Notes

Compound Names:

In the early days of chemistry, scientists used common names, such as salt, baking soda, and potash to identify chemical compounds. However, they eventually realized that it was impossible for anyone to memorize the names of the increasing number of compounds being discovered. So, they devised a system in which compounds are named according to their chemical composition.

This system is based on a set of rules established by the International Union of Pure and Applied Chemistry (IUPAC). These rules are like the rules of soccer or a board game you play with friends and family. Without rules, you wouldn't understand the game correctly.

In this lesson, you will study the rules for naming chemical compounds and review how to write formulas for different types of compounds.
An ionic compound contains negatively and positively charged ions that combine in certain proportions such that the net charge of the compound is zero. The toothpaste you use every morning, for example, probably contains an ionic compound called sodium fluoride (NaF), which consists of a sodium ion (Na+) and a fluoride ion (F-). Sodium and fluoride ions are called monatomic ions because they consist of a single atom and have a charge.

Metals in Groups 1A, 2A, and 3A of the periodic table form positively charged cations. Nonmetals from Groups 5A, 6A, and 7A, on the other hand, form negatively charged anions. Recall that metals tend to lose electrons and nonmetals tend to gain electrons to achieve full valence shells. According to the octet rule, the filled valence shell of an atom should contain eight electrons, with the exception of hydrogen and helium, whose valence shells contain only two electrons.
Cations within a specific group in the periodic table have a similar charge that is equal to the group number. For example, the metals in Group 1A lose one electron when they react, to form cations with a 1+ charge; those in Group 2A lose two electrons, and form cations with a 2+ charge.

Some transition metals, which belong to Groups 1B to 8B, form more than one cation. The charges of these cations are determined by the number of electrons lost by the element. For example, iron (Fe) can form two cations, Fe2+ and Fe3+. Fe2+ has lost two electrons and has taken on a charge of 2+, Fe3+ has lost three electrons and takes of a charge of 3+.

A cation is generally named using the element that it originates from, followed by the word ion. For example, Mg2+ is a magnesium ion. For elements that can have more than one charge, each cation is represented by the element's name followed by its charge, written in Roman numerals within parentheses.
The charge of anions can also be determined by their position in the periodic table. Just as cations lose electrons, anions in the Main group (A groups) gain electrons to attain the same number of electrons as a noble gas. For example, phosphorus needs to gain three electrons to have the same number of electrons as argon. Likewise, chlorine needs to gain one electron to have the same number of electrons as argon.

The name of an anion is the element's name with the
suffix –ide added. For example, the bromine ion is named bromide and the oxygen ion is oxide.
Since cations and anions have opposite charges, they attract each other and combine to form ionic compounds. When two monatomic ions combine, they form a binary ionic compound. For example, the monatomic cation sodium combines with the monatomic anion fluoride to form the binary ionic compound sodium fluoride, which is toothpaste. Likewise, a sodium cation combines with a chloride anion to form the binary ionic compound sodium chloride, common table salt.

The names of binary ionic compounds begin with the cation's (metal's) name followed by the anion's (nonmetal's) name. So, the combination of the sodium cation and chloride anion produces the binary ionic compound sodium chloride. Similarly, iron (II) and iron (III) cations combine with chloride anions to form iron (II) chloride and iron (III) chloride, respectively. There are two methods for naming a binary ionic compound: the classical method and the Stock system. Let's see what they are.
In the classical system, the cation is named using a root word with a suffix. The suffix –ous is used for the cation with the lower charge and –ic is used for the cation with the higher charge. For example, the root word for iron is ferr. So, the Fe2+ cation is named as the ferrous ion, and the Fe3+ cation is named as the ferric ion. When a ferric ion combines with a chloride anion, it forms a compound named ferric chloride. However, this classical naming system does not work for elements with more than two ions because it doesn't indicate the numerical value of the charge.

To indicate the numerical value of a charge, you can use the Stock system. In this system, which you have already seen, the charge is written in Roman numerals within the parentheses. For example, Cr2+ is written as chromium (II) and Cr3+ is written as chromium (III). So, when chromium (III) combines with the fluoride anion, it forms chromium (III) fluoride.
The name of the compound Cu3N2 is copper (II) nitride because it contains a copper (II) cation and a nitride anion.
Cations and anions form neutrally charged ionic compounds when they bond in such a way that their charges cancel each other. These ionic compounds are expressed in chemical formulas. These formulas indicate the number of atoms that are needed by each element to form a neutrally charged compound. The atoms are expressed in the simplest whole ratio. For example, the chemical formula for potassium chloride is KCl. The potassium cation has a charge of 1+ and the chloride anion has a charge of 1-. So, when one atom of potassium combines with one atom of chlorine, their charges cancel each other to form a neutral compound. This means that the ions in the compound combine in the simplest whole ratio 1:1. Since the ratio is 1:1, there are no subscripts in the formula.

Not all ionic compounds occur in a 1:1 ratio. For example, the atoms in calcium fluoride (CaF2) bond in the ratio 1:2. We know this because the absence of a subscript next to Ca indicates the presence of one calcium atom. The subscript 2 indicates the presence of two fluorine atoms. Now, a calcium cation has a charge of 2+ and a fluoride anion has a charge of 1-. So, when one calcium cation combines with two fluoride anions, their charges cancel each other to produce a neutral compound.
Now that you know that chemical formulas indicate the ratio in which cations and anions combine, let's see how we can come up with a formula of a compound using the charges of the ions. Take the example of aluminum oxide. We'll write the formula using the "crisscross" method.

The first step in the crisscross method is to write the symbols of the elements that form the compound. We write the symbol of aluminum followed by the symbol of oxygen. The symbol for aluminum is Al and that for oxygen is O.

Next, write the charge of each ion next to its symbol. The aluminum cation has a 3+ charge and the oxide anion has a 2- charge. So you write them as Al3+ and O2-.

Then, move the charge of the cation to the subscript position of the anion, and move the charge of the anion to the subscript position of the cation. So, since the charge of aluminum cation is 3+, we move it to the subscript position of the oxide anion and get O3. Likewise, take the charge of the oxide anion, 2-, and move it to the subscript position of the aluminum cation to get Al2. The resulting chemical formula is the name of the cation followed by the name of the anion: Al2O3
The subscripts in Al2O3 indicate the ion ratio in the compound. You can see that aluminum oxide has two aluminum ions and three oxygen ions. Since the ratio 2:3 cannot be reduced further, this ratio is the smallest whole ratio of the ions in aluminum oxide. And so, the chemical formula of aluminum oxide is Al2O3.

You can determine whether the chemical formula is correct by checking whether the final compound is neutral. In the case of aluminum oxide, the aluminum ion has a charge of 3+ and the oxide ion has a charge of 2-. By using the crisscross method when you multiply the charge 3+ of aluminum with its subscript 2, you get 6+. Similarly, when you multiply the charge 2- of oxygen with its
subscript 3, you get 6-. Now, adding 6+ and 6-, you get a net charge of 0. This shows that you have a neutral compound.
In the example of Al2O3, the ion ratio 2:3 of aluminum cations to oxide anions cannot be reduced further. However, try writing the formula for lead (IV) oxide. Following the steps described in the example of aluminum oxide, we move the charge of the cation to the subscript position of the anion and the charge of the anion to the subscript position of the cation. So, moving the 4+ charge of the lead cation to the subscript position of the oxide anion, we get O4. Then, moving the 2- charge of the oxide anion to the subscript position of the lead cation, we get Pb2. Finally, we get Pb2O4.

However, this formula indicates that the ion ratio in the compound is 2:4. We know that a chemical formula is written with the simplest whole ion ratio. Since the simplest whole ratio of 2:4 is 1:2, we rewrite the formula, to get, Pb1O2. Finally, since the subscript of Pb is 1, we can drop it and get PbO2
The formula of chromium oxide is Cr2O3. You get this formula by crisscrossing the charges of the cation to the subscript position of the anion and vice versa.
Ternary Ionic Compounds with Polyatomic Ions
Ternary ionic compounds consist of three elements, which usually include a metallic cation and a polyatomic ion. Polyatomic ions are tightly bound groups of atoms that behave as a unit and carry a charge. For example, a nitrate ion is a polyatomic ion in which one nitrogen atom bonds with three oxygen atoms. The overall charge of a nitrate ion is 1-.

If a single atom of an element can combine with different numbers of oxygen atoms, it forms different polyatomic ions. Most polyatomic ions contain oxygen atoms and are called oxyanions. Depending on the number of oxygen atoms they contain, these ions have names ending with the suffix –ite or –ate. For example, a single sulfur atom combines with three oxygen atoms to form sulfite (SO32-) and four oxygen atoms to form sulfate (SO42-). In both cases, the overall charge of the ion is 2-.
So, we can conclude that polyatomic ions with the suffix –ite have fewer oxygen atoms than the ones with the suffix –ate. Also, if an element combines with oxygen to form more than two polyatomic ions, then the prefix hypo– is added to the ion that has the least number of oxygen atoms and the prefix per– is added to the ion that has the highest number of oxygen atoms. For example, a single chlorine atom combines with a different number of oxygen atoms to form ClO-, ClO2-, ClO3-, and ClO4- polyatomic ions. Now, among these ions ClO- has the least number of oxygen atoms, so the prefix hypo– is added to its name, giving us hypochlorite. In the same way, ClO4- has the highest number of oxygen atoms, which means we add per– to its name to get perchlorate.

Sometimes polyatomic ions contain a hydrogen atom. In this case, the polyatomic ion's name is preceded by the word hydrogen. For example, HCO3- is hydrogen carbonate. Other examples of polyatomic ions containing hydrogen are hydrogen sulfate (HSO4-) and
hydrogen phosphate (HPO4-).

Now that you know how to name polyatomic ions, you can name ternary ionic compounds that contain polyatomic ions. For this, you write the name of the cation followed by the name of the polyatomic ion.
The crisscross method that we used to determine the formulas of binary ionic compounds can also be used for polyatomic compounds. Remember that these compounds consist of polyatomic ions that contain more than one atom. For example, calcium phosphate is an ionic compound that has a polyatomic anion. In this compound, the calcium cation has a 2+ charge. One phosphorus (P) atom and four oxygen (O) atoms bond to form a phosphate (PO43-) ion, which has a 3- charge.

To write the formula of calcium phosphate, move the charge of the calcium ion to the subscript position of the phosphate ion, and move the charge of the phosphate ion to the subscript position of the calcium ion. We must add parentheses around the phosphate ion to indicate the number of polyatomic ions in the compound. So, we get the following expression:
Ca3(PO4)2
Next, ensure that the subscripts are in the smallest possible
whole–number ratio. Finally, drop any subscript with the value 1. In this example, the subscripts can't be reduced further, and there aren't any subscripts with the value 1. So, the formula for calcium phosphate is Ca3(PO4)2.

You can check the correctness of the formula by determining whether it has a neutral charge. Each of the three calcium ions has a charge of 2+, so the total charge of the calcium ions is 6+. Each of the two phosphate ions has a charge of 3-, so the total charge of the phosphate ions is 6-. The net charge of the compound is, therefore, zero, which indicates that the formula is correct.
One barium cation combines with two polyatomic chlorate anions to give barium chlorate Ba(CIO3)2.
The formula of magnesium phosphate is Mg3(PO4)2. You get this formula by crisscrossing the charges of the cation to the subscript position of the anion and vice versa.
In a covalent compound, atoms of two nonmetallic elements share one or more electrons to achieve full valence shells. In doing so, they form covalent bonds. When two or more atoms are linked by covalent bonds, they form a molecule. The number and types of atoms that constitute a molecule are indicated by the molecular formula of the compound. For example, methane has the molecular formula CH4, which indicates that it has one carbon atom and four hydrogen atoms.

Elements combine covalently in different proportions. For example, one carbon atom combines with one oxygen atom to form carbon monoxide (CO), and with two oxygen atoms to form carbon dioxide (CO2). You release carbon dioxide into the air when you breathe, whereas carbon monoxide is a lethal gas that can kill you if inhaled. Although both these compounds are formed from carbon and oxygen, they have very different properties, so it's critical to distinguish them by their names
To name a covalent compound, you begin by identifying the elements in the compound and then determine the number of atoms of each of these elements. Based on the position of the elements in the chemical formula, you write the name of each element by adding numerical prefixes, such as mono, di, and tri. The prefix used for each element depends on the number of atoms present in a single molecule of the compound. The name of the second element in the covalent compound always ends with the suffix –ide.

In the case of CO, the prefix mono– is added to oxide to indicate one atom of oxygen. In the case of CO2, the prefix di– is added to indicate two atoms of oxygen: carbon dioxide. The prefix tri– indicates three atoms, tetra– indicates four atoms, penta–, five atoms, and so on.

Take CCl4: this compound has one carbon atom and four chlorine atoms, so it's named carbon tetrachloride. If the first element has only a single atom, the prefix mono is dropped. Also, prefixes ending in a or o drop this letter if the name of the element succeeding it begins with a vowel. For example, in CO, the second o in mono is dropped before attaching it to
oxide: the compound name is carbon monoxide. In the case of N2O4, the a in tetra– is dropped because oxide begins with a vowel. So, the name of the compound becomes dinitrogen tetroxide.
You now know how to write the names of covalent compounds by adding prefixes to the element names. To write the formula of a compound, we use the prefixes in the name to derive the number of atoms of each element present in the compound. The number of atoms of an element corresponds to its subscript in the compound. We simply write the chemical symbols of the elements along with their appropriate subscripts to get the formula.
You now know how to write the names of covalent compounds by adding prefixes to the element names. To write the formula of a compound, we use the prefixes in the name to derive the number of atoms of each element present in the compound. The number of atoms of an element corresponds to its subscript in the compound. We simply write the chemical symbols of the elements along with their appropriate subscripts to get the formula.

Let's write the formula for dinitrogen trioxide. The prefix di– indicates two nitrogen atoms are in the compound. The prefix tri– indicates three oxygen atoms. The chemical symbol for nitrogen is N and the symbol for oxygen is O. Writing the symbols along with their subscripts, we get the formula N2O3. Here, the subscripts 2 and 3 indicate the number of nitrogen and oxygen atoms. Unlike binary and ternary compounds, these numbers don't indicate the ion ratio. So, you don't need to reduce them further.


silver (II) fluoride=AgF2
diphosphorous trisulfide=P2S3
silicon tetrafluoride=SiF4
magnesium perchlorate=Mg(ClO4)2
potassium sulfite=K2SO3