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Counting valence electrons for main group elements

How to determine the number of valence electrons and draw Lewis structures for main group elements starting from the electron configuration. Created by Jay.

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  • aqualine tree style avatar for user raineem6597
    Ho do we know what an equation yields if we are give the first part?

    Na(Co3)+H2O→
    (14 votes)
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  • leaf green style avatar for user Manahil  Ahmed
    If helium is in group 8 why does it not have 8 valence electrons . And when two hydrogen atoms combine they make a molecule and then have a total of 2 valence electrons if they are then stable then why are the elements in group 2 not stable with 2 valence electrons ?
    (13 votes)
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    • piceratops ultimate style avatar for user Just Keith
      Helium is not truly a member of Group 18 (or Group VIII in the older system). We just place it there because it is unique and does not really fit into any group. It is placed there because it is practically inert, like the other noble gases.

      Not all elements follow the octet rule. H, He, Li, and Be never follow the Octet Rule. Boron usually does not follow the octet rule, though it sometimes does. Other elements, such as S and P, sometimes do not follow the octet rule. So, the octet rule is not a strict law of science that is always followed.

      Hydrogen does not follow the octet rule, so that is part of why H₂ is stable. Note: H₂ is stable, but it is quite reactive.

      The octet rule follows from the energetic favorability of having both the s and p subshells either completely empty or completely full. There is no p subshell in shell number 1, so that is part of why the lightest elements do not follow the octet rule (there are other reasons).

      There are many considerations as to what does or does not make an atom with a particular electron configuration reactive or nonreactive. So, while the octet rule is a convenient rule of thumb, the reality of chemistry is much, much more complicated.

      So, with chemistry, there are many competing factors that affect how elements behave, so you cannot just take a single rule and insist or suspect that every element must necessarily follow that rule. It doesn't work that way. You have to take all of the factors into consideration to predict how an element might behave.

      NOTE: The Pauli exclusion principle is one of the few laws that all of the elements always obey. But the octet rule is not always followed.
      (21 votes)
  • aqualine tree style avatar for user Julius Alphonso
    In one of the previous videos, Sal said that an unstable atom would share its valence electrons. But here, Jay says that the electrons are donated. Do the atoms share the electrons (and therefore form a bond)? Or do the lose electrons, and form bonds because of their opposite charge?
    (11 votes)
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    • female robot grace style avatar for user Rashmi
      Both the types of attaining stability are possible. An atom may donate an electron and attain octet configuration or it may gain electron and attain octet configuration. It is also possible that the atom shares its electron(as in O2) and attain stability. The bond formed by donating or gaining electron is called ionic bond. The bond formed by sharing of electrons is called covalent bond
      Hope that helped.
      (12 votes)
  • mr pants purple style avatar for user JB
    I'm also having a little trouble on understanding the 1s, 2s, and 3s concepts. Are those just stating the ring number level? Is the 1s the first level, because it seems to always be 1s-squared?
    (4 votes)
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    • mr pants purple style avatar for user Ryan W
      1s^2 doesn't mean squared here, it means that there are 2 electrons in the 1s orbital.

      I don't know what level of education you're at so I don't want to confuse you too much, but we know that electrons do not really exist in circular orbits around the nucleus but rather there are certain regions of space around an atom where we are likely to find electrons, these are called orbitals.
      (5 votes)
  • winston baby style avatar for user Christopher Lindsey
    How did he get Na1s^2 2s^2 2p^6 3s^1?
    I do not get it at all.
    (2 votes)
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  • leaf green style avatar for user Karan Bhat
    what will be the electron configration of lithium
    (3 votes)
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  • marcimus purple style avatar for user Wafaullah Mirbahar
    What is s and p in the video?https://youtu.be/akm5H2JsccI
    (2 votes)
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  • marcimus purple style avatar for user Wafaullah Mirbahar
    After losing or gaining electrons, does element remain same as it was at the beginning?
    (2 votes)
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  • leaf red style avatar for user Andy Ho
    Where did he get 1s^12s^2.... etc! How do I find those measurements?
    (2 votes)
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  • aqualine ultimate style avatar for user hrishikeshkulkarni2001
    Carbon has 4 valance electrons. It forms covalent bond with other elements. So why doesn't it form any ionic bond? What will happen if it does so? please answer
    (2 votes)
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    • marcimus pink style avatar for user Munzah Shah
      To form ionic bonds, Carbon molecules must either gain or lose 4 electrons &This is highly unfavorable. therefore, carbon molecules share their 4 valence electrons through single, double, and triple bonds so that each atom can achieve noble gas configurations
      (2 votes)

Video transcript

Now that we've classified our elements into groups on the periodic table, let's see how to determine the number of valence electrons. And so for this video, we're only talking about the valence electrons for elements in the main groups. When we talk about the main groups, you're using the one through eight system for classifying groups. So one, two, three, four, five, six, seven, and eight. So we're going to ignore the other way to number the groups. And so therefore, we're going to ignore groups three through 12 for this video. And so if we're talking about the main groups, the valence electrons are the electrons in the outermost shell or the outermost energy level. And so let's see if we can figure out how many valence electrons sodium has. So for sodium, if I wanted to write an electron configuration for sodium-- I assume you already know how to do these-- so you would say it is 1S2, 2S2, 2P6. And that takes you all the way over here to neon. And then that brings you to the third period or the third energy level. And you have one more electron to worry about. And so that electron would go into a 3S orbital. So the full electron configuration is 1S2, 2S2, 2P6, and 3S1. When I want to figure out how many valence electrons sodium has, the number of valence electrons would be equal to the number of electrons in the outermost shell, the outermost energy level. For sodium, sodium has the first energy level, second energy level, and the third energy level. The outermost energy level would, of course, the third energy level. So if I see how many electrons sodium has in its outermost energy level, it's only one this time. So that means that sodium has one valence electron. And that's very convenient, because sodium is found in group one. And so we can say that for main groups, if you want to figure out how many valence electrons you have, it's just equal to the group number. So the group number is equal to the number of valence electrons. And so that makes everything really easy. And so if I wanted to represent a neutral atom of sodium with its one valence electron, I could draw sodium here, and I could draw one valence electron next to sodium like that. All right. Let's go ahead and write the electron configuration for chlorine next. So here's chlorine over here. And so if I wanted to write the electron configuration for chlorine, it would be 1s2, 2s2, 2p6, and once again, that takes me all the way to neon. And so now, I'm over here in the third energy level, or the third period. I can see that I would fill 3s2-- so 3s3. And that puts me into my P orbitals. So how many electrons are in my P orbitals? One, two, three, four, five-- so I'm in the third energy level, I'm in P orbitals, and I have five electrons. And so that would be the electron configuration for chlorine. If I want to figure out how many valence electrons chlorine has, I have to look for the electrons in the outermost shell, or the outermost energy level. So I have, once again, the first energy level, the second energy level, and the third energy level. So I want the total number of electrons in the outermost energy level. So how many electrons are in the third energy level? Well, there's two and five, for a total of seven. So chlorine has seven valence electrons. And once again, that's very convenient, because chlorine is in group seven. And so let's go ahead and draw chlorine with its seven valence electrons. So here is chlorine. So one, two, three, four, five, six, and seven, like that-- and so the reason I picked sodium and chlorine is, of course, because the sodium and chlorine will react together to form sodium chloride. And let's analyze what happens using our electron configurations. And so sodium is going to lose one electron. So a neutral atom of sodium has equal numbers of protons and electrons. But if sodium loses its one valence electron-- so it's going to lose its one valence electron, and I can show its one valence electron, actually, is moving over here to the chlorine. So now, when I draw sodium, I have to represent it as an ion, a cation. Sodium used to have equal numbers of protons and electrons, but it just lost one electron. Therefore, it's left with an unbalanced number of protons. So it has one more proton than electrons. So it's a plus one charge. So Na+ is the sodium cation. The sodium cation is stable. And the reason why has to do with the resulting electron configuration. So if I look at the resulting electron configuration-- let me go ahead and use yellow here-- it would be 1s2, 2s2, 2p6. And so the electron configuration for the sodium cation is the same as neon, which is a noble gas. And we know that noble gases are generally unreactive, and that has to do with the fact that their electron configurations are full in their outermost energy level. So the sodium cation is stable, because it has an electron configuration like that of a noble gas. So for chlorine, if we think about how chlorine reacts, chlorine has seven valence electrons. And let's find it on our periodic table here. So here is chlorine. Chlorine has seven valence electrons. If chlorine gets one more, then chlorine would have an electron configuration like a noble gas, like that of argon. So chlorine will gain an electron here. So let's go ahead and write the new electron configuration. If a neutral atom of chlorine picks up an electron, well, the electron would add right in here. So instead of 3p5, we would write 3P6. And so the electron configuration for the chloride anion would be 1s2, 2s2, 2p6, 3s2, 3p6. Let me just go ahead and highlight that-- 1s2, 2s2, 2p6, 3s2 and then 3p6. Let's go ahead and draw it. So we're no longer talking about a neutral chlorine atom here. We're talking about a chloride anion that picked up one electron. So it took that electron from sodium. So I'm going to show that electron in red-- has moved over here to chlorine, like that. And so chlorine gains an electron. So it used to be overall neutral. It used to have an equal number of positive charges and negative charges. But it just added one more electron. So that gives chlorine a negative charge. So it's now the chloride anion. And so you have an anionic bond that forms between the sodium cation and the chloride anion here. So the attraction of these opposite charges forms an ionic bond. And so this is an example of a group one alkali metal reacting with a halogen. So in our video on the periodic table, we talked about elements. We talked about these being our alkali metals. And since these alkali metals are all in group one, they all have one valence electron. And we talked about our halogens over here as also being extremely reactive. And the reason they are so reactive is if they add one more electron, they have the electron configuration of a noble gas. And so drawing the electron configurations, thinking about valence electrons and thinking about the resulting electron configurations allows you to figure out how these things react. And so that is the reason why we can say that group one metals are so reactive, and why we can say that group seven halogens, or 17, are so reactive. It's because of this concept of electron configurations and drawing out your valence electrons. And so we could figure out how many valence electrons something else has, right? So let's say we were asked to figure out how many valence electrons oxygen has. So all we would need to do is look at the group number, right? So this would be-- oxygen is in group six. And so therefore, oxygen has six valence electrons. And so if you wanted to represent oxygen with its six valence electrons, you could go ahead and draw in six valence electrons like that. And so it's a very useful thing to think about that if you want to find the number of valence electrons, think about the group number for main group elements.