What Is an Ion?

Lesson Transcript
Adrianne Baron

Adrianne has a master's degree in cancer biology and has taught high school and college biology.

Expert Contributor
Matthew Bergstresser

Matthew has a Master of Arts degree in Physics Education. He has taught high school chemistry and physics for 14 years.

Learn what an ion is and how it forms. We will look at some common ions and get to know the terms for the two types of ions. Take the quiz at the end to see how well you understand the lesson. Updated: 05/28/2020

Definition of an Ion

Let's recap a few things you probably learned a long time ago. There are 3 subatomic particles that are found in the atoms of an element. They are the positively charged protons, negatively charged electrons, and neutrons, which have no charge. Elements normally have the same number of protons and electrons in each of its atoms. This being the case, the atoms of the elements are neutral, meaning that they don't have a net positive or negative charge.

There are some instances when an atom may have a positive or negative charge. When the atom has a charge, it is now known as an ion. Atoms will pick up or lose electrons, which creates this charge. Electrons move around to reach the goal of having a full outermost shell within the atom in order to be stable.

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  • 0:00 Definition of an Ion
  • 0:47 Formation and Types of Ions
  • 2:19 Examples of Ions
  • 4:14 Lesson Summary
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Formation and Types of Ions

The positively charged protons always remain the same in an atom. The negatively charged electrons, however, can move from one atom to another. Ions form when an atom loses or gains electrons. Let's look at this a different way for a minute. You are making a decision, and you are weighing out your pros and cons. The pros are positive, and the cons are negative. You are at a point in your list where you have the same number of pros as cons. This would be the same as how the atom normally occurs.

Suddenly, you think of another con to add to the list. Now you have 1 more con than you do pros. This makes the decision more negative than it is positive. This same thing happens when an atom gains an electron. It now has more negatively charged particles than it does positively charged particles. A negatively charged atom has now become an ion known as an anion.

After thinking about things even more, you realize that you already have a solution for the con you just added, so you erase it. You are back to being even on your decision. Then you see that another con has also been resolved so you erase it as well. Now you have 1 more pro than you do cons. At this point, the decision is more positive than it is negative. This is the same as the neutral atom losing a negatively charged electron. It now has more positively charged particles than negatively charged ones. A positively charged atom that has now become an ion is known as a cation.

Examples of Ions

There are several commonly occurring ions. Let's look at some common anions first. Hydrogen usually has 1 proton and 1 electron. It sometimes gains an electron so that it has 2 electrons and still just 1 proton. It is now an anion named hydride.

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Additional Activities


Ions are charged atoms, meaning a neutral atom has lost or gained electrons giving the atom an electric charge. Gaining electrons makes the atom negatively charged, and losing electrons makes the atom positively charged. Positive ions are called cations, and negative ions are called anions. For example, lithium loses 1 electron to become Li+1. The +1 indicates the ion has a +1 charge. Nitrogen gains 3 electrons and becomes N-3. The reason atoms gain or lose electrons is to either complete an outer energy level or to lose an outer energy level. Losing an outer energy level is easier for atoms that have only 1, 2 or 3 electrons in an energy level that typically needs 8 electrons to be full.

There are also polyatomic ions that contain multiple atoms that are covalently bonded together. An example of a polyatomic ion is NH4 +1.

Practice Problems

Here are some more practice problems involving ions.

  1. Magnesium loses two electrons. What is its symbol?
  2. Oxygen gains two electrons. What is its symbol?
  3. Sodium loses one electron. What is its symbol?
  4. Carbonate is a polyatomic ion with a -2 charge. It has one carbon atom and three oxygen atoms. What is its formula?


  1. Mg+2
  2. O-2
  3. Na+1
  4. CO3 -2

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