3.1: Two Types of Bonding

Skills to Develop

• Define the octet rule.
• Describe how ionic bonds are formed.

Atoms can join together by forming a chemical bond, which is a very strong attraction between two atoms. Chemical bonds are formed when electrons in different atoms interact with each other to make an arrangement that is more stable than when the atoms are apart.

What causes atoms to make a chemical bond with other atoms, rather than remaining as individual atoms? A clue comes by considering the noble gas elements, the rightmost column of the periodic table. These elements—helium, neon, argon, krypton, xenon, and radon—do not form compounds very easily, which suggests that they are especially stable as lone atoms. What else do the noble gas elements have in common? Except for helium, they all have eight valence electrons. Chemists have concluded that atoms are especially stable if they have eight electrons in their outermost shell. This useful rule of thumb is called the octet rule, and it is a key to understanding why compounds form.

Of the noble gases, only krypton, xenon, and radon have been found to make compounds.

There are two ways for an atom that does not have an octet of valence electrons to obtain an octet in its outer shell. One way is the transfer of electrons between two atoms until all atoms have octets. Because some atoms will lose electrons and some atoms will gain electrons, there is no overall change in the number of electrons, but individual atoms acquire a nonzero electric charge. Those that lose electrons become positively charged, and those that gain electrons become negatively charged. Charged atoms are called ions. Because opposite charges attract (while like charges repel), these oppositely charged ions attract each other, forming ionic bonds. The resulting compounds are called ionic compounds and are the primary subject of this chapter.

The second way for an atom to obtain an octet of electrons is by sharing electrons with another atom. These shared electrons simultaneously occupy the outermost shell of more than one atom. The bond made by electron sharing is called a covalent bond.

Despite our focus on the octet rule, we must remember that for small atoms, such as hydrogen, helium, and lithium, the first shell is, or becomes, the outermost shell and hold only two electrons. Therefore, these atoms satisfy a “duet rule” rather than the octet rule.

Example \(\PageIndex{1}\)

A sodium atom has 11 electrons: 2 in the first shell, 8 in the second shell, and 1 in the third shell. Is sodium more likely to gain or lose electrons to obtain an octet?

SOLUTION

Sodium could gain 7 electrons in the third shell to have an octet (like argon, which has 18 total and 8 valence electrons). Seven extra negative charges is a bit too much, so is not likely.

Instead, sodium would lose its one electron from the third shell. Since the third shell would be empty, now the second shell is the valence shell, and thus it would have an octet (like neon, which has 10 total and 8 valence electrons). When that happens, it becomes an ion with a net positive charge. This can be illustrated as follows:

Sodium Atom (no charge)		Sodium Ion (net positive)
11 protons	11+	11 protons	11+
11 electrons	11−	10 electrons	10−
	0 overall charge		+1 overall charge

Example \(\PageIndex{2}\)

Is nitrogen more likely to gain or lose electrons to obtain an octet? How many electrons?

SOLUTION

Nitrogen has 7 total electrons, 2 in the first shell and 5 in the second shell. To obtain the same number of electrons as a noble gas, it could gain 3 to be like neon or lose 5 to be like helium. Gaining 3 electrons is closer and thus more likely.

After gaining three, it would have 10 total electrons, 2 in the first shell and 8 in the second shell (an octet!). When that happens, it becomes an ion with a net charge of negative three. This can be illustrated as follows:

Nitrogen Atom (no charge)		Nitrogen Ion (net negative three)
7 protons	7+	7 protons	7+
7 electrons	7−	10 electrons	10−
	0 overall charge		-3 overall charge

Key Takeaways

• Atoms have a tendency to have eight electrons in their valence shell.
• The attraction of oppositely charged ions is what makes ionic bonds.

Contributors

• Anonymous