Which is a correct Lewis structure for hydrogen cyanide, HCN?
a)H-CEN
b)H-C=N:
c)H-CEN:
d)H-CEN:
The Correct Answer and Explanation is:
The correct Lewis structure for hydrogen cyanide (HCN) is:
b) H-C≡N:
Explanation:
To understand why option b) H-C≡N: is the correct Lewis structure for HCN, let’s go through the steps involved in constructing a Lewis structure for HCN.
- Count the Valence Electrons:
- Hydrogen (H) has 1 valence electron.
- Carbon (C) has 4 valence electrons.
- Nitrogen (N) has 5 valence electrons. In total, HCN has ( 1 + 4 + 5 = 10 ) valence electrons.
- Determine the Skeleton Structure:
The central atom is carbon (C), since hydrogen (H) typically forms only one bond and cannot be placed at the center. Nitrogen (N) will form a bond with carbon, and hydrogen will bond to carbon. The skeleton structure is: H – C – N. - Distribute the Electrons:
- First, form bonds between atoms. Each single bond requires two electrons, so placing single bonds between H and C, and C and N uses 4 electrons (2 electrons per bond × 2 bonds).
- This leaves 6 electrons to be distributed as lone pairs.
- Place Remaining Electrons as Lone Pairs:
After placing the bonds, there are 6 electrons left. These will be placed as lone pairs on nitrogen and carbon. Nitrogen needs to satisfy the octet rule, so it will get three lone pairs, and carbon will get one lone pair. - Form Multiple Bonds:
Nitrogen has only 2 electrons in its bond with carbon, and it needs 6 more to complete its octet. Therefore, a triple bond (≡) is formed between carbon and nitrogen. This uses 6 electrons, and nitrogen now has a full octet. After the triple bond, hydrogen’s 1 electron pairs with carbon’s 4 electrons, satisfying hydrogen’s duet rule. - Final Structure:
The final Lewis structure is: [
H – C \equiv N:
]
where carbon shares three electrons with nitrogen, forming a triple bond (≡), and nitrogen has a lone pair.
Why Other Options Are Incorrect:
- a) H-CEN: is incorrect because “EN” is not a valid element and this does not follow the typical bonding patterns of HCN.
- c) H-C≡N: is incomplete, as it does not show the lone pair on nitrogen.
- d) H-CEN: is similar to option a, and it’s not a valid structure.
In summary, option b) H-C≡N: is the correct Lewis structure for hydrogen cyanide. It satisfies the valence electron count and bonding requirements for each atom involved.