1. Step 1. Count number of valence electrons
QUESTION:
Which of the following has a double bond? A. HCl, B. O22-, C. N2, D. PF
Step 1. Count number of valence electrons
SCRIPT: 3-5-1
Which of the following diatomic species has a double bond? ….
Hydrogen chloride…. Peroxide ion…. Nitrogen…, or phosphorus fluoride
PAUSE 2 seconds
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The first step in determining the Lewis structure of a diatomic molecule is to count the number of valence electrons.
To do that, we first locate the atoms in the periodic table.
Hydrogen is located is in group “one A”.
CLICK Therefore, it has one valence electron.
CLICK, CLICK
Fluorine and chlorine belong to group “seven A”
Therefore, each Fluorine or chlorine atom has seven valence electrons
CLICK,
Oxygen belongs to group “Six A”
Therefore, each oxygen atom has six valence electrons
Finally, phosphorus and nitrogen belong to group “five A”
CLICK Therefore, every atom of nitrogen or phosphorus has five valence electrons.
To summarize,…
HIGHLIGHT Numbers
Hydrogen has one valence electron…
Chlorine has 7
Oxygen has 6
Nitrogen has 5
Phosphorus has 5
And Fluorine has 7
Therefore, HCl has a total of eight valence electrons. One plus seven equals eight
Peroxide has a total of 14 valence electrons. Each oxygen has six. Six times two is Twelve. But since peroxide has a negative 2 charge, that means two additional electrons. Twelve plus Two equals 14.
The N two molecule has a total of 10 valence electrons. Five for each nitrogen.
And the phosphorus fluoride molecule has 12 valence electrons. Five from phosphorus…. Plus seven from fluorine.
For diatomic molecules or ions, there are only five unique ways of satisfying the octet or duet rule.
CLICK and here they are.
If both atoms follow the duet rule, then the both atoms share two electrons in a single bond. Only hydrogen and helium atoms follow the duet rule.
HIGHLIGHT STRUCTURE 1
An example of this is the H TWO molecule, or the H E H PLUS ion.
None of the choices given has only two valence electrons…
Let’s consider the second possibility. If one atom follows the duet rule, and the other atom follows the octet rule, then the the total number of valence electrons must be eight. The atoms share two electrons in a single bond, and the atom that requires an octet has three lone pairs.
HIGHLIGHT STRUCTURE 2
HCl has 8 valence electrons, so its Lewis structure looks like this.
Let’s look at the third possibility. If both atoms follow the octet rule and they have 14 valence electrons, then they share two electrons in a single bond, and each atom has three lone pairs.
HIGHLIGHT STRUCTURE 3
Peroxide ion has 14 valence electrons, so its Lewis structure looks like this. Note that we have enclose the structure in a square bracket and indicate the charge as a superscript. We do this for ions.
Next, we look at the fourth possibility. f both atoms follow the octet rule and they have 12 valence electrons, then they share FOUR electrons in a double bond, and each atom has two lone pairs.
HIGHLIGHT STRUCTURE 4
Phosphorus fluoride has 12 valence electrons, so its Lewis structure looks like this.
This is the correct answer. The question is asking for the molecule or ion that has a double bond.
What about the last possibility. If both atoms follow the octet rule and they have 10 valence electrons, then they share SIX electrons in a triple bond, and each atom has one lone pair.
HIGHLIGHT STRUCTURE 5
This is the case for the nitrogen molecule. It has ten valence electrons. So its Lewis structure can be described as having a triple bond between the two atoms, and a lone pair on each of the atoms.
END RECORDING

3. QUESTION:
Which of the following has a double bond? A. HCl, B. O22-, C. N2, D. PF
Step 1. Count number of valence electrons H-1, Cl-7, O-6, N-5, P-5, F-7
HCl: 1+7 = O22-: = 14
N2: = PF: = 12

4. QUESTION:
Which of the following has a double bond? A. HCl, B. O22-, C. N2, D. PF
Step 1. Count number of valence electrons HCl: 8, O22-: 14, N2: 10, PF: 12
Step 2. Compare with five possibilities of satisfying the octet/duet rule for diatomic molecules/ions.

5. 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