Ions: Predicting Formation, Charge, and Formulas of Ions

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  • 0:06 How Ions Are Formed
  • 0:35 Cation and Anion
  • 2:19 Predicting Charge of an Ion
  • 4:05 Lesson Summary
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Lesson Transcript
Instructor: Amy Meyers

Amy holds a Master of Science. She has taught science at the high school and college levels.

Learn how ions are formed using the octet rule. Use the periodic table to predict the charge an atom will have when it becomes an ion. Learn whether an ion is a cation or anion and how to write the formula depending on what charge the ion has.

How Ions Are Formed

As we've learned before, atoms like to be stable. They feel most stable when their outer electron shells are full. They become full when they have eight electrons in them. This is called the octet rule, which says that atoms like to have full valence shells of eight electrons. Remember that the valence electrons are the electrons in the outermost energy shell of an atom. They get eight electrons by either borrowing some from or giving some to another atom. Let's look at how this works.

Atoms get eight valence electrons by giving electrons to another atom or by accepting electrons from another atom.
How ions are formed

Cations and Anions

Atoms start out electrically neutral because they have the same number of negatively charged electrons and positively charged protons. An ion is an atom that has gained or lost one or more electrons and therefore has a negative or positive charge. A cation is an atom that has lost a valence electron and therefore has more positive protons than negative electrons, so it is positively charged. An anion is an atom that has gained a valence electron and is negatively charged.

Take the element sodium, Na. With one valence electron, it is very unstable in its single form. It just has that one electron in its outermost shell, and it wants very badly to get rid of it. Look at chlorine (Cl) over there in Group VII. It has seven valence electrons in its outermost shell, and it badly wants to gain an electron to become full and happy. Sodium will easily lose that extra electron. When it does, it becomes unbalanced. It now has more protons than electrons, so it is positively charged. That is why it is usually written as Na+. Chlorine, on the other hand, has seven valence electrons and wants to add an electron to fill its outer shell. When it adds an electron, it becomes negatively charged (more electrons than protons) and forms an anion, usually written as Cl-.

Atoms can gain or lose more than one electron at a time. If they do, they are written with the superscript of what they have gained or lost. Ca2+, for instance, has lost two electrons. I know it seems a bit confusing since it lost electrons but became positively charged. As you'll remember, though, atoms are neutral to start with, so if they lose a negatively charged electron, they become positively charged.

Ions are written with the superscript of the number of electrons they have gained or lost.
Gaining and losing electrons

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