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# Drawing Lewis diagrams

A Lewis diagram shows how the valence electrons are distributed around the atoms in a molecule. Shared pairs of electrons are drawn as lines between atoms, while lone pairs of electrons are drawn as dots next to atoms. When constructing a Lewis diagram, keep in mind the octet rule, which refers to the tendency of atoms to gain, lose, or share electrons until they are surrounded by eight valence electrons (an octet). Created by Sal Khan.

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• What is the chemistry behind the least electronegative atom being central?
• Atoms which have lower electronegativities hold onto their electrons less tightly and therefore are more prone to share their electrons. The central atom of a molecule needs to be sharing its electrons with multiple atoms which is easier to do so with a less electronegative atom which isn't as reluctant to share its electrons.

(pasted from this same question elsewhere)
• Why does every line in a Lewis diagram represent two electrons? Does that mean covalent bonds always share even numbers of electrons?
• Yes, covalent bonds come in pairs which are represented by lines in Lewis structures. One line is a single bond with 2 bonding electrons, two lines is a double bond with 4 bonding electrons, and three lines is a triple bond with 6 bonding electrons. Covalent bonds form when two atoms react such that they share electrons in a bond between them and each atom donates half of the electrons which forms the bond from their original valence electrons. So yes each covalent bond will be a pair of electrons because each atom contributes 1 electron to a bond (And 1+1=2).

Hope that helps.
• I tried the Lewis structure on BeF2, and I came out with each F being double bonded to the Be,with extra electrons around each F, fulfilling the octet rule. But their answer had each F single bonded to the Be, with extra electrons around the F. Could you please explain this? Thanks.
• When 2 atoms share electrons, what is actually happening is both atoms take one electron to share, making a pair of electrons shared. It isn't just that one atom shares one electron with another (that is not how it works).

So we have Beryllium (Be) which has 2 valence electrons and then we take 2 Fluorine (F) which both have 7 valence electrons.

If we make a single bond of F with anything else, then the Fluorine atom now has 8 total valence electrons, so a neutral fluorine atom will never make more than one covalent bond because otherwise it would have more than 8 valence electrons & overflow into the next shell.

Thus once the Fluorine atoms each make a single bond with Beryllium then they no longer have a reason to share anymore so Beryllium must make due with what it has. If Beryllium isn't completely full then it may try to get more electrons making making additional bonds to make the overall molecule even larger.

Hope this helps,
- Convenient Colleague
• Is the reason that the central atom is the least electronegative except Hydrogen because Hydrogen wants only 2 valence electrons so it won't form that many bonds? Or is it because Hydrogen is electronegative?
(1 vote)
• The central atom of a molecules is going to be one sharing its electrons with all the other atoms. An atom is more likely to be able to share electrons if it’s not holding onto its electrons as tightly. Since electronegativity is a measure of how strongly an atom attracts electrons to itself, low electronegativity values signify an atom more willing to share its electrons. So having the atom with the lowest electronegativity makes it more likely for multiple covalent bonds to form.

Hydrogen wants to gain noble gas electron configuration like all other elements since noble gases are more stable. The nearest noble gas is helium with two valence electrons, so hydrogen wants to also have two valence electrons. A single hydrogen atom has one valence electron, so it only needs to react with one other atom covalently with a single bond to get a second electron.

So hydrogen is never a true central atom of a molecule simply because its limited in its bonding abilities. You would never create a molecule any more complicated than a diatomic molecule with hydrogen as the central atom. You could argue that hydrogen is the central atom in diatomic molecules like H2 or HF. But again, we would just be limited to just these diatomic examples, and we know that there are molecules which are much larger than these which means hydrogen isn’t acting as the central atom in these larger molecules.

Hope that helps.
• Do you have to do this for metallic bonds?
• You can use lewis structures to visualize covalent and ionic bonds, but not so much metallic bonds because electrons don't behave the same-- they are delocalized between all the atoms. They are still a very nice tool though.
• Does the Lewis structure apply to both covalent and ionic bonds?
(1 vote)
• Lewis structures are mostly applied to covalent molecules, and while it is exceedingly uncommon you should do the same for ionic compounds.

It's just for ionic compounds electrons aren't shared so you won't have things like single bonds between atoms. Instead ionic compounds stick together through electrostatic forces (different electrically charged ions) which we usually represent with brackets and the charge in the upper right corner. Additionally ionic compounds don't exist as individual molecules (as their formula unit suggests) but as a repeating pattern of these formula units in a lattice. So you can use Lewis structures for ionic compounds too, but the bonding is different enough from covalent compounds that it's simply not used to the same amount.

Hope that helps.
• Is there a trick to remember the valence electrons of various elements without taking help of the periodic table?
(1 vote)
• You could memorize the number of valence electrons for every element, but that's unproductive and unnecessary unless you're working with that element a lot. The periodic table is the best way to remember the valence electrons for the elements in my opinion since it partially organizes elements by their valence electrons. It's a good tool which doesn't make us burden our memories with minor facts.
• Is every element trying to reach 8 in its outer shell? does that number ever change
(1 vote)
• Not every element follows the octet rule and strives to reach eight valence electrons. A simplest exception to this rule is hydrogen and helium and the first period. They strive to reach two valence electrons and hence follow the duet rule. Another exception are the transition metals which follow an 18-elecron rule. Primarily the octet rule is followed by main block elements (groups 1-2 & 13-18) and even then there are plenty of exceptions. For example phosphorus usually would want to follow the octet rule, but in a chemical like phosphorus pentachloride it has ten valence electrons. Elements in period 2 is where the octet rule best applies. A lot of chemistry is learning simple rules and finding out about all the exceptions.

Hope that helps.