Which one of the following compounds does not follow the octet rule?
a) NF3
b) CF4
c) SF4
d) PH3
e) HCl
I know I can eliminate A and B because nitrogen and carbon follow the octet rule. I know I can eliminate E because the total amount of valence electrons in HCl is 8. I am not sure about C and D.
We can't draw structures on this forum but these two links should help you decide.
https://terpconnect.umd.edu/~wbreslyn/chemistry/Lewis-Structures/lewis-structure-for-PH3.html
https://terpconnect.umd.edu/~wbreslyn/chemistry/Lewis-Structures/lewis-structure-for-SF4.html
So it looks like SF4 does not follow the octet rule?
That's right.
To determine whether compounds follow the octet rule, we need to count the number of valence electrons for each atom and then distribute them to satisfy the octet rule.
Let's examine compounds C and D:
c) SF4:
To determine the number of valence electrons in SF4, we add the valence electrons of sulfur (group 16) and fluorine (group 17):
Sulfur (S) has 6 valence electrons, and there are 4 fluorine (F) atoms, each contributing 7 valence electrons.
6 (Sulfur) + 4 * 7 (Fluorine) = 34 valence electrons
Next, we distribute the valence electrons to satisfy the octet rule. We place the remaining electrons around each atom and then check if each atom has achieved an octet:
F | S | F | F
7 | | 7 | 7 | 7
| - 2 |
5 | | 6 | 6 | 6
In this case, all the atoms have achieved an octet, so SF4 follows the octet rule.
d) PH3:
To determine the number of valence electrons in PH3, we add the valence electrons of phosphorus (group 15) and hydrogen (group 1):
Phosphorus (P) has 5 valence electrons, and there are 3 hydrogen (H) atoms, each contributing 1 valence electron.
5 (Phosphorus) + 3 * 1 (Hydrogen) = 8 valence electrons
Next, we distribute the valence electrons to satisfy the octet rule. We place the remaining electrons around each atom and then check if each atom has achieved an octet:
H | P | H | H
1 | | 5 | 1 | 1
| - 2 |
3 | | 3 | 3 | 3
In this case, phosphorus has not achieved an octet, as it only has 6 electrons. Therefore, PH3 does not follow the octet rule.
From the given compounds, the one that does not follow the octet rule is d) PH3.