Chapter 2: Ionic and Covalent Compounds

In general compounds (pure substances containing two or more different elements) are classified as either ionic or covalent. In class we took some time to list a few compounds and to classify them as ionic or covalent. Ionic compounds which are generally composed of a metallic element and a nonmetallic element, contain ions (which carry either a positive..cations or negative charge..anions). Covalent compounds are composed of nonmetallic elements contain neutral molecules, which are not electrically charged.

So let's begin with a discussion of ionic compounds, substances which contain a metallic element and a nonmetallic element. Ionic compounds are composed of ions, cations and anions. The metal is always the cation and the nometal, or as we will soon learn the polyatomic species, is typically the anion. An ion is a positively or negatively charged atom or molecule. Ions are formed when an atom gains or loses an electron. How many electrons do atoms liek to gain or lose? Well, that will depend on the element. The easiest way to predict the number of electrons gained or lost by an atom is to consider its proximity to its nearest noble gas.

The alkali metals (Group IA...Li, Na, K, Rb, Cs and Fr) like to lose an electron. Sodium is an example. As the neutral atom sodium has 11 protons and 11 electrons. The loss of one electron gives the ion 10 electrons, same as the noble gas neon, and Na becomes Na⁺ . All of the alkali metals prefer to lose an electron when they combine with other elements.

Alkaline earth metals (Group IIA...Mg, Ca, Sr, Ba) lose two electrons, giving the ion the same number of electrons as the noble gas neon, forming doubly charged cations, i.e. Mg²⁺. In general metals lose electrons, forming cations, which have as many electrons as the preceeding noble gas element.

Aluminum, in Group IIIA, will lose three electrons forming the +3 cation.

If we now consider the halogens (Group VIIA...F, Cl, Br, I) chlorine, Cl, will gain one electron, forming Cl^-, to have as many as argon. Nonmetals generally gain electrons, forming anions, to have as many electrons as the nearest noble gas element. Oxygen, in Group VIA, will gain two electrons forming the 2- ion, nitrogen gains three electrons forming the 3- ion.

Writing formulas consists of balancing the charges on the cation and anion to produce a formula with a balance of positive and negative charge. Sodium and chlorine form a compound sodium chloride with a formula of NaCl. Sodium likes to lose one electron and chlorine likes to gain one electron. A formula of NaCl balances Na ⁺ and Cl ^-. A compound containing calcium and bromine would have a formula of CaBr₂. Calcium has a +2 charge and bromide has a -1 charge. Therefore we need two bromide ions to balance the +2 charge on calcium.

Polyatomic ions are characterized as ions, charged species, with more than one type of atom. Some examples include NO₃^-, SO₄^2-, OH^-,and NH₄⁺. What would be the formula of a compound containing aluminum and nitrate? Aluminum, as a metal, loses electrons to have as many electrons as the nearest noble gas. In this case aluminum forms a stable 3+ cation. Nitrate, NO₃^-, combines with Al³⁺. The formula of the compound is Al(NO₃)₃. The parenthese are needed to clearly indicate the presence of three nitrate ions. What is the formula for a compound containing calcium and phosphate? Ca loses two electrons to form Ca²⁺. Phosphate has the formula PO₄^3-. Manipulating the charges to achieve electroneutrality means three calcium ions balance two phosphate ions and the formula is Ca₃(PO₄)₂.

Remember the charge on an ion is a result of the lose or gain of electrons, metals lose electrons to form cations, nonmetals gain electrons to form anions. Metals generally lose a number of electrons equal to their group number. Group IA elements lose one electron, Group IIIA elements lose three electrons. Nonmetals gain electrons equal to the group number minus 8. Chlorine, in Group VIIA, gains 8 - 7 = 1 electron. Oxygen gains two electrons. The formula for ionic compounds is determined by balancing the positive charge of the cations and the negative charge of the anion to form an electrically neutral compound.

Transition metals, the B elements, form a variety of possible charged ions. Since they are metals they lose electrons to form cations. But where the Group IA and IIA element lose 1 or 2 electrons, respectively, the transition metals lose 2 or 3 electrons. As an example consider iron. It can form several cations. The most common, for our purposes, are the +2 and the +3 ions. The same occurs fo rthe other transition metal elements. However there are a few exceptions. The exceptions include, copper, zinc and silver. Coper and zinc will (for our purposes) only form +2 cations, while silver will only form a +1 ion. The reamining transition metals can form either +2 or +3 cations. The last exception is lead. Though it is in Group IV it only forms +2 cations.

Have you ever wondered about the chemical formula of lava?

Writing formulas and naming binary ionic compounds

While we will learn how to names many compounds as we progress through the semester we will begin now with the simplest types of compounds, the binary compounds. These are compounds which contain only two elements. There are two types of binary compounds that I want you to be familiar with at this point in the course.

I. Binary compounds containing metals and nonmetal elements.

The correct chemical name is composed of the name of the metallic element followed by the name of the nonmetal which has been modified to the identifying stem plus an -ide ending.

For example,

NaCl

sodium chloride

PbI₂

lead iodide

Al₂O₃

aluminum oxide

BaS

barium sulfide

Writing formulas of binary covalent compounds

II. Binary compounds containing only nonmetal elements.

The element written first is named first and the second element retains the modified binary ending of -ide. In addition a Latin or Greek prefix is associated with the name of each element to indicate the number of atoms of each element in the periodic table. For example,

Several examples of the names for binary compounds containing only nonmetal atoms include;

CO₂

carbon dioxide

CO

carbon monoxide

N₂O

dinitrogen monoxide

CCl₄

carbon tetrachloride

P₂O₅

diphosphorus pentoxide

Writing formulas for ternary compounds

Now we'll begin discussing ternary compounds. The first group of ternary compounds are the acids. While we'll discuss acids and their properties in more detail this semester, right now it is worth knowing that acids are covalent compounds which will react with many metals to produce H₂ gas and taste sour (do not taste any acids in the laboratory). The important acids which you must memorize are;

Another important characteristic of an acid is its ability to donate hydrogen ions when dissolved in water. What is a hydrogen ion? We've heard of hydride ion, which is H^-. Hydrogen ion is the other ion, H⁺. So for example when sulfuric acid is added to water we detect the presence of ions! Yes, ions. Even though sulfuric acid is a covalent compound it forms ions when added to water. In fact all of the acids listed above form ions when added to water.

You are probably wondering what are the ions that are formed when sulfuric acid is added to water. If sulfuric acid donates two hydrogen ions there has to be an anion to balance the cations. Since two positive hydrogen ions are formed we know there has to be two negative charges on another specie or species. In this case the anion is SO₄^2- which is called sulfate. So sulfuric acid produces 2H⁺ ions and SO₄^2- ions. We can even write an equation to represent the ions formed;

H₂SO₄ (aq) ---> 2H⁺(aq) + SO₄^2-(aq)

Notice we've generated a new anion, SO₄^2-. This anion is an example of a polyatomic anion. As you might guess there are more polyatomic anions which can be obtained from the other acids listed above when the hydrogen ions are removed. Below is a table of these very important polyatomic anions.

From this new group of anions we can now generate a whole new list of ionic compounds to name. For example,

Na₂SO₄ is sodium sulfate

KClO₄ is potassium perchlorate

NaNO₂ is sodium nitrite

NaHCO₃ is sodium hydrogen carbonate, or sodium bicarbonate.

magnesium acetate is Mg(C₂H₃O₂)₂

sodium phosphate is Na₃PO₄

There is another group special group of binary covalent compounds which we need to mention at this point. They are the following;

Why do I bring up these particular binary covalent compounds now, why did I not mention them when we discussed naming of binary covalent compounds. First you can see we did not use mono as a prefix before the halogen (F, Cl, Br, or I) or before sulfur. So this is a special group of binary covalent compounds. But something even more interesting arises when we add these particular five substances to water. They all form ions when added to water, they all donate hydrogen ions! This behavior is characteristic of acids. Also these particular binary covalent compounds have slightly different names when added to water. How do we know whether these substances are in water or not. If they have the '(aq)' next to their formula, that's how.

Like the acids we discussed above these binary acids will forms ions when added to water.

HCl(aq) ---> H⁺(aq) + Cl^-(aq)

Another group of compounds which have similar properties are called bases. There are ionic and covalent bases, but we will focus on ionic bases and only include one covalent base.