How many lone pairs are on the central atom of the HCN molecule?
The Correct Answer and Explanation is:
The correct answer is 0.
The HCN molecule, hydrogen cyanide, contains three atoms: hydrogen (H), carbon (C), and nitrogen (N). To determine the number of lone pairs on the central atom, we must first identify which atom occupies the central position. In HCN, carbon is the central atom because it can form four bonds and is less electronegative than nitrogen.
Hydrogen contributes one valence electron, carbon has four, and nitrogen brings five, totaling ten valence electrons to distribute in the molecule. Carbon forms a single bond with hydrogen and a triple bond with nitrogen. This bonding configuration satisfies the octet rule for both carbon and nitrogen.
In the Lewis structure, hydrogen uses its one valence electron to form a single bond with carbon. Carbon, using all four of its valence electrons, forms a single bond with hydrogen and three bonds with nitrogen. This means all four of carbon’s electrons are involved in bonding, leaving no electrons as non-bonding pairs. Therefore, carbon has zero lone pairs.
Nitrogen completes its octet by sharing three electrons in the triple bond with carbon and retaining one lone pair. The lone pair resides exclusively on nitrogen, not on carbon.
The molecular geometry of HCN is linear, with a bond angle of approximately 180 degrees. This is consistent with the electronic structure, as there are no lone pairs on the central carbon atom to repel the bonding pairs and distort the shape.
Understanding the lone pair distribution is essential for predicting molecular geometry, polarity, and reactivity. In HCN, the absence of lone pairs on carbon reinforces the molecule’s linear shape and impacts how it interacts with other molecules, particularly in biological or industrial contexts where its polarity plays a role in reactivity.
