Naming Compounds-Rules PDF

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This document explains all the rules a student should remember when naming chemical compounds...

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Naming Compounds The naming of elements is relatively easy since you use the elements name without changing it (Example: Al is aluminum). When elements combine to form compounds, the naming of the compound is derived from the names of the elements in the compound. Since there are an incredibly large number of combinations of elements, naming compounds requires a system to express not only the elements that join together, but also the ratio of atoms (or ions) of each element. This document is designed to help you learn how to write the name of a compound given the symbolic formula of the compound and how to write the symbolic formula of the compound given its name. For example, you should be able to write sodium chloride given NaCl or write Fe2O3 given iron (III) oxide. First Step The first step in naming a compound is to identify what type of compound you are naming. Compounds can be divided into two broad categories: ionic compounds and covalent compounds (or molecular compounds). Ionic compounds can be broken down further into binary ionic compounds and polyatomic ionic compounds. Covalent compounds can also be broken down into three categories: binary covalent, acids, and organics. Acids can be broken now even further into binary acids and oxyacids. And—you guessed it—each category has its own naming system. To better see the classifications, look at the following diagram. Compounds Ionic (Metal & Nonmetal) (Metal & Polyatomic) (Polyatomic & Nonmetal) (Two Polyatomics)

Binary (only 2 elements)

Polyatomic (3 or more)

Covalent (Nonmetals, including H)

Binary (Only 2 elements)

Binary (Only 2)

Acid (Starts with H)

Organic (C, H, O, N)

Oxyacid (Has O)

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To determine the correct category for any compound, you must examine the formula for that compound. Binary Ionic If the formula contains a metal, then the compound is ionic (other than alloys, which we don’t name in this course). If the metal is joined with only one other type of element, then it is a binary ionic compound. Examples of binary ionic compounds include NaCl, MgF2, Fe2O3, and Li2O. Note that binary means that it has only two types of elements in the formula (not just two atoms/ions). Polyatomic Ionic If the formula contains a polyatomic ion (anion or cation or both), it will be a polyatomic ionic compound. If you have trouble recognizing polyatomic ions, you can also recognize many polyatomic ionic compounds by noticing the first element listed is a metal and the other two are nonmetals. Examples of polyatomic ionic compounds include NaCN, Mg(NO3)2, Li3PO4, NH4Cl, and NH4NO3. (Most polyatomic ionic compounds have a metal ion as the cation; however, NH4+ is also a common polyatomic cation.) Binary Covalent If the formula contains only two types of nonmetals, then it will be a binary covalent compound. Examples include CO, CO2, N2O5, P4O10, and SO3. Note that many binary covalent compounds may look like polyatomic ions; however, they will NOT have a charge. Binary Acid For this course, if the formula contains an H as the first element listed, then you have an acid. If there is only one other type of element present, then you have a binary acid. Examples include HCl and H2S. Oxyacid For this course, if the formula contains an H as the first element listed, then you have an acid. If there is another element plus oxygen present, then you have an oxyacid. Examples include HNO3, HClO4, and H3PO4. (Many oxyacids contain H with a polyatomic anion.) Organic Compound If the formula contains only C and H or a combination of C, H, O, and/or N, you probably have an organic compound. Having multiple C atoms within the compound is also a good tip that you have an organic compound.

Each category has its own set of rules for naming compounds. The rules are designed to allow us to know the type and number of each element in a compound. Sometimes (especially with ionic compounds) the number of each element is determined from known properties of the element and not from the name itself.

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For each category, I will give you the summary for how to convert formulas into names and names into formulas. Both conversions have challenges that will take some practice to master. Binary Ionic Compounds You can use the following system for transforming a binary ionic compound’s formula into a written name. metal name (metal charge as roman numeral) root of anion name “-ide” Do NOT write the metal charge if the metal has only one possible charge. Examples: NaCl is sodium chloride Al2O3 is aluminum oxide FeO is iron (II) oxide Fe2O3 is iron (III) oxide So… why is the system designed this way… The suffix “-ide” is used to distinguish the anion from its elemental form. Writing NaCl as sodium chlorine can get confusing… are you talking about sodium ions and “chlorine” ions or sodium metal and chlorine gas. The “-ide” ending indicates anion. The metal name is left alone since its pairing with an anion indicates that it would be a cation. The formula for a binary ionic compound must express the small whole number ratio of elements in the compound. Since the ratio is expressed in the formula (subscripts) and the overall charge of the compound must be zero, the charges of the ions in the compound can be determined from the ratio. The naming system of binary ionic compounds must contain enough information in order that the charges on the ions can be determined, especially since many elements can form ions with different charges (Example: Iron can for a +2 and +3 ion). Determining the charge on an ion must be done using the Periodic Table and the predictable charges for some elements. The following elements form ions with only one specific charge: Group 1A metals form +1 ions, Group 2A form +2 ions, Group 3B form +3 ions, Ag forms a +1 ions, Zn and Cd from +2 ions, and Al, Ga, and In form +3 ions. While the list seems tough to remember, it does form a pattern on the Periodic Table.

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+1 +2 +3

+1 +2 +3

While the above ions have ONLY one possible charge, there are many non-metals that will have predictable charges when they are anions in a binary ionic compound. These are the ones in the figure below.

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It is these charges that give binary ionic compounds the mole ratios seen in their formulas. NaCl is one Na+ ion with one Cl— ion. MgO is also a one to one ratio, but it is an Mg2+ and an O2— that have come together. Li2O is two Li+ ions with one O2— ion. In all cases, the total charge of an ionic compound is zero. Transforming a binary ionic compound name into a formula requires that you determine what the mole ratios are based on the charges of the ions. Here are some examples. Magnesium fluoride contains Mg2+ ions and F— ions. In order to get a zero overall charge, there must be one Mg2+ ion for every two F— ions, thus the formula is MgF2. Calcium nitride contains Ca2+ ions and N3— ions. In order to get a zero overall charge, there must be three Ca2+ ions and two N3— ions, thus the formula is Ca3N2 Manganese (IV) oxide contains Mn4+ ions and O2— ions. In order to get a zero overall charge, there must be one Mn4+ ions and two O2— ions, thus the formula is MnO2 Polyatomic Ionic Compounds You can use the following system for transforming a binary polyatomic compound’s formula into a written name. metal name (metal charge as roman numeral) polyatomic name Do NOT write the metal charge if the metal has only one possible charge. Examples: NaNO3 is sodium nitrate Fe2(SO3)3 is iron (III) sulfite NH4Cl is ammonium chloride (NH4)3PO4 is ammonium phosphate So… why is the system designed this way… The system follows the same rules as binary ionic compound except that polyatomic ions have specific names already established. You do not have to add any suffix to indicate the difference between the element and the ion since all polyatomic ions would NOT have elemental names. Everything else follows from the same reasons seen in binary ionic compounds. The polyatomic ion names you need to know are in the following table.

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Transforming a polyatomic ionic compound name into a formula requires that you determine what the mole ratios are based on the charges of the ions, just like with binary ionic compounds. If the polyatomic ion needs to have a mole ratio greater than one, then enclose the polyatomic ion in parenthesis and put the ratio number outside the parenthesis. Here are some examples. Magnesium nitrate contains Mg2+ ions and NO3— ions. In order to get a zero overall charge, there must be one Mg2+ ion for every two NO3— ions, thus the formula is Mg(NO3)2. Calcium phosphate contains Ca2+ ions and PO43— ions. In order to get a zero overall charge, there must be three Ca2+ ions and two PO43— ions, thus the formula is Ca3(PO4)2.

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Ammonium sulfate contains NH4+ ions and SO42— ions. In order to get a zero overall charge, there must be two NH4+ ions and one SO42— ion, thus the formula is (NH4)2SO4. To make it easier to remember some of the polyatomic ions, knowing a few rules helps reduce the number of ions you must memorize. The two main rules deal with hydrogen being included in the anion and the different number of oxygen atoms with the same central element. For example, PO43— is phosphate. Adding one hydrogen to phosphate causes the charge to increase (become more positive) by one and you add the word “hydrogen” before “phosphate.” Adding a second hydrogen causes an additional increase in charge and you add the numeric prefix “di-“ to the “hydrogen” in the name. This same rule applies to carbonate, sulfate, and sulfite, so if you memorize the base name, you can modify it with “hydrogen” when needed. For changes in the number of oxygens, you can see the pattern in the polyatomic anions containing chlorine. ClO— ClO2— ClO3— ClO4—

Hypochlorite Chlorite Chlorate Perchlorate

“hypo-“ root “-ite” root “-ite” root “-ate” “per-“ root “-ate”

This same pattern is used for polyatomic anions that have the same root element and varying numbers of oxygen atoms bonded to it. If you memorize the “-ate” anions, you can figure out the rest of them from that one. One less oxygen than the “-ate” form… change “-ate” to “-ite” Two less oxygens than the “-ate” form… add “hypo-“ and use “-ite” One more oxygen than the “-ate” form… add “per-“ to the “-ate” name This can be used with nitrate, chlorate, and sulfate. Using these rules, you now have to memorize only 18 polyatomic ions instead of 28, and the ones that are very similar in name don’t get as confusing.

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Binary Molecular Compounds You can use the following system for transforming a binary molecular compound’s formula into a written name. Prefix- elemental name prefix- root name “-ide” Do NOT use “mono-“ for the first prefix. If prefix ends in “a” or “o” and root starts with “o” then only write one “o”. (there are exceptions to this rule, put typically only one vowel between prefix and root) Examples: CO2 is carbon dioxide N2O4 is dinitrogen tetroxide SF6 is sulfur hexafluoride CO is carbon monoxide So… why is the system designed this way… Molecular compounds, unlike ionic compounds, can have large numbers of differing ratios between atoms forming the compounds. For example, N2O, NO, NO2, N2O4, and N2O5 are all existing combination for nitrogen and oxygen. The naming system must be detailed enough to describe all the possible combinations accurately (and we can’t use charges to determine ratios like we do in ionic compounds). So, numeric prefixes are used in the naming system. The numeric prefixes you will need to know are as follows. mono = 1 di = 2 tri = 3 tetra = 4 penta = 5

hexa = 6 hepta = 7 octa = 8 nona = 9 deca = 10

The only time you do not use a numeric prefix is when you have “mono-“ on the first element. You do not have monocarbon monoxide, it is just carbon monoxide. This also brings up the point that when two “o”s come together, you drop one of the “o” from the name (it is NOT carbon monooxide).

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Binary Acids You can use the following system for transforming a binary acid’s formula into a written name. “hydro-“ root name “acid” Binary acids are named as binary ionic compound when they are in solid state. Only use above naming system when acid is in aqueous state. Examples: HCl is hydrochloric acid H2S is hydrosulfuric acid So… why is the system designed this way… The “hydro-“ prefix indicates the binary nature of the acid. It distinguishes hydrochloric acid (HCl) from chloric acid (HClO3). Since hydrogen always has a plus one charge, naming prefixes are not required (since number of hydrogen atoms can be determined from charge on anion).

Oxyacids You can use the following system for transforming an oxyacid’s formula into a written name. root name with correct suffix plus “acid” The correct suffix is based on the polyatomic anions name. Anion names ending in “-ate” are changed to “-ic.” Anion names ending in “-ite” are changed to “-ous.”

Examples: HNO3 is nitric acid H2SO3 is sulfurous acid So… why is the system designed this way… Some polyatomic anions have prefixes (ClO— is hypochlorite), thus no prefixes are used with oxyacids. And, to distinguish the acid from any other compound that contains the polyatomic anion, the suffix is changed. Again, hydrogen always has a +1 charge in acids, so no naming prefixes are needed since the number of hydrogens in the acid can be determined from the anions charge.

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Organic Compounds You can use the following system for transforming an organic compound’s formula into a written name. (We will only be learning the most basic names for organic compounds. Chapter 25 in your textbook gives a much more detailed description of how to name organic compounds.) Base name (based on number of carbon atoms) suffix (type of bonding and/or functional groups) This chapter only describes alkanes (all single bonds between carbons) and simple functional groups. For purposes of exam material, you would only be required to name only alkanes with NO functional groups. However, you should be able to identify a functional group in a molecule. Examples: CH4 is methane C3H8 is propane C8H18 is octane So… why is the system designed this way… Organic compounds can become very complex, and the naming system for the more complex organic compounds will be addressed during an organic chemistry class. I will only require you to learn the naming system for some basic organic compounds since some of those compounds will be used during the semester. The base name for organic compound comes from the number of carbon atoms in the molecule (this is a simplification, but it will work for this semester). The base names you must know are as follows. meth = 1 eth = 2 prop = 3 but = 4 pent = 5

hex = 6 hept = 7 oct = 8 non = 9 dec = 10

Note that these are similar to, but NOT identical to, the prefixes for molecular compounds. The functional groups that you are expected to recognize are the four given in the textbook. Key to recognizing functional groups is the arrangement of the atoms in that part of the molecule. (See Table 2.8 on next page.)

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The notation above has the bond going into the group is typically coming from a carbon atom. For example, Ethanol is an alcohol with only one carbon. What makes it an alcohol is the OH group off of the carbon atom.

H

H

H

C

C

H

H

O

H

The aldehyde is similar to the alcohol in that the O atom is a key characteristic. The O atom in an aldehyde is NOT bonded to a H atom, but it is double bonded to the carbon. Ethanal, also known as acetylaldahyde, also has two carbon atoms, but two less hydrogen atoms since the oxygen is double bonded to the carbon.

H H

C H

O C H

A carboxylic acid contains both an oxygen double bonded to a carbon and an OH group bonded to that same carbon atom. The amine group is characterized by the nitrogen atom being singled bonded to a carbon atom (similar to OH in alcohol).

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SUMMARY Knowing an example or two can be the best way to remember the rules! Binary Ionic Compounds metal name (metal charge as roman numeral) root of anion name “-ide” Do NOT write the metal charge if the metal has only one possible charge. Examples: NaCl is sodium chloride Al2O3 is aluminum oxide FeO is iron (II) oxide Fe2O3 is iron (III) oxide Polyatomic Ionic Compounds metal name (metal charge as roman numeral) polyatomic name Do NOT write the metal charge if the metal has only one possible charge. Examples: NaNO3 is sodium nitrate Fe2(SO3)3 is iron (III) sulfite NH4Cl is ammonium chloride (NH4)3PO4 is ammonium phosphate Binary Molecular Compounds Prefix- elemental name prefix- root name “-ide” Do NOT use “mono-“ for the first prefix. If prefix ends in “a” or “o” and root starts with “o” then only write one “o”. Examples: CO2 is carbon dioxide N2O4 is dinitrogen tetroxide SF6 is sulfur hexafluoride CO is carbon monoxide Binary Acids “hydro-“ root name “acid” Binary acids are named as binary ionic compound when they are in solid state. Only use above naming system when acid is in aqueous state. Examples: HCl is hydrochloric acid H2S is hydrosulfuric acid

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Oxyacids root name with correct suffix “acid” The correct suffix is based on the polyatomic anions name. Anion names ending in “-ate” are changed to “-ic.” Anion names ending in “-ite” are changed to “-ous.” Examples: HNO3 is nitric acid H2SO3 is sulfurous acid Organic Compounds Base name (based on number of carbon atoms) suffix (type of bonding and/or functional groups) I will require that you get names for only alkanes. I want you to be able to recognize functional groups, but not required to name them. Examples: CH4 is methane C3H8 is propane C8H18 is octane

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