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Predicting bond type (electronegativity)

One way to predict the type of bond that forms between two elements is to compare the electronegativities of the elements. In general, large differences in electronegativity result in ionic bonds, while smaller differences result in covalent bonds. Created by Sal Khan.

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  • stelly blue style avatar for user aniketprasad123
    i really dont understand how metallic bonds are form ??
    (6 votes)
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    • leaf red style avatar for user Richard
      Metallic bonds are more less like covalent bonds however the electrons are free to move between metal atoms instead of being localized as what happens in normal covalent bonding. This free movement of electrons is often referred to as a "sea of electrons" and is responsible for metal's high electrical conductivity. Hope that helps.
      (16 votes)
  • leaf green style avatar for user Yu Aoi
    why most of the elements in the 7th period has no electronegativity value?
    (4 votes)
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    • leaf red style avatar for user Richard
      Well the electronegativity values on the Pauling scale are calculated using bond dissociation energies between elements. Bond dissociation energy basically being the energy input required to break a chemical bond. So this requires us to be able to make various chemical bonds between the elements of interest.

      So the notable exceptions to the Pauling scale where there are gaps are the noble gases helium, neon, and argon. As well as elements in the 7th period. For the noble gases this can have two possible causes. One explanation being that these elements simply have values of 0 for their electronegativities indicating that they have no attraction for other atom's electrons. I find this least probable considering this would mean the noble gas's electrons are perfectly shielding other electrons from the positive charge of their atom's protons in the nucleus. There's going to be at least some degree of attraction felt by outside electrons, even if that attraction is small. The other explanation which I find more probable is that the bonds which needed to be formed for the bond dissociation energies could not be formed for the noble gases. And this is a reasonable assumption considering molecules like diatomic helium basically impossible to create. So it's likely the case that these noble gas elements have gaps in the Pauling scale because no data was able to be collected on these elements.

      The 7th period likely have a similar cause for their gaps, but for a different reason. Elements beyond atomic number 92 (Uranium) are known as synthetic elements, of which most periodic 7 elements are apart of, which means that they are not found naturally and must be created artificially by humans. The methods to create synthetic elements vary but generally we've been able to do so through either nuclear tests like nuclear weapons, or particle accelerators which slam subatomic particles of smaller elements together as fast speeds and fuse them into the nuclei of new larger elements. You can imagine these expensive and difficult tasks to accomplish which means that a very limited number of atoms of these super heavy synthetic elements have ever been created. So that limits the number of samples which can be used to create the bonds necessary for Pauling scale bond dissociation calculations. Additionally the synthetic atoms created through these methods are extremely radioactive and unstable meaning they fall apart very quickly (which would partly explain why we don't find them naturally). So you have a limited number of atoms which decompose on you so fast that it's hard to measure their presence let alone make them form chemical bonds. So this is another case where no data can be collected on their electronegativities and would explain their absences in the Pauling scale.

      Hope that helps.
      (15 votes)
  • piceratops ultimate style avatar for user Sayan Mondal
    even though water are formed by covalent bonds then why is it polar??
    (2 votes)
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    • leaf red style avatar for user Richard
      A covalent bond is essentially atoms sharing electrons, 1 from each atom. If they share them equally we call this non-polar. And if they share them unequally we call this polar. The oxygen in water has two single bonds to two hydrogens and each bond is polar because oxygen is more electronegative than hydrogen. Oxygen loves electrons more than hydrogen does and so the electrons in the water molecule spend more time around the oxygen than they do the hydrogen. Since electrons are negative in charge, oxygen having more negative charge at any given time means it will have partial negative charge. Hydrogen not having those electrons as much will have a partial positive charge. There being a positive and a negative end of a molecule makes something polar as is the case with water because of its two polar covalent bonds. Hope this helps.
      (7 votes)
  • leaf green style avatar for user Devansh Mittal
    Well, why do the Noble Gases need to hog electrons so badly, why are they so electronegative.??. As indicated by the following table above.
    (1 vote)
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    • leaf red style avatar for user Richard
      The issue here is that if you use the Pauling definition of electronegativity noble gases aren’t very electronegative. It’s complicated. Electronegativity, as defined by Pauling, is the attractive force an element feels for bonding electrons. This requires the element’s atom to be in a covalent compound where it can compete for the bonding electrons with another element’s atom. With noble gases this is difficult because they seldom, if ever, like to bond with atoms and instead prefer to remain as monatomic atoms. This is because the ground state electron configurations of noble gases give them an ideal stability which dissuades them from trying to bond and change the number of electrons they have.

      So the electronegativity values for noble gases we have are for the higher atomic number ones like krypton, xenon, and radon because their electrons are less tightly held by the nucleus being so far from it in high electron shells compared to the smaller noble gases. And only then we’re able to bond them to the more reactive elements like fluorine and oxygen. The smaller atomic number noble gases like helium, neon, and argon we’ve been unsuccessful in creating compounds where they bond to other elements so we can directly measure their electronegativity. That’s why they have no data for their electronegativities. We have been able to mathematically predict what their electronegativities might be, but we’ve never been able verify these numbers with real compounds.

      Now there are other electronegativity scales such as the Mulliken and Allen scales which do not require that the elements be bonded in a covalent compound for us to measure their electronegativities. Using these scales, the Allen scale specifically, the noble gases do indeed have the highest electronegativities.

      Hope that helps.
      (6 votes)
  • blobby green style avatar for user kbainemail
    why do non-metals form both ionic and covalent compounds, but metals usually form ionic compounds?
    does it have something to do with their relative electronegativity as well?...
    (1 vote)
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    • leaf red style avatar for user Richard
      So for an element to be able to form a covalent bond as opposed to be an ionic bond, it has to have similar electronegativity to the bonding atom's element. Metals generally have lower electronegativities compared to nonmetals and so easily lose their electrons and become cations when paired with nonmetals.

      It is also possible for metals to form metallic bonds which are bonds with other metals who have similar electronegativity values. Metallic bonding is somewhat similar to covalent bonding except the electrons are much more delocalized.

      There's also dative bonding with metals that can happen which is essentially a covalent bond, but all the bonding electrons are provided by the non-metal atom.

      Hope that helps.
      (3 votes)
  • blobby green style avatar for user secretwood01
    What is the threshold in a difference in electronegativities for two atoms to bond ionically, polar covalently, etc? Thanks.
    (2 votes)
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    • blobby green style avatar for user roycerajanlal0181
      I think it is usually based on the physical properties of an atom or the multiple atoms that create bonds. If the two atoms that are bonding are in a gas form but have a sizable difference in electronegativity, they will form polar covalent bonds. If it is a metal and a non-metal bonding, ionic bonds will usually be created because the metals are looking to lose electrons while gases are looking to gain electrons. Hope that helps!
      (1 vote)
  • blobby green style avatar for user neschism
    Ok, this question is off-topic, but do you know what type of periodic tables are given to students at college for test taking? If so, do you have a PDF link? Thanks!
    (1 vote)
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  • mr pants teal style avatar for user mariana
    woudlnt it be like the elements on the right are more covalent and the left are more ionic? he said it the other way around and i was confsued
    (1 vote)
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  • blobby green style avatar for user jamesrife
    In other periodic tables focused on electronegativity, Fluorine is listed as having a value of 4.00. Why does the table in the video show Fluorine's value as 3.98? Aren't all the other values based off of Fluorines value of 4.00?
    (1 vote)
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  • blobby green style avatar for user Pensive Rabbit
    I understand that polar covalent bonds possess a greater difference of electronegativity than typical covalent bonds. As such, due their difference in electronegativity, would chemicals bonded by polar covalent bonds have dipole-dipole bonds?
    (1 vote)
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    • leaf red style avatar for user Richard
      Dipole-dipole isn’t a bonding type in the way covalent and ionic are, rather it’s referred to an intermolecular force, or a force which exists between molecules (as opposed to covalent bonding which is an intramolecular force, or within a molecule). Dipole-dipole is the force of attraction which exists between polar molecules. So for dipole-dipole to be present we need a polar molecule.

      A molecule is polar when it has a distinct negative side with an excess of electrons and a positive side with a lack of electrons. Using electronegativity, this polar behavior arises because the more electronegative atoms attract the electrons to themselves causing them to be negative, and simultaneously cause the less electronegative atoms to be more positive.

      For a molecule to be polar, we need polar bonds and the right geometry. A polar bond is simply a polar covalent bond where a very electronegative atom is bonded to a less electronegative atom. This creates a dipole moment which is essentially a vector that points in the direction of the more electronegative atom. We also need a geometry for the molecule where the dipole moments of the polar covalent bonds do not cancel each other out. If the dipole moments do cancel each other out, we can a molecule with polar bonds, but still be a nonpolar molecule.

      Hope that helps.
      (2 votes)

Video transcript

- In other videos, we had started talking about the types of bonds that might form between atoms of a given element. For example, if you have two metals forming a bond, well, you are going to have a metallic bond. If you have two nonmetals, engaged in some type of bonding activity, this is likely to be a covalent bond. And the general rule of thumb is if you have one metal, and one nonmetal, that this is likely to be an ionic bond. These are the general rules of thumb. What I wanna do in this video is to better appreciate that bonding is really more of a spectrum. There are bonds, and we've talked about things like polar covalent bonds, that start to look a little bit more and more ionic in nature. And so that's what we're gonna talk about in this video and think about it in the context of electronegativity. Just as a reminder, we talk about electronegativity in many videos, but this is the property of an atom that's in a bond to hog electrons, to want the electron density to be closer to it for the electron pairs to spend more time around that particular atom. So, something with a high electronegativity is going to be greedier with the electrons than something with a low electronegativity. We can think about the spectrum between at this end you have ionic, and at this end you have covalent. And one way to think about it is at the extreme left end, you don't have much difference in electronegativities. Both atoms that are participating in the bond are roughly equal in how badly they want the electrons. While in an ionic bond, you have a very big difference in electronegativities, so much so that one of the atoms swipes an electron from the other. So, one way to think about it is, let me draw a little bit of an arrow here, so this is increased electronegativity difference as you go from left to right. And some place in the middle, or as you go from left to right, you're becoming more and more polar covalent. So for example, if you have a bond between oxygen and hydrogen, these are both nonmetals. So this will be a covalent bond by just our general rule of thumb. And actually the division between metals and nonmetals, I'm gonna make it right over here, it's this blue line is one division you could view, although things that straddle it are a little bit more interesting. But oxygen and hydrogen are both nonmetals, but you have a pretty big difference in electronegativities. This right over here is electronegativity measured on a Pauling scale, named after the famous biologist and chemist, Linus Pauling, and you can see on that scale oxygen is a 3.44, one of the most electronegative atoms. Electronegativity trends, we talk about in other videos, goes from bottom left to top right. The things at the top right that are not the noble gases, these are the ones that really are greedy with electrons. And oxygen is one of the greediest. While hydrogen, it's not not electronegative, but it's lower, at 2.20. So in this scenario, those electrons are going to spend more time around the oxygen. If they spent an equal amount of time, that oxygen might be neutral, but since they're spending a little bit more time here, we'll say that has a partial negative charge, the Greek lowercase letter delta, and on the hydrogen side because the shared electrons are spending more time around the oxygen than around the hydrogen, you would have a partially positive charge right over there. And so this would be a polar covalent bond. Maybe on the spectrum it sits right over there, depending on how you wanna, how you view this scale. Now the other question you say is okay, this is a spectrum between covalent and ionic, what about metallic? Well, metallic bonds are in general going to be formed if you have two things that are not so different in electronegativity, and they both have reasonably low electronegativities. So that's why things on the bottom left right over here, if you have two of these forming bonds with each other somehow, that you're likely to have metallic bonds. And that makes sense because in metallic bonds you have all the electrons kind of mixing in in a shared pool, which gives some of the properties like conductivity. And so if you have a lot of things that are fairly similar in electronegativity, and they're all low in electronegativity, they might be more willing to share those valence electrons in a communal pool.