Earlier in this course, you learned about the process of ionization, in which atoms gain or lose electrons to form ions. The valence electrons are the electrons that are gained or lost in the formation of ions. View this slideshow to learn why atoms form positive and negative ions, and how ion formation relates to an element's position on the periodic table.
Recall that an ion is an atom with a charge, due to having an unequal number of protons and electrons. The two types of ions that can form are summarized in the table.
| Cation |
A cation is a positively charged ion. A cation is formed when an atom loses electrons, so that it has more positive protons than negative electrons, giving it an overall positive charge. |
|---|---|
| Anion |
An anion is a negatively charged ion. An anion is formed when an atom gains electrons, so that it has more negative electrons than positive protons, giving it an overall negative charge. |
Elements are most stable when they have a full valence shell (i.e., when the s and p subshells are full). Recall that the s subshell can hold a maximum of two electrons and the p subshell can hold a maximum of six electrons. Therefore, a full valence shell holds eight total electrons. This leads us to one of the most important principles in chemistry: the octet rule.
The Octet Rule
The octet rule states that atoms tend to gain, lose, or share electrons to achieve a full valence shell (the configuration of a noble gas).
The prefix “oct-” means eight (like in octopus or octagon), so the term “octet” refers to the fact that a full valence shell holds eight electrons.
Note that hydrogen and helium are exceptions to the octet rule. They are complete with only two valence electrons.
What group of elements on the periodic table has a full valence shell?
The noble gases (Group 18) have full valence shells. This explains why they are inert (chemically stable) and generally do not react with other elements.
When an atom gains or loses electrons to form a valence octet, the new particle has an unequal number of protons and electrons and therefore has a net charge.
You can use the octet rule along with an element's group number on the periodic table to determine whether it will gain or lose electrons (and how many) to form a stable octet. Click the periodic table to learn how the group number and the type of ion formed by representative elements are related.
Let's Practice
Complete this activity to practice determining whether elements will gain or lose electrons to form ions and identifying what charge those ions have. Choose the answer that correctly describes each ion.
If you need a periodic table, click below to open an interactive periodic table or to download a PDF.
Which statement describes how calcium (Ca) will form an ion?
- Calcium will gain two electrons to form a 2+ ion.
- Calcium will lose two electrons to form a 2+ ion.
- Calcium will gain two electrons to form a 2- ion.
- Calcium will lose two electrons to form a 2- ion.
Metals lose their valence electrons to form positive ions.
Metals lose their valence electrons to form positive ions.
Metals lose their valence electrons to form positive ions.
Metals lose their valence electrons to form positive ions.
Which statement describes how nitrogen (N) will form an ion?
- Nitrogen will lose five electrons to form a 5+ ion.
- Nitrogen will gain five electrons to form a 5- ion.
- Nitrogen will gain three electrons to form a 3- ion.
- Nitrogen will lose three electrons to form a 3+ ion.
Nonmetals gain enough electrons to fill their valence shell according to the octet rule. Gaining electrons makes a negatively charged ion.
Nonmetals gain enough electrons to fill their valence shell according to the octet rule. Gaining electrons makes a negatively charged ion.
Nonmetals gain enough electrons to fill their valence shell according to the octet rule. Gaining electrons makes a negatively charged ion.
Nonmetals gain enough electrons to fill their valence shell according to the octet rule. Gaining electrons makes a negatively charged ion.
Which statement describes how bromine (Br) will form an ion?
- Bromine will gain one electron to form a 1- ion.
- Bromine will lose one electron to form a 1+ ion.
- Bromine will gain seven electrons to form a 7- ion.
- Bromine will lose seven electrons to form a 7+ ion.
Nonmetals gain enough electrons to fill their valence shell according to the octet rule. Gaining electrons makes a negatively charged ion.
Nonmetals gain enough electrons to fill their valence shell according to the octet rule. Gaining electrons makes a negatively charged ion.
Nonmetals gain enough electrons to fill their valence shell according to the octet rule. Gaining electrons makes a negatively charged ion.
Nonmetals gain enough electrons to fill their valence shell according to the octet rule. Gaining electrons makes a negatively charged ion.
A mystery element formed an ion with a 1+ charge. What does this tell you about this element?
- The mystery element has seven valence electrons.
- The mystery element is in Group 17 on the periodic table.
- The mystery element gained one electron.
- The mystery element has one valence electron.
A positive one charge means the element lost one valence electron.
A positive one charge means the element lost one valence electron.
A positive one charge means the element lost one valence electron.
A positive one charge means the element lost one valence electron.
A new mystery element formed an ion with a 2- charge. What does this tell you about this element?
- The mystery element lost two electrons.
- The mystery element is in Group 16 on the periodic table.
- The mystery element is in Group 2 on the periodic table.
- The mystery element has two valence electrons.
A negative two charge means the element gained two electrons to fill its valence shell. A full valence shell has eight electrons in it, so the neutral element must have had six valence electrons.
A negative two charge means the element gained two electrons to fill its valence shell. A full valence shell has eight electrons in it, so the neutral element must have had six valence electrons.
A negative two charge means the element gained two electrons to fill its valence shell. A full valence shell has eight electrons in it, so the neutral element must have had six valence electrons.
A negative two charge means the element gained two electrons to fill its valence shell. A full valence shell has eight electrons in it, so the neutral element must have had six valence electrons.
Summary
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