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Electron affinity: period trend

AP.Chem:
SAP‑2 (EU)
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SAP‑2.A (LO)
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SAP‑2.A.2 (EK)
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SAP‑2.A.3 (EK)
Electron affinity is the energy change that results from adding an electron to a gaseous atom. For example, when a fluorine atom in the gaseous state gains an electron to form F⁻(g), the associated energy change is -328 kJ/mol. Because this value is negative (energy is released), we say that the electron affinity of fluorine is favorable. Created by Jay.

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  • leafers seedling style avatar for user Ruchi Vashist
    Lithium is a metal, and it is electropositive while boron is more electronegative than lithium. Even then it has more electron affinity than boron. It is so confusing!
    Is there a way in which I can correctly compare the periodic properties of elements without fault?
    (2 votes)
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    • piceratops ultimate style avatar for user Just Keith
      Boron is a very unusual element, with complicated properties. That is why Boron is not usually studied at an introductory level. So, that is part of the issue.

      Unlike electronegativity, the electron affinity does not have a strong periodic value. The electron affinity measures the energy released when an electron is captured by the atom (or a molecule), forming an anion with a 1− charge. This is not necessarily directly related to the "willingness" for the element to actually acquire an electron when forming a compound (as measured by electronegativity). There is some correlation, of course, but it is not a strict correspondence.

      So, these are rather different situations. With electron affinity you expose the atom to a source of electrons and see how much energy it releases when it gains an electron (note, gains an electron, not forms a molecule). That is quite different from the situation with electronegativity where you are concern with how much the atom "wants" an electron as determined by the molecules it makes.
      (17 votes)
  • female robot amelia style avatar for user Dayita Chaudhury
    amongst ionization energy and electron affinity energy, which one is endothermic and which one exothermic?
    (4 votes)
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    • duskpin ultimate style avatar for user Davide Ghazal
      IE is always endothermic (energy supplied to remove electrons, precisely one mole of electrons) EA is not always an exothermic process (energy released). Only the first EA is always exothermic the successive ones are endothermic because adding an electron to an anionic species (species negatively charged) requires energy!
      (8 votes)
  • old spice man blue style avatar for user Hannah Chorlton
    So... energy is released when we add an electron to an atom. The electron affinity is 0 on Nitrogen as it would take more energy to add an electron to an orbital which already has an electron within it. I am confused when it comes to Oxygen. Surely to add another electron into Oxygen, it would also take energy for us to throw an electron into an orbital which already has an electron within it? Why is the electron affinity 0 on Nitrogen but not on Oxygen... or even Fluorine? as their P orbitals already have an electron within it?
    (5 votes)
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    • male robot hal style avatar for user Meowtzy
      My understanding is that since there are already 2 electrons in 1 of the orbitals in oxygen and fluorine has 2 P orbitals with 2 electrons in them. they are now attempting to reach the magic number of 8.

      Affinity has to do with the electron configurations.

      Another way to discribe this is that to add an electron to Nitrogen requires a higher energy level whereas oxygen and fluorine already have that higher energy level and are still looking to get to 8 electrons.
      (7 votes)
  • blobby green style avatar for user pawadekhushal
    why is energy is released when electron is added
    (7 votes)
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    • old spice man green style avatar for user Matt B
      You can think of it as an electron losing potential energy. It takes energy to lift it away from the nucleus (higher potential energy) and there when you add electrons it goes down toward the nucleus and loses this potential energy.
      (2 votes)
  • leaf green style avatar for user hozz
    Why would you want to add an election to the element?
    (2 votes)
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  • leaf green style avatar for user dyoffis
    when the narrator talks about how the effective nuclear charge for neon is 0, wouldnt it be true for all the other neutral atoms too if we tried to add another electron? since by the formula, lithium has 3 protons and 3 electrons shielding the new incoming electron, well?🤷‍♀️
    (3 votes)
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    • leaf red style avatar for user Richard
      So effective nuclear charge is the attractive electric force an electron feels from the positively charged protons in the nucleus minus the repulsive electric force experienced from other electrons (called the shielding effect). The shielding effect is technically felt by electrons in lower shells than the electron in question AND other electrons in the same shell, but here on KA Jay is giving a simplified version where only the electrons in lower levels are considered (called core electrons if we're concerned with electrons in the valance shell). The formula is given as: Zeff = Z - S, where Zeff is effective nuclear charge, Z is the atomic number (number of protons), and S is the number of core electrons.

      So if we were to add an electron to a neutral atom of neon, neon has 10 protons, and 10 core electrons, so Zeff = 10 - 10 = 0. So an electron being added to the third shell (the 3s orbital) would feel no attraction to the nucleus and not remain easily bound to the neon atom.

      But if we do the same to a neutral atom of lithium with 3 protons, lithium has 3 total electrons, but 2 are core electrons in the first shell and 1 is a valance electron in the second shell. So the effective nuclear charge felt by a new valance electron to a neutral lithium atom is: Zeff = 3 - 2 = 1. Even though the lithium atom is neutral (that is has the same number of protons as electrons) a new valance electron feels an effective nuclear charge from the protons because only two of the total electrons are engaging in that shielding effect and therefore the new valence electron would remain bound to the lithium atom.

      So being neutral isn't deterring an electron from being added to the atom, rather if it has an effective nuclear charge which is based on the number of protons and core electrons, not the total number of electrons.

      Hope that helps.
      (5 votes)
  • leafers sapling style avatar for user Kshitij Singh
    Why would Na give off energy when given an electron when it actually would prefer to lose one? by adding an electron it gets further from Ne.
    (4 votes)
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  • starky tree style avatar for user BP
    Is there a pattern for periods or groups regarding election affinity?
    (2 votes)
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  • piceratops ultimate style avatar for user Saaim Khan
    Wouldn't the EA of Lithium be 1 because there are 2 shielding electrons and a +3 charge in the nucleus, so 3-2=1, right?
    (2 votes)
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    • male robot donald style avatar for user Kaavinnan Brothers
      good question Saalm...
      the EA is -59.6 KJ/mole for lithium........
      " **Electron affinity is the change in energy** when a neutral atom in its gaseous form gains an electron to form a negative ion. For example, when a neutral fluorine atom in its gas state gains an electron, it releases energy to form a fluoride ion"-Mr.Jay
      It is a factor depended upon nuclear charge, atomic size, electronic configuration etc. and your prediction is wrong and "cannot be found out so simply!!"
      Hope this helps and you get it man...... : )
      I would recommend you to watch the marvelous Jay's video again for better clarity...
      (3 votes)
  • purple pi purple style avatar for user Safwan S. Labib
    Why is the electron affinity of Nitrogen 0? I thought it was supposed to attract electrons to fill up the 2p subshell?? Could someone help?
    (1 vote)
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    • piceratops seed style avatar for user RogerP
      Electron affinity is different to the concept of electronegativity and electron affinity refers to the desire of a neutral atom, in the gaseous state, to accept an electron to create an negatively charged ion. Nitrogen doesn't want to accept an electron under these circumstances, for the reasons that Jay explained.

      However, nitrogen does have high electronegativity, which refers to the power of an atom in a molecule to attract electrons to itself. Nitrogen likes to complete an octet of electrons by forming covalent bonds.

      It does also form anions, such as sodium amide (Na+ NH2-). These are strong bases and highly reactive.
      (4 votes)

Video transcript

- [Instructor] Before we get into electron affinity, let's really quickly review ionization energy. And let's start with a neutral lithium atom, with an electron configuration of one s two, two s one. A lithium atom has three protons in the nucleus, so a positive three charge. Two electrons in the one s orbital, so here are the two electrons in the one s orbital, or our core electrons, and one electron in a two s orbital. This is our outermost electron, our valence electron. The valence electron is shielded from the full positive three charge of the nucleus by the presence of the core electrons. So like charges repel, and these core electrons repel this outer electron, and shield it from the full positive three charge. But there still is an attractive force between the positively charged nucleus and this outermost electron. So opposite charges attract, and our outermost electron still feels a pull from the nucleus. Therefore, since the outer electron is attracted to the nucleus, it takes energy to completely pull away this valence electron from the neutral atom. So if we pull away the outermost electron, we lose our valence electron, and we're left with a lithium ion, with a positive charge. Positive one charge, because we still have three protons, but only two electrons now, so overall a plus one charge. Since this ionization takes energy to rip away the electron, the energy, the ionization energy, is positive, and is measured in kiloJoules per mol. Let's compare that with electron affinity. So, in electron affinity, let's say we're starting with our neutral lithium atom again, but this time, instead of taking an electron away, we are adding an electron. So, we would add an electron to the two s orbital. So we started off with three electrons in the neutral lithium atom, and we're adding one more. So the electron configuration for the lithium ion would be one s two, two s two. So still three positive charges in the nucleus, two electrons in the one s orbital, but now we've added an electron, so we have four electrons total, two in the two s orbital. So let me highlight the electron that we added in magenta. So this is the electron that we added to a neutral lithium atom. And this electron, we know, is shielded from the full positive three charge of the nucleus by our two core electrons in here, right? So like charges repel. It's also going to be repelled a little bit by this electron, that's also in the two s orbital. So this electron's going to repel this one as well. But, there is an attractive force between our positively charged nucleus and our negative charge on the electron. So this electron that we added still feels an attractive force that's pulling on it from the nucleus. And so, if you add this fourth electron, energy is given off. And since energy is given off, this is going to have a negative value for the electron affinity. For adding an electron to a neutral lithium atom, it turns out to be -60 kiloJoules per mol. So energy is released when an electron is added, and that is because the electron that we added is still able to be attracted to the charged nucleus. So if the nucleus has an attraction for the added electron, you're going to get a negative value for the electron affinity. Or that's one way to measure electron affinity. Note that the lithium anion is larger than the neutral lithium atom. It's just hard to represent it here with those diagrams. So as long as the added electron feels an attractive force from the nucleus, energy is given off. Let's look at one more comparison between ionization energy and electron affinity. In ionization energy, since the outer electron here is attracted to the nucleus, we have to work hard to pull that electron away. It takes energy for us to rip away that electron. And since it takes us energy, we have to do work, and the energy is positive, in terms of ionization energy. But for electron affinity, since the electron that we're adding is attracted to the positive charge of the nucleus, we don't have to force this, we don't have to do any work. Energy is given off in this process, and that's why it's a negative value for the electron affinity. Electron affinities don't have to be negative. For some atoms, there's actually no attraction for an extra electron. Let's take neon, for example. Neon has an electron configuration of one s two, two s two, and two p six. So there's a total of two plus two plus six, or 10 electrons, and a positive 10 charge in the nucleus for a neutral neon atom. So let's say this is our nucleus here, with a positive 10 charge, 10 protons. And then we have our 10 electrons here, surrounding our nucleus. So this is our neutral neon atom. If we try to add an electron, so here let's add an electron. So we still have our 10 protons in the nucleus. We still have our 10 electrons, which would now be our core electrons. To add a new electron, this would be the neon anion here, so one s two, two s two, two p six. We've filled the second energy level. To add an electron, we must go to a new energy level. So it would be the third energy level, it would be an s orbital, and there would be one electron in that orbital. So, here is, let's say this is the electron that we just added. So we have to try to add an electron to our neon atom. But if you think about the effective nuclear charge that this electron in magenta feels, alright, so the effective nuclear charge, that's equal to the atomic number, or the number of protons, and from that, you subtract the number of shielding electrons. Since we have 10 protons in the nucleus, this would be 10. And our shielding electrons would be 10, as well. So those 10 electrons shield this added electron from the full positive 10 charge of the nucleus. And for a quick calculation, this tells us that the effective nuclear charge is zero. And this is, you know, simplifying things a little bit, but you can think about this outer electron that we tried to add, of not having any attraction for the nucleus. These 10 electrons shield it completely from the positive 10 charge. And since there's no attraction for this electron, energy is not given off in this process. Actually, it would take energy to force an electron onto neon. So if you wrote something out here, and if we said, alright, I'm trying to go from, I'm trying to add an electron to neon, to turn it into an anion. Instead of giving off energy, this process would take energy. So we would have to force, we would have to try to force this to occur. So it takes energy to force an electron on a neutral atom of neon. And we say that neon has no affinity for an electron. So it's unreactive, it's a noble gas, and this is one way to explain why noble gases are unreactive. This anion that we intended isn't going to stay around for long. So it takes energy to force this onto our neutral neon atom. So you could say that the electron affinity is positive here, it takes energy. But usually, you don't see positive values for electron affinity, for this sort of situation. At least, most textbooks I've looked at would just say the electron affinity of neon is zero, since I believe it is hard to measure the actual value of this. Here we have the elements in the second period on the periodic table, and let's look at their electron affinities. We've already seen that adding an electron to a neutral atom of lithium gives off 60 kiloJoules per mol. Next, we have beryllium, with a zero value for the electron affinity. That means it actually takes energy. So this number is actually positive, and it takes energy to add an electron to a neutral atom of beryllium. So if you think about going from a neutral atom of beryllium to form the beryllium anion, when we look at electron configurations, neutral atom of beryllium is one s two, two s two, and so, to form the negatively charged beryllium anion, it would be one s two, two s two, and to add the extra electron, it must go into a two p orbital, which is of higher energy. And so, this is actually the same thing, or very similar, to neon, which we just discussed. For neon, the electron configuration was one s two, two s two, two p six, and to add an extra electron, we had to go to the third energy level. We had to open up a new shell. And the electron that we added was effectively screened from the full nuclear charge by these other electrons. And a similar thing happens here for the beryllium anion. To add this extra electron, we have to open up a higher energy p orbital. This electron is, on average, further away from the nucleus, and is effectively shielded from the full positive charge of the nucleus, and therefore, there's no affinity for this added electron. So there's no affinity for this electron, so it takes energy to form the beryllium anion. And that's why we see this zero value here for beryllium. Beryllium has no value for electron affinity, or, it's actually a very positive value, and we just say it has a zero value. Next, let's look at boron here. So this gives off -27 kiloJoules per mol. And we can see a little bit of a trend here, as we go from boron to carbon to oxygen to fluorine. So as we go across the period on the periodic table, more energy is given off, and therefore, fluorine has the most affinity for an electron. So as we go across a period, we get an increase in the electron affinity. So the negative sign just means that energy is given off, so we're really just looking at the magnitude. More energy is given off when you add an electron to a neutral atom of fluorine, than if you add an electron to a neutral atom of oxygen. And we can explain this general trend in terms of effective nuclear charge. As we go across our period, we also have an increase in the effective nuclear charge. And if the added electron is feeling more of a pull from the nucleus, which is what we mean here by increased effective nuclear charge, more energy will be given off when we add that electron. So this idea explains the general trend we see for electron affinity, as you go across a period, we get an increase in the electron affinity. We've already talked about beryllium as an exception, neon as an exception, but what about nitrogen in here? We can see that nitrogen doesn't really have an affinity for an electron, and you'll see many different values for this one, depending on which textbook you're looking in. But if we look at some electron configurations really quickly, we can try to explain this. So, for nitrogen, the electron configuration is one s two, two s two, and then two p three. So if we draw out our orbitals, let's just say this is the two s orbital, and then these are the three two p orbitals. So we'll just do these electrons here. Two electrons in the two s orbital, and then we have three electrons in the p orbital. So let's draw those in there. If we try to add an electron to a neutral nitrogen atom, we're adding an electron to one of these orbitals, which already has an electron in it. So adding an electron to one of these orbitals, right, the added electron would be repelled by the electron that was in there to start with. And this is the reason that you usually see in textbooks for the fact that this does not follow the trend. Nitrogen doesn't have an affinity for one added electron. So after going through all of that, it's obvious that electron affinity is a little more complicated than ionization energy. In ionization energy, we had a pretty clear trend, and it was a little easier to explain why. For electron affinity, going across a period on the periodic table, we see a little bit of a trend, but there are many exceptions to this, and perhaps our explanations are a little bit too simplistic to explain actually what's going on. But across a period, you do see a little bit of a trend. Going down a group is much harder. You see more inconsistencies, and it's not really even worth going over a general trend for that.