If you're seeing this message, it means we're having trouble loading external resources on our website.

If you're behind a web filter, please make sure that the domains *.kastatic.org and *.kasandbox.org are unblocked.

Main content

Introduction to ions

An atom with an electric charge is called an ion. A positively charged ion (a cation) has more protons than electrons. A negatively charged ion (an anion) has more electrons than protons.

Want to join the conversation?

  • primosaur seedling style avatar for user ISHITA
    What is the proper definition of ions?
    (26 votes)
    Default Khan Academy avatar avatar for user
  • duskpin tree style avatar for user SunnyWolf
    Hi! Here are my biggest takeaways from the video. Feel free to correct me if I get anything wrong :)

    Atom vs Ion
    Atoms are neutral, which means there is an equal number of protons and electrons.
    Ions are atoms WITH a charge. The charge can be positive or negative. In other words, the atom gains or loses an electron. The proton never changes (if the proton changes, it is a different element)! See here for Sal's explanation:

    Ion - unequal amount of protons and electrons in an atom.

    As mentioned above, there are positive and negative ions.
    Anion - negatively charged ion
    Cation - positively charged ion
    Here is my way of remembering which charge is with anion and cation: "A" (in Anion) becomes before "C" (Cation) in the alphabet. The word "*n*egative" comes before "*p*ositive" in the dictionary. I don't know if that helps XD

    **Remember, the number of electrons determines whether an atom is an ion (if there is an equal amount of electrons and protons, the atom is neutral). If the number of protons change, it is a different element!**

    I hope this helps! Remember to also watch the video if you haven't already, because Sal goes into greater detail!
    (72 votes)
    Default Khan Academy avatar avatar for user
  • blobby green style avatar for user Adhithi Sriram
    so an element is only an ion if it loses or gains electrons (in order to have a full valence shell) correct? Why would an element need to be stable?
    (15 votes)
    Default Khan Academy avatar avatar for user
    • piceratops ultimate style avatar for user skofljica
      that is correct. everything around you wants to get as stable as possible because that means having potential energy as low as possible. that's why book falls on the ground when you put it in mid-air or why electrons move from area with high electric potential to area with low electric potential (in wires or between storm clouds). when elements gain or lose electrons they lower their potential energy.
      (32 votes)
  • aqualine ultimate style avatar for user pradeepthi
    how can anyone know that protons have positive charge
    (0 votes)
    Default Khan Academy avatar avatar for user
  • orange juice squid orange style avatar for user Cheol Ho Kwag
    When does a stable atom become an ion. I want practical example.
    (2 votes)
    Default Khan Academy avatar avatar for user
    • duskpin ultimate style avatar for user von luger
      When a stable atom gains or loses an electron, it becomes an ion. For example, when chlorine takes an electron from sodium, and sodium gives that electron to chlorine, they become ions and form NaCl. Sodum is a cation (lost an electron and became positive) and chlorine is an anion (gained an electron and becomes negative) in this situation.
      (25 votes)
  • duskpin ultimate style avatar for user J. L. MC 101
    If Sal Kahn said that a positively charged ion is a cation, what is the term used for a negatively charged ion?
    (5 votes)
    Default Khan Academy avatar avatar for user
  • hopper cool style avatar for user NerdLord28
    How do atoms gain or lose electrons to become ions? Can atoms gain/lose electrons themselves?
    (6 votes)
    Default Khan Academy avatar avatar for user
  • duskpin seedling style avatar for user Maria
    Can the oxygen we breathe become an ion? And if so, what would happen if we inhaled it?
    (3 votes)
    Default Khan Academy avatar avatar for user
    • leaf red style avatar for user Richard
      Yep, oxygen, and any other molecule or atoms we breathe in can become ions. It’s simple to become an ion since all a neutral atom/molecule has to do is gain or lose an electron. If one of these airborne ions is breathed in, it’ll simply transfer its charge to atoms/molecules inside our body.

      We create ions and transfer them every day without us noticing. One obvious example would be if you walk on carpet with enough friction and touch a metal doorknob. Your body will accumulate charge (also referred to as static electricity) and when you touch the doorknob you discharge that charge through your fingertips into the doorknob. The doorknob example involves much higher concentration of ions than what you’ll ever breathe in and even then the worst thing that happens is a shock.

      Another source of airborne ions are cosmic rays from space which ionize the air upon entering Earth's atmosphere. Ionized air can also be created through radioactive atoms, thunderstorms, and even waterfalls. So we exist with ions every day and breathe them in without any adverse effects to our health. Even right now you're breathing in ions.

      Interestingly you can purchase certain air purifiers which function by creating airborne negatively charged ions. They use a high voltage to ionize the air and release the ions into the air. The idea is that the ions will cling to pollutant particles in the air and make the pollutants stick to surfaces thereby removing them from the air and preventing you from breathing in the pollutants. However, they also produce ozone which toxic to humans and is also a pollutant, just a different kind.

      Hope that helps.
      (12 votes)
  • leafers seedling style avatar for user ashley fung
    so basing on my understanding of this so far, an atom is an element that has the same number of protons and neutrons, an ion is an element that has different numbers of protons and electrons, an ion is positive when it has more protons than electrons and negative when it has more electrons than ions . while isotopes are the same element with the same number of protons and different mass because of the neutrons... is that right?
    (5 votes)
    Default Khan Academy avatar avatar for user
    • boggle blue style avatar for user Davin V Jones
      No, an element is where all atoms have the same number of protons.
      An ion is an atom that has different number of protons and electrons.
      An isotope is an atom of an element with a specific number of neutrons. Two different isotopes of the same element will have two different neutron counts.
      (6 votes)
  • male robot hal style avatar for user Bill the Mather
    How can an ion turn back into a stable atom?
    (3 votes)
    Default Khan Academy avatar avatar for user

Video transcript

- [Instructor] So my apologies. I just had a deviated septum surgery yesterday. What we're going to talk about in this video is the notion of an ion. So before we talk about ions we're just going to talk about the idea of an element. For example, if I have carbon, carbon is an element. And let's say I have one atom of carbon. What do we know about that one atom of carbon? Well, by definition an atom of carbon has six protons. So it's going to have six protons. And if it is neutral carbon it is going to have the same number of electrons. It is going to have six electrons and that's what makes it neutral. You have the six positive charges and the six negative charges. Now you could have a carbon ion, although they aren't that typical. The way you get an ion is if you don't have an equal amount of protons and electrons. So for example, if you had six protons and five electrons what would that be? Well, we still have six protons. And remember, protons are what define what element we're dealing with. So we're still dealing with carbon, but now we have one more positive charge than we have a negative charge. So this will be carbon, you can write it with a one plus charge like that or you could even write it like this. And this you would just view as a carbon atom. This you would now call an ion because it has that net charge. It has a different number of protons and electrons. What if you were to go the other way around? What if you were to have five protons, five protons and six electrons? What would this be? Well remember, protons define what element you're dealing with, so now if you look at what element has five protons we're dealing with boron. So this is going to be boron. Neutral boron would have five protons and five electrons. But this one has one extra electron, so it has one extra negative charge. So you can write it like this, one minus. Or you could just say it has a negative charge. So this is a boron ion right over here. As soon as you have an imbalance between protons and electrons you no longer would call it an atom, you would call it an actual ion. Now let's do an example question dealing with this. So our question tells us... Our question ... our question tells us ... An atom of platinum has a mass number of 195. So let's just look up platinum on our periodic table. Platinum is sitting right over here if you can see it. So an atom of platinum has a mass number of 195. And 195 looks pretty close to that atomic mass we have there. And it contains 74 electrons. 74 electrons. How many protons and neutrons does it contain and what is its charge? Alright, so let's think about this a little bit. So we're dealing with platinum. So by definition platinum has 78 protons, so we know that. It has 78 protons. They're telling us it has 74 electrons. 74 electrons. So just from that we see that we have four more protons than electrons. So you're going to have a positive four charge. Four more of the positive thing than you have of the negative things. So you could write this as platinum with a plus four charge. This is a platinum ion, a positive platinum ion. The general term when we're talking about a positive ion, we're talking about a cation. That is a positive ion. Up there when we talked about boron being negative, a negative ion, that is an anion. This is just to get ourselves used to some of the terminology. But we're not done answering the question. They say an atom of platinum has a mass number of 195 and contains 74 electrons. How many protons and neutrons does it contain and what is its charge? We figured out its charge. We know the protons. By definition platinum has 78 protons. But what about its neutrons? Well protons plus neutrons is going to be equal to our mass number. It's going to be equal to 195. So we have 78 protons plus I'll say N for neutrons is equal to 195. Subtract 78 from both sides and what do you get? The number of neutrons is equal to 1 ... let's see if I subtract 70 I'll get 125, minus eight I have 117 neutrons. So 117 neutrons, and we're done. But the big idea is, and we say atom when we have the same number of electrons and protons, that's when we are neutral. But as soon as we become non-neutral, we have either more electrons or more protons, and this is true of an atom or a molecule, we will then call it an ion. And if you wanna be a little bit more specific, a positive ion is called a cation and a negative ion is called an anion.