Introduction: Ionic Bonds

In the previous tutorials in this series, we learned about the structure of atoms. Now we’ll look at how atoms combine to form compounds and molecules. In this tutorial, we start with one of the major types of chemical bonds: the ionic bond.

1. Atoms are only “happy” when they have complete outer energy levels

1.1. As a biology student, you can get by with a simple model of chemical bonding that works very well to explain what happens when atoms combine to form molecules and compounds. Just pretend that atoms have feelings, and remember these two rules:

1. 

   Atoms “like” to have their outer shells (or “energy levels”) be full. That means that atoms are “happy” when

   1. Two electrons are in the first energy level
   2. Eight electrons are in the second energy level
   3. Eight electrons are in the third energy level
2. If these energy levels aren’t full, then atoms will trade or share electrons so that their outer shells become full.

1.2.Let’s start by looking at a situation when atoms trade electrons. To do this, we’ll examine how table salt forms.

Table salt’s chemical name is sodium chloride, and its formula is NaCl (“Na” is the symbol for sodium, and “Cl” is the symbol for chlorine). Start by solving the challenge below.

Make sure you’ve clicked “show the answer” and checked your work before going on.

That outer, lone electron in sodium’s third shell makes it an “unhappy” atom. Sodium can become “happy” by giving up that outer electron. But who will it give it up to?

Make sure you’ve clicked “show the answer” and checked your work before going on.

And just to double-check that you’re getting this, answer the questions below.

Sodium and chlorine can make each other happy. The way they do this is through trading electrons. The next section explains how this works. But first, let’s look at a few atoms to make sure that we can distinguish between “happy” and “unhappy ones.”

2. Ionic Bonding Involves Trading Electrons

Introduction: Unhappy atoms (ones that don’t have a complete outer shell of electrons) can achieve happiness by trading electrons with other atoms. As they do, they become charged ions (atoms that have a positive or negative charge). The attraction between positive and negatively charged ions creates a chemical bond called an ionic bond. 

Here’s how this works in the formation of table salt (sodium chloride).

2.1. Sodium gives up its outer electron to chlorine.

2.2: As a result, both sodium and chlorine have complete outer energy levels. Note that sodium has lost its outermost (third) electron energy level.

2.3: Sodium’s charge is now +1 because it lost an electron. It now has one more proton (with a positive charge) than electrons (with a negative charge).

Correspondingly, chlorine originally had 17 protons and 17 electrons. Now it has 18 electrons. With an extra electron, it has an overall negative charge of minus 1. To make these charges clear, I’ve added a plus and a minus sign to the diagram. Read the “+” as “plus one.” Read the “-” as “minus one.”

2.4: This difference in charges is the essence of the bond between sodium and chlorine. The expression “opposites attract” is completely true here. The sodium, with its positive charge, will stick to the chlorine, with its negative charge, just like the opposite poles of a magnet attract each other.

2.5:This type of bond, where atoms trade electrons, creating charged particles that “stick” to one another because of their opposite charges, is called an ionic bond.

2.6: The newly charged atoms are called ions. Sodium has become a positive ion. Chlorine has become a negative ion. And while the system naming ions is beyond the scope of this tutorial, you should note that when chlorine gains an electron and becomes a charged ion, its name changes to “chloride.”

2.7: Atoms connected by ionic bonds form ionic compounds. Study the diagram below to make sure you understand this before proceeding.

2.8: It just takes a bit of imagination to get from two connected ions to the crystals that make up table salt.

A crystal consists of billions of ions regularly arranged in three dimensions. Take a look at the image below.

The opposite charges between the positive sodium ions (purple) and the negative chloride ions (green) hold the salt crystal together.

 3. A Few More Points about Ionic Bonding

3.1: Atoms will trade one, two, or, less frequently, three electrons fairly readily. But they never trade four, five, six, or seven electrons.

3.2: The charge on an ion is easy to figure out. For every electron that an atom gains, it gains a negative charge. For every electron that an atom loses, it loses a negative charge and gains a positive charge.

3.3: For example, when fluorine, with seven electrons, gains one more electron to complete its outer shell, it will have a charge of -1. Why? Because it now has one more electron than protons.

3.4: Similarly, if an atom loses two electrons, it will have a charge of + 2. This is what happens to magnesium (which starts with 12 protons and 12 electrons) when it becomes an ion (with 12 protons and 10 electrons). You can see this in the diagram below

3.5: It’s not always two atoms that make each other “happy” by trading electrons. You can have three atoms that swap electrons in a way so that they’re all happy. This is what happens in the compound sodium sulfide (Na₂S). Study the diagram and caption below.

4. Checking Understanding

5. The Importance of Ions and Ionic Bonding in Living Things

As you learn about cells, you’ll see ions playing important roles in many biological processes.

5.1: Ions and nerve cells

When you learn about how nerve cells transmit messages, for example, you’ll see that nerve impulses are waves of ions moving along the length of a nerve cell. That’s what’s happening in the image below.

5.2. Ions and the structure of molecules

Ionic bonds play a key role in determining the structure of molecules.

• As we’ll see later in this course, a molecule’s shape defines its function.
• The job of the myoglobin molecule shown at the left is to store oxygen in muscle cells.
• Myoglobin can only do its job if it’s folded in the exact way shown.
• One of the reasons why it’s folded is because of an ionic bond within the molecule.

You’ll learn lots more about ions as we progress through this course.

6. Next steps

If you need more practice, please scroll up to the top and work through this tutorial again. Otherwise, proceed to Covalent Bonds (the next basic chemistry tutorial).