Atoms can join together by forming a chemical bond, which is a strong, persistent 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 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.
Note
Of the noble gases, chemists have synthesized compounds of krypton, xenon, and radon only.
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 to transfer electrons between 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. Since an atom is neutral by definition, a new word ion is used to refer to an atom or small group of atoms that carries a charge. An atom that loses electron(s) becomes a positively-charged cation. An atom that gains electrons becomes a negatively-charged anion. Note that the charges for ions are always due to gain or loss of electrons, never gain or loss of protons which are at the center of the atom and key to the atom’s identity. Because opposite charges attract, anions and cations 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. Covalent bonding and covalent compounds will be discussed in Chapter 4 “Covalent Bonding and Simple Molecular Compounds”.
Note
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 1
A sodium atom has one valence electron. Do you think it is more likely for a sodium atom to lose one electron or gain seven electrons to obtain an octet?
Skill-Building Exercise
- A fluorine atom has seven valence electrons. Do you think it is more likely for a fluorine atom to lose seven electrons or gain one electron to obtain an octet?
Concept Review Exercises
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What is the octet rule?
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How are ionic bonds formed?
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.
Exercises
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Why is an ionic compound unlikely to consist of two positively charged ions?
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Why is an ionic compound unlikely to consist of two negatively charged ions?
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A calcium atom has two valence electrons. Do you think it will lose two electrons or gain six electrons to obtain an octet in its outermost electron shell?
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An aluminum atom has three valence electrons. Do you think it will lose three electrons or gain five electrons to obtain an octet in its outermost electron shell?
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A selenium atom has six valence electrons. Do you think it will lose six electrons or gain two electrons to obtain an octet in its outermost electron shell?
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An iodine atom has seven valence electrons. Do you think it will lose seven electrons or gain one electron to obtain an octet in its outermost electron shell?
