Lewis Diagrams Made Easy: How to Draw Lewis Dot Structures

Welcome to Lewis Diagrams Made Easy with Ketzbook Today we are going to learn how to draw Lewis Diagrams for atoms and simple molecules, but before we begin, let’s start with a question. How many valence electrons does chlorine have? In order to answer that question, we need
to look at the Periodic Table. Remember that within any given column, all the elements have the same number of valence electrons. To get that number, all we do is count the
columns starting from the left. Skip the transition metals, and remember that the only exception to this is helium, which has only 2 valence electrons, not 8. Now, find chlorine in the Periodic Table. Remember that it’s symbol is Cl. See it in the 7th column? That tells us that it has 7 valence electrons. Knowing the number of valence electrons an element has is critical, and in Lewis diagrams, we use dots to represent valence electrons. So, the Lewis diagram of chlorine is the symbol Cl with 7 dots around it. When you draw the dots, don’t just put them anywhere. Instead, imagine a square around the element’s symbol. The dots should be neatly drawn on the four
sides of the square with no more than two dots on any side. Practice drawing the Lewis diagrams of a few
elements just to make sure you’ve got it. This is the Lewis diagram of hydrogen, which
has only one valence electron. This is carbon, which has four valence electrons,
and this is oxygen, which has 6 valence electrons. Where you put the dots doesn’t really matter
as long as you neatly draw them along the sides of an imaginary square and never put
more than two dots on one side. Lewis diagrams are often used to represent
covalent bonding in molecules and ions. In covalent bonding, atoms share valence electrons
in order to get a full octet or duet, that is every non-metal element wants 8 valence
electrons, except for hydrogen, which only wants two valence electrons. The simplest molecule possible is that of
hydrogen, H2. A hydrogen atom has one valence electron,
but it wants to have two. So, in order to satisfy its desire for another
electron, two hydrogen atoms will share their electrons with each other. And the crazy thing is that in the wonderful
world of atoms, the shared electrons are counted as owned by both atoms. That means that both hydrogen atoms are happy
because they both satisfy the octet rule. Now, we normally draw hydrogen and other molecules
like this with lines to represent shared electrons and dots only for non-bonding electrons. These two diagrams of a hydrogen molecule are equivalent because one line, which is a single bond, represents 2 shared electrons. In the same way, two lines between atoms would be a double bond and would be the sharing of 4 electrons. An example of a molecule with a double bond
is oxygen, which looks like this. Notice that in this Lewis diagram both oxygen
atoms have 8 valence electrons, 4 from the double bond and 4 from the lone pairs of electrons. By the way, 2 dots together are called a lone
pair of electrons. Some molecules contain triple bonds, which
we write using 3 lines that represent the sharing of 6 electrons. An example of a molecule with a triple bond
is nitrogen, which looks like this. Once again, notice that both nitrogen atoms
have 8 valence electrons, 6 from the triple bond and 2 from the lone pairs of electrons. Okay, how do you actually draw the Lewis diagram
of a molecule? Let’s start with water, H2O. There are 5 important steps that you need
to follow when drawing the Lewis diagram of a molecule. First, count all of the valence electrons
in the molecule. For water, each hydrogen has one electron
and we multiply that by two because there are two hydrogens in the molecule. The oxygen has 6 valence electrons. Add those all up, and we have a total of 8
valence electrons for the water molecule. Step 2, determine the central atom. The central atom is the one that all of the
other atoms will be bonded to. It is usually the element that there is only
one of. In the case of H2O, because there are 2 hydrogens
and only one oxygen, we choose oxygen as the central atom and write it in the middle. Step 3, draw single bonds to the central atom. Step 4, put all of the remaining valence electrons
on atoms as lone pairs. For H2O, we started with 8 valence electrons;
we have used four electrons for the two single bonds, so that leaves 4 more electrons left
over. We put all 4 of those remaining electrons
on oxygen instead of hydrogen because hydrogen is already happy with 2 valence electrons. Remember, never give hydrogen more than 2
valence electrons. Everyone else wants 8 electrons, but hydrogen
only wants 2. Step 5, turn lone pairs into double or triple
bonds to give every atom an octet or (duet for hydrogen). Pause the video and see if all the atoms in
our H2O are happy. Because each hydrogen has 2 electrons, and
the oxygen has 8 electrons, everyone is happy, and there is no need for double or triple
bonds, which means that our Lewis diagram of water is now complete. All right, let’s try one more example just
to make sure we’ve got this. Draw the Lewis diagram of sulfur trioxide. Once again, the first step is to count all
the valence electrons. There is one sulfur with six valence electrons,
and there are 3 oxygens with 6 valence electrons each. 6 times 3 gives us 18 valences electrons for
the 3 oxygens. The total would be 6 + 18, which equals 24
valence electrons. Step 2, determine the central atom. This time, sulfur is the central atom because
there is only one sulfur in the molecule. We write sulfur in the middle with the three
oxygen atoms all around it. Step 3, draw single bonds to the central atom. Step 4, we started with 24 valence electrons,
and we have used 6 electrons to make the 3 single bonds. This means that we have 24 minus 6 or 18 valence
electrons remaining. We now put those remaining 18 valence electrons
on atoms as lone pairs. Count by two’s when adding them. 2, 4, 6, 8, 10, 12, 14, 16, and 18. That’s it, we’ve now used up all of our
valence electrons. Step 5, first pause the video and check to
see if any atom does not have an octet or duet. That’s right, the sulfur is unhappy because
it only has 6 valence electrons. So, what can we do to make it happy? We cannot just give it more electrons, since
we don’t have any more. Instead, one of the oxygen atoms can take
one of its lone pairs and share those two electrons with sulfur in another bond. The double bond between oxygen and sulfur
now makes every atom happy with 8 valence electrons. Be sure to follow these steps when drawing
Lewis diagrams. Thanks for watching. Please comment, vote, subscribe, or check
me out at ketzbook.com.

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