How to find formal charge of C2H2
The correct answer and explanation is:
How to Find the Formal Charge of C₂H₂ (Acetylene)
Step 1: Draw the Lewis Structure of C₂H₂
- Carbon (C) has 4 valence electrons.
- Hydrogen (H) has 1 valence electron.
- C₂H₂ has 2 carbon atoms and 2 hydrogen atoms.
- The structure is: H–C≡C–H (a triple bond between the two carbons, and each carbon bonded to one hydrogen).
Step 2: Count the Electrons Around Each Atom in the Lewis Structure
- Each carbon forms one single bond with hydrogen (2 electrons shared).
- Each carbon forms a triple bond with the other carbon (6 electrons shared).
- Total bonding electrons around each carbon = 2 (H–C bond) + 6 (C≡C bond) = 8 electrons shared.
Step 3: Use the Formal Charge Formula
Formal charge (FC) = Valence electrons of atom – Nonbonding electrons – ½(Bonding electrons)
Calculate for each atom:
- For Carbon:
- Valence electrons = 4
- Nonbonding electrons = 0 (no lone pairs on carbon in C₂H₂)
- Bonding electrons = 8 (one single bond + one triple bond)
- For Hydrogen:
- Valence electrons = 1
- Nonbonding electrons = 0 (no lone pairs)
- Bonding electrons = 2 (one single bond)
Final result:
- Formal charge on each carbon atom = 0
- Formal charge on each hydrogen atom = 0
The overall formal charge on the molecule C₂H₂ is zero, which means it is a stable, neutral molecule.
Explanation (300 words):
Formal charge is a theoretical charge assigned to individual atoms in a molecule, which helps us understand the distribution of electrons and stability of a molecule. It is calculated assuming electrons in bonds are shared equally between atoms, regardless of electronegativity.
In the acetylene molecule (C₂H₂), each carbon is bonded to one hydrogen and the other carbon through a triple bond. Carbon has four valence electrons, and in this structure, all are involved in bonding. Each carbon shares three pairs of electrons with the other carbon and one pair with a hydrogen, totaling four pairs (or eight electrons) around each carbon.
Calculating the formal charge involves subtracting the number of electrons assigned to an atom in the Lewis structure from the number of valence electrons in the free atom. Electrons in lone pairs are assigned fully to that atom, while bonding electrons are split equally between the two atoms involved.
For C₂H₂, carbon atoms have no lone pairs, so all their electrons are bonding electrons. Since each carbon shares eight electrons (four bonds × 2 electrons per bond), they are assigned half of those (4 electrons). Subtracting this from the four valence electrons carbon naturally has gives a formal charge of zero. Similarly, hydrogens share two electrons in a single bond, so they are assigned one electron, which matches their valence count, giving a formal charge of zero.
Because all atoms have a formal charge of zero, the molecule is electrically neutral and considered stable in this structure. This is consistent with the known properties of acetylene, which has a linear geometry and strong triple bonds.
Understanding formal charges helps chemists determine the most plausible Lewis structures and predict reactivity or stability of molecules.