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Electronegativity - Concept
Teaching at a top-ranked high school in SF
She teaches general and chemistry at a top-ranked high school in San Francisco. Prior to that, she lead and published a number of research studies and lectured at SF State University.
Electronegativity is a quantitative measure of how tightly an atom holds onto its electrons. Bond polarity results from imbalanced electronegativities of the atoms involved in bonding. Electronegativity is a periodic trend which increases as you go up and to the right along the periodic table.
So let's take a moment and talk about electronegativity. So what actually is electronegativity? It describes the unequal sharing of electrons between atoms. So once you hear that sharing between atoms, it should bring up some thoughts about what you know about bonding thus far.
So electronegativity more specifically describes the ability of an atom in a molecule to attract a shared electron to itself. So the higher the value of the electro- of electronegativity, the more the electron in the bond resides near the atom. So it's almost like a popularity contest for electrons if you'd like to think about it that way.
So we basically measure electronegativity by the polarities of the bonds between various atoms and there's actually a numerical value associated with electronegativity and it ranges from 4.0 to 0.7 and so this schematic here is kind of you imagine it's a, a full periodic table if you will and so electronegativity increases as you go to the right towards the non-metals with fluorine being the most electronegative atom with the value of 4.0 and electronegativity decreases as you go down the periodic table with cesium having the lowest electronegativity of 0.7. So if you think about that and what you know about these respective elements at this point, you know fluorine loves loves loves negative energy, so it loves electrons. So it's electron loving, so that's why it is the most electronegative atom. Versus cesium it's kind of large and pretty soft and it prefers to lose its electron so that it can gain a positive charge and become like the noble gases.
Okay. So let's kind of look at that, a little in I guess in a diagram form if you will. So here I've depicted a bond between hydrogen and hydrogen. So remember that this line here is representative of two electrons. Alright so, here in this bond, these two elements are the same. No one is more popular than the other with the electron so the two electrons here are equally shared as depicted here, okay? And so therefore they form a covalent bond. In a covalent bond the electrons are equally shared between the two participants.
Going a step further, hydrogen and chlorine being bonded. Okay, so now you have hydrogen which we know likes to form a proton and chlorine which we know likes to form an anion and so these guys have kind of a a push-pull here with the electrons that are being shared in this bond. Chlorine definitely is dominating having this electron closer to it, so it looks more like this. It's got the negative energy surrounding it and not so much here for hydrogen. So we call that kind of bond a polar covalent bond to indicate that the electrons are unequally shared between the hydrogen and the chlorine.
So we take it a step further and go with an even more electronegative element like fluorine bonded to hydrogen. So definitely here there is almost no sharing of electrons here even though I've drawn this as a bond. So as soon as possible these guys are going to want to separate and fluorine is going to want to take the electron with it and have b and f minus and hydrogen is going to want to become an H+. And so we call that bond an ionic bond where there is essentially no sharing of electrons. Okay so here again we're just looking at kind of the progression of electronegativity with these halogen elements here.
So one thing that can come up when you're thinking about electronegativity is this other term that's electron affinity. So let me just go ahead and say the definition of those two things so that you're clear that they're not the same. So, again electronegativity is the numerical value that's associated with an atom's ability to form a covalent bond. Okay, so it just tells you who likes the electrons more and who can have them kind of gravitate towards it in a bond versus electron affinity is the amount of energy that is released when an electron attaches to an atom. Okay, so like our halogens have high electron affinities because they love negative energy being towards them but they're different numerical values and that is electronegativity.
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