iodide _____________

b. carbon tetraoxide _____________

c. calcium bromate _____________

d. copper (II) phosphide _____________

e. dinitrogen tetraoxide _____________

f. ammonium sulfide _____________

g. aluminum sulfite _____________

2. For each of the following, correctly write the name in the space provided.

a. SiCl4 __________________________

b. Li2O __________________________

c. Sn(ClO)2 __________________________

d. (NH4)2O __________________________

e. PCl5 ________________________________________

f. Ba(OH)2 ________________________________________

g. AgCl __________________________

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