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  • - 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.

- In other videos, we had started talking

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預測鍵型(電負性)|AP化學|可汗學院 (Predicting bond type (electronegativity) | AP Chemistry | Khan Academy)

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    林宜悉 發佈於 2021 年 01 月 14 日
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