Lewis Diagrams Made Easy: How to Draw Lewis Dot Structures — Transcript

Full transcript

1. 0:02Welcome to Lewis Diagrams Made Easy with Ketzbook
2. 0:05Today we are going to learn how to draw Lewis Diagrams for atoms and simple molecules,
3. 0:09but before we begin, let's start with a question.
4. 0:12How many valence electrons does chlorine have?
5. 0:14In order to answer that question, we need to look at the Periodic Table.
6. 0:18Remember that within any given column, all the elements have the same number of valence electrons.
7. 0:22To get that number, all we do is count the columns starting from the left.
8. 0:27Skip the transition metals, and remember that the only exception to this is helium,
9. 0:31which has only 2 valence electrons, not 8.
10. 0:34Now, find chlorine in the Periodic Table.
11. 0:37Remember that it's symbol is Cl.
12. 0:39See it in the 7th column?
13. 0:41That tells us that it has 7 valence electrons.
14. 0:44Knowing the number of valence electrons an element has is critical, and in Lewis diagrams,
15. 0:49we use dots to represent valence electrons.
16. 0:52So, the Lewis diagram of chlorine is the symbol Cl with 7 dots around it.
17. 0:58When you draw the dots, don't just put them anywhere.
18. 1:01Instead, imagine a square around the element's symbol.
19. 1:03The dots should be neatly drawn on the four sides of the square with no more than two
20. 1:07dots on any side.
21. 1:09Practice drawing the Lewis diagrams of a few elements just to make sure you've got it.
22. 1:13This is the Lewis diagram of hydrogen, which has only one valence electron.
23. 1:17This is carbon, which has four valence electrons, and this is oxygen, which has 6 valence electrons.
24. 1:23Where you put the dots doesn't really matter as long as you neatly draw them along the
25. 1:27sides of an imaginary square and never put more than two dots on one side.
26. 1:32Lewis diagrams are often used to represent covalent bonding in molecules and ions.
27. 1:36In covalent bonding, atoms share valence electrons in order to get a full octet or duet,
28. 1:42that is every non-metal element wants 8 valence electrons,
29. 1:46except for hydrogen, which only wants two valence electrons.
30. 1:50The simplest molecule possible is that of hydrogen, H2.
31. 1:53A hydrogen atom has one valence electron, but it wants to have two.
32. 1:57So, in order to satisfy its desire for another electron,
33. 2:01two hydrogen atoms will share their electrons with each other.
34. 2:04And the crazy thing is that in the wonderful world of atoms,
35. 2:08the shared electrons are counted as owned by both atoms.
36. 2:12That means that both hydrogen atoms are happy because they both satisfy the octet rule.
37. 2:18Now, we normally draw hydrogen and other molecules like this with lines to represent shared electrons
38. 2:24and dots only for non-bonding electrons.
39. 2:27These two diagrams of a hydrogen molecule are equivalent
40. 2:31because one line, which is a single bond, represents 2 shared electrons.
41. 2:36In the same way, two lines between atoms would be a double bond
42. 2:39and would be the sharing of 4 electrons.
43. 2:42An example of a molecule with a double bond is oxygen, which looks like this.
44. 2:47Notice that in this Lewis diagram both oxygen atoms have 8 valence electrons,
45. 2:514 from the double bond and 4 from the lone pairs of electrons.
46. 2:57By the way, 2 dots together are called a lone pair of electrons.
47. 3:01Some molecules contain triple bonds, which we write using 3 lines that represent the
48. 3:05sharing of 6 electrons.
49. 3:07An example of a molecule with a triple bond is nitrogen, which looks like this.
50. 3:12Once again, notice that both nitrogen atoms have 8 valence electrons,
51. 3:176 from the triple bond and 2 from the lone pairs of electrons.
52. 3:21Okay, how do you actually draw the Lewis diagram of a molecule?
53. 3:25Let's start with water, H2O.
54. 3:28There are 5 important steps that you need to follow when drawing the Lewis diagram of
55. 3:33a molecule.
56. 3:34First, count all of the valence electrons in the molecule.
57. 3:37For water, each hydrogen has one electron and we multiply that by two because there
58. 3:44are two hydrogens in the molecule.
59. 3:46The oxygen has 6 valence electrons.
60. 3:49Add those all up, and we have a total of 8 valence electrons for the water molecule.
61. 3:55Step 2, determine the central atom.
62. 3:57The central atom is the one that all of the other atoms will be bonded to.
63. 4:01It is usually the element that there is only one of.
64. 4:03In the case of H2O, because there are 2 hydrogens and only one oxygen, we choose oxygen as the
65. 4:10central atom and write it in the middle.
66. 4:13Step 3, draw single bonds to the central atom.
67. 4:16Step 4, put all of the remaining valence electrons on atoms as lone pairs.
68. 4:22For H2O, we started with 8 valence electrons; we have used four electrons for the two single
69. 4:29bonds, so that leaves 4 more electrons left over.
70. 4:32We put all 4 of those remaining electrons on oxygen instead of hydrogen because hydrogen
71. 4:39is already happy with 2 valence electrons.
72. 4:41Remember, never give hydrogen more than 2 valence electrons.
73. 4:46Everyone else wants 8 electrons, but hydrogen only wants 2.
74. 4:51Step 5, turn lone pairs into double or triple bonds to give every atom an octet
75. 4:58or (duet for hydrogen).
76. 4:59Pause the video and see if all the atoms in our H2O are happy.
77. 5:04Because each hydrogen has 2 electrons, and the oxygen has 8 electrons, everyone is happy,
78. 5:09and there is no need for double or triple bonds, which means that our Lewis diagram
79. 5:13of water is now complete.
80. 5:15All right, let's try one more example just to make sure we've got this.
81. 5:21Draw the Lewis diagram of sulfur trioxide.
82. 5:24Once again, the first step is to count all the valence electrons.
83. 5:27There is one sulfur with six valence electrons, and there are 3 oxygens with 6 valence electrons
84. 5:34each.
85. 5:356 times 3 gives us 18 valences electrons for the 3 oxygens.
86. 5:39The total would be 6 + 18, which equals 24 valence electrons.
87. 5:44Step 2, determine the central atom.
88. 5:46This time, sulfur is the central atom because there is only one sulfur in the molecule.
89. 5:51We write sulfur in the middle with the three oxygen atoms all around it.
90. 5:56Step 3, draw single bonds to the central atom.
91. 6:01Step 4, we started with 24 valence electrons, and we have used 6 electrons to make the 3
92. 6:06single bonds.
93. 6:07This means that we have 24 minus 6 or 18 valence electrons remaining.
94. 6:13We now put those remaining 18 valence electrons on atoms as lone pairs.
95. 6:18Count by two's when adding them.
96. 6:212, 4, 6, 8, 10, 12, 14, 16, and 18.
97. 6:30That's it, we've now used up all of our valence electrons.
98. 6:36Step 5, first pause the video and check to see if any atom does not have an octet or duet.
99. 6:42That's right, the sulfur is unhappy because it only has 6 valence electrons.
100. 6:47So, what can we do to make it happy?
101. 6:50We cannot just give it more electrons, since we don't have any more.
102. 6:54Instead, one of the oxygen atoms can take one of its lone pairs and share those two
103. 6:59electrons with sulfur in another bond.
104. 7:04The double bond between oxygen and sulfur now makes every atom happy with 8 valence electrons.
105. 7:10Be sure to follow these steps when drawing Lewis diagrams.
106. 7:13Thanks for watching.
107. 7:14Please comment, vote, subscribe, or check me out at ketzbook.com.