How many of the following molecules are polar? CH4, CH3F, CH2F2, CHF3, CF4
The correct answer and explanation is:
To determine which molecules are polar, we need to consider both the molecular geometry and the electronegativity difference between the atoms involved.
- CH4 (Methane):
- Methane has a tetrahedral geometry with four hydrogen atoms symmetrically arranged around a central carbon atom. Since the electronegativity of carbon (2.55) and hydrogen (2.20) is very similar, there is no significant dipole moment.
- Conclusion: Methane is nonpolar.
- CH3F (Fluoromethane):
- Fluoromethane has a tetrahedral geometry, but one of the hydrogen atoms is replaced by a fluorine atom. Fluorine is highly electronegative (3.98), which creates a significant dipole moment. The electronegativity difference between carbon (2.55) and fluorine (3.98) causes the molecule to have a polar bond.
- Conclusion: Fluoromethane is polar.
- CH2F2 (Difluoromethane):
- Difluoromethane also has a tetrahedral geometry, with two fluorine atoms attached to the carbon. Fluorine’s high electronegativity causes polar bonds with carbon. The two fluorine atoms are positioned on opposite sides, creating an uneven distribution of electron density.
- Conclusion: Difluoromethane is polar.
- CHF3 (Trifluoromethane):
- In trifluoromethane, three fluorine atoms are attached to the carbon atom. The molecule has a tetrahedral geometry, but the electronegativity difference between carbon and fluorine results in polar bonds. The presence of three highly electronegative fluorine atoms leads to a significant overall dipole moment in the molecule.
- Conclusion: Trifluoromethane is polar.
- CF4 (Carbon Tetrafluoride):
- CF4 has a tetrahedral geometry, but all four bonds are between carbon and fluorine. The electronegativity difference between carbon and fluorine creates polar bonds. However, because of the symmetrical arrangement of the fluorine atoms around the central carbon, the dipoles cancel out, and there is no overall dipole moment.
- Conclusion: Carbon tetrafluoride is nonpolar.
Final Answer: 3 molecules are polar: CH3F, CH2F2, CHF3.