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Learning Objectives
- Define valence electrons
- Define the octet rule.
- Determine the most likely ion of an element.
A chemical reaction involves either electron removal, electron addition, or electron sharing in order to break or make bonds between atoms. The path that a specific element will take in a reaction depends on where the electrons are in the atom and how many there are.
Valence Electrons
In the study of chemical reactivity, electrons in the outermost principal energy level are very important and so are given a special name. Valence electrons are the electrons in the highest occupied principal energy level of an atom. In the second period elements, the two electrons in the \(1s\) sublevel are called inner-shell electrons and are not involved directly in the element's reactivity, or in the formation of compounds. Lithium has a single electron in the second principal energy level, and so we say that lithium has one valence electron. Beryllium has two valence electrons. How many valence electrons does boron have? Recognize that the second principal energy level consists of both the \(2s\) and the \(2p\) sublevels, and so the answer is three. In fact, the number of valence electrons goes up by one for each step across a period, until the last element is reached. Neon, with its configuration ending in \(2s^2 2p^6\), has eight valence electrons.
Valence Shell Stability
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.
Note
Krypton, xenon, and radon are the only noble gases found to make compounds with other atoms.
There are three 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 to another atom (losing electrons). This will result in an atom with a net positive charge known as cation. Another option is to capture electrons from another atom (gaining electrons).This will result in an atom with a net negative charge known as anion. A third option 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. Whether an atom decides to capture, lose, or share electrons with another atom depends on the chemical properties of both atoms, and that will result in different types of chemical bonds
Note
Despite the 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 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?
Solution
Although either event is possible, a sodium atom is more likely to lose its single valence electron. When that happens, it becomes an ion with a net positive charge. This can be illustrated as follows:
| Sodium atom | Sodium ion | ||
|---|---|---|---|
| 11 protons | 11+ | 11 protons | 11+ |
| 11 electrons | 11− | 10 electrons | 10− |
| 0 overall charge | +1 overall charge | ||
Exercise \(\PageIndex{1}\)
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? Write the formula of the resulting ion.
- Answer
-
The process that involves less number of electrons is more favorable. Fluorine would gain one electron. The formula of the resulting ion is F−.
Types of chemical bonds
There four types of chemical bonds, depending on the nature of the atoms forming the bond (metallic or nonmetallic):
Summary
- Valence electrons are the outer-shell electrons of an atom.
- Valence electrons determine the reactivity of an atom.
- Atoms have a tendency to have eight electrons in their valence shell.
- The attraction of oppositely charged ions is what makes ionic bonds.
Exercise \(\PageIndex{2}\)
1. What is the octet rule?
2. How are ionic bonds formed?
3. Why is an ionic compound unlikely to consist of two positively charged ions?
4. Why is an ionic compound unlikely to consist of two negatively charged ions?
5. 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? Write the formula of the resulting ion.
6. 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? Write the formula of the resulting ion.
7. 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? Write the formula of the resulting ion.
8. 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? Write the formula of the resulting ion.
- Answer 1
-
The octet rule is the concept that atoms tend to have eight electrons in their valence electron shell.
- Answer 2
-
Ionic bonds are formed by the attraction between oppositely charged ions.
- Answer 3
-
Positive charges repel each other, so an ionic compound is not likely between two positively charged ions.
- Answer 4
-
Negative charges repel each other also.
- Answer 5
-
Ca atom is more likely to lose two electrons. It will become Ca2+ ion.
- Answer 6
-
An Al atom is more likely to lose three electrons. It will become Al3+ ion.
- Answer 7
-
Selenium is more likely to gain two electrons. It will become Se2− ion.
- Answer 8
-
Iodine is more likely to gain one electron. It will become I− ion.
