Chemistry Essentials for Dummies

Chapter 5. Ionic Bonding

In This Chapter

· Finding out why and how ions are formed

· Understanding how ions create chemical bonds

· Deciphering the formulas of ionic compounds

· Naming ionic compounds

· Connecting conductivity and ionic bonds

In this chapter, I introduce you to ionic bonding, the type of bonding that holds salts together. I discuss simple ions and polyatomic ions: how they form and how they combine. I also show you how to predict the formulas of ionic compounds and how chemists detect ionic bonds.

Forming Ions: Making Satisfying Electron Trades

In nature, achieving a filled (complete) valence energy level is a driving force of chemical reactions, because when that energy level is full, elements become stable, or “satisfied” — stable elements don’t lose, gain, or share electrons.

The noble gases — the VIIIA elements on the periodic table — are extremely nonreactive because their valence energy level (outermost energy level) is filled. However, the other elements in the A families on the periodic table do gain, lose, or share valence electrons to fill their valence energy level and become satisfied.

REMEMBER. Because filling the valence energy level usually involves filling the outermost s and p orbitals, it’s sometimes called the octet rule — elements gain, lose, or share electrons to reach a full octet (eight valence electrons: two in the s orbital and six in the p orbital).

In this section, I explain how atoms gain or lose electrons to form ions and achieve stability. I also explain how ions can consist of single atoms or a group of atoms. (For info on achieving stability by sharing electrons, flip to Chapter 6.)

Gaining and losing electrons

REMEMBER. When an atom gains or loses an electron, it develops a charge and becomes an ion. In general, the loss or gain of one, two, or sometimes even three electrons can occur, but an element doesn’t lose or gain more than three electrons.

Losing an electron to become a cation: Sodium

REMEMBER. Ions that have a positive charge due to the loss of electrons are called cations. In general, a cation is smaller than its corresponding atom. Why? The filled energy level determines the size of an atom or ion, and a cation gives up enough electrons to lose an entire energy level.

Consider sodium, an alkali metal and a member of the IA family on the periodic table. Sodium has 1 valence electron and 11 total electrons, because its atomic number is 11. It has an electron configuration of 1s²2s²2p⁶3s¹. (See Chapter 2 for a review of electron configurations.)

By the octet rule, sodium becomes stable when it has eight valence electrons. Two possibilities exist for sodium to become stable: It can gain seven more electrons to fill energy level 3, or it can lose the one 3s electron so that energy level 2 (which is already filled at eight electrons) becomes the valence energy level.

So to gain stability, sodium loses its 3s electron. At this point, it has 11 protons (11 positive charges) and 10 electrons (10 negative charges). The once-neutral sodium atom now has a single positive charge [11 (+) plus 10 (-) equals 1+]. It’s now an ion, an atom that has a charge due to the loss or gain of electrons. You can write an electron configuration for the sodium cation:

Na⁺: 1s²2s²2p⁶

TIP. Note that if an ion simply has 1 unit of charge, positive or negative, you normally don’t write the 1; you just use the plus or minus symbol, with the 1 being understood.

Atoms that have matching electron configurations are isoelectronic with each other. The positively charged sodium ion (cation) has the same electron configuration as neon, so it’s isoelectronic with neon. So does sodium become neon by losing an electron? No. Sodium still has 11 protons, and the number of protons determines the identity of the element.

There’s a difference between the neutral sodium atom and the sodium cation: one electron. As a result, their chemical reactivities are different and their sizes are different. Because sodium loses an entire energy level to change from a neutral atom to a cation, the cation is smaller.

Gaining an electron to become an anion: Chlorine

REMEMBER. Ions with a negative charge due to the gain of electrons are called anions. In general, an anion is slightly larger than its corresponding atom because the protons have to attract one or more extra electrons. The attractive force is slightly reduced, so the electrons are free to move outward a little.

Chlorine, a member of the halogen family — the VIIA family on the periodic table — often forms anions. It has seven valence electrons and a total of 17 electrons, and its electron configuration is 1s²2s²2p⁶3s²3p⁵. So to obtain its full octet, chlorine must lose the seven electrons in energy level 3 or gain one at that level.

Because elements don’t gain or lose more than three electrons, chlorine must gain a single electron to fill energy level 3. At this point, chlorine has 17 protons (17 positive charges) and 18 electrons (18 negative charges). So chlorine becomes an ion with a single negative charge (Cl^-). The neutral chlorine atom becomes the chloride ion. The electronic configuration for the chloride anion is

Cl^-: 1s²2s²2p⁶3s²3p⁶

The chloride anion is isoelectronic with argon. The chloride anion is also slightly larger than the neutral chlorine atom. To complete the octet, the one electron gained went into energy level 3. But now there are 17 protons attracting 18 electrons, so the electrons can move outward a bit.

Looking at charges on single-atom ions

TIP. In the periodic table, the roman numerals at the top of the A families show the number of valence electrons in each element. Because atoms form ions to achieve full valence energy levels, that means you can often use an element’s position in the periodic table to figure out what kind of charge an ion normally has. Here’s how to match up the A families with the ions they form:

✓ IA family (alkali metals): Each element has one valence electron, so it loses a single electron to form a cation with a 1+ charge.

✓ IIA family (alkaline earth metals): Each element has two valence electrons, so it loses two electrons to form a 2+ cation.

✓ IIIA family: Each element has three valence electrons, so it loses three electrons to form a 3+ cation.

✓ VA family: Each element has five valence electrons, so it gains three electrons to form an anion with a 3- charge.

✓ VIA family: Each element has six valence electrons, so it gains two electrons to form an anion with a 2- charge.

✓ VIIA family (halogens): Each element has seven valence electrons, so it gains a single electron to form an anion with a 1- charge.

Determining the number of electrons that members of the transition metals (the B families) lose is more difficult. In fact, many of these elements lose a varying number of electrons so that they form two or more cations with different charges.

Seeing some common one-atom ions

Table 5-1 shows the family, element, ion name, and ion symbol for some common monoatomic (one-atom) cations.

Table 5-1. Common Monoatomic Cations

Table 5-2 gives the same information for some common monoatomic anions.

Table 5-2. Common Monoatomic Anions

Possible charges: Naming ions with multiple oxidation states

The electrical charge that an atom achieves is sometimes called its oxidation state. Many of the transition metal ions (the B families) have varying oxidation states because these elements can vary in how many electrons they lose. Table 5-3 shows some common transition metals that have more than one oxidation state.

Table 5-3. Common Metals with More than One Oxidation State

REMEMBER. Notice that these cations can have more than one name. Here are two ways to name cations of elements that have more than one oxidation state:

✓ Current method: Use the metal name, such as chromium, followed by the ionic charge written as a roman numeral in parentheses, such as (II). For example, Cr²⁺ is chromium (II) and Cr³⁺ is chromium (III).

✓ Traditional method: An older way of naming ions uses -ous and -ic endings. When an element has more than one ion, do the following:

· Give the ion with the lower oxidation state (lower numerical charge, ignoring the + or -) an -ous ending.

· Give the ion with the higher oxidation state (higher numerical charge) an -ic ending.

So for chromium, the Cr²⁺ ion is named chromous and the Cr³⁺ ion is named chromic.

Grouping atoms to form polyatomic ions

Ions can be polyatomic, composed of a group of atoms. For example, take a look at Table 5-3 in the preceding section. Notice anything about the mercury (I) ion? Its ion symbol, Hg₂²⁺, shows that two mercury atoms are bonded together. This group has a 2+ charge, with each mercury cation having a 1+ charge. The mercurous ion is classified as a polyatomic ion.

Similarly, the symbol for the sulfate ion, SO₄^2-, indicates that one sulfur atom and four oxygen atoms are bonded together and that the whole polyatomic ion has two extra electrons: a 2- charge.

Polyatomic ions are treated the same as monoatomic ions (see “Naming ionic compounds,” later in this chapter). Table 5-4 lists some important polyatomic ions.

Table 5-4. Some Important Polyatomic Ions