1. QUESTION:
Consider the following partially-drawn structure of 2-butene (C4H8) where only the C atoms are shown:
C-C=C-C
When this structure is completed following the octet/duet rule, how many H atoms are bonded to the first C?
A. 1, B. 2, C. 3, D. 4 H H H H
Consider the following partially-drawn structure of 2-butene (C4H8) where only the C atoms are shown.
When this structure is completed following the octet/duet rule, how many H atoms are singly-bonded the carbon atom on the left?
A. 1, B. 2, C. 3, D. 4
CLICK
Let’s make sure that all the carbon atoms are surrounded by an octet.
The first carbon atom is shown here with a single bond. To complete the octet, we need to surround this atom with three more pairs.
CLICK CLICK CLICK
The second atom is already surrounded by six electrons.
HIGHLIGHT single bond and double bond
So, this atom needs only one more pair of electrons in order to have an octet.
Similarly, the third carbon atom needs just one more pair of electrons.
And the fourth carbon atom needs three more pairs.
Now, let’s add hydrogen atoms.
We know that hydrogen atoms follow the duet rule. They can only form one single bond.
We have 8 hydrogen atoms, so let’s bond each of these atoms with a carbon atom.
...1
CLICK ...2
CLICK ...3
...4
...5
...6
...7
...8
Let’s do one final check... we need to make sure that the total number of valence electrons is correct.
Each carbon atom contributes 4 electrons, so the number of valence electrons from carbon is times 4 equals 16.
and each hydrogen atom contributes 1 electron, so the number of valence electrons from hydrogen is times 1 equals 8.
The total number of valence electrons shown in the structure should be equals 24.
Let’s count.
Here we have eight electrons.
Here we have another eight electrons
And here we have eight more
for a total of 24.
Therefore, the correct answer is choice C.
There are three hydrogen atoms bonded to the first carbon atom
HIGHLIGHT the three H atoms as you count
and 3....
CLICK PAUSE 2 seconds
END RECORDING H H H H
Number of valence electrons: 24 C: 4 x 4 = H: 8 x 1 = 8

2. Video ID: © 2008, Project VALUE (Video Assessment Library for Undergraduate Education), Department of Physical Sciences Nicholls State University Author: Glenn V. Lo Funded by Louisiana Board of Regents Contract No. LA-DL-SELECT-13-07/08