We’re being asked to determine which distance would be longer, Na-Cl or K-F.

To estimate the lattice energy (or ionic bond energy) of an ionic compound, we will use the following equation:

$\overline{){\mathbf{lattice}}{\mathbf{}}{\mathbf{energy}}{\mathbf{=}}\frac{\left|\mathbf{cation}\mathbf{}\mathbf{charge}\mathbf{\times}\mathbf{anion}\mathbf{}\mathbf{charge}\right|}{\mathbf{cation}\mathbf{}\mathbf{radius}\mathbf{}\mathbf{+}\mathbf{}\mathbf{anion}\mathbf{}\mathbf{radius}}}$

**Radius = period number**

NaCl and KF have the same crystal structure. The only difference between the two is the distance that separates cations and anions.

Compound | Lattice Energy kJ/mol | Compound | Lattice Energy kJ/mol | |

LiF | 1030 | MgCl_{2} | 2326 | |

LiCl | 834 | SrCl_{2} | 2127 | |

LiI | 730 | |||

NaF | 910 | MgO | 3795 | |

NaCl | 788 | CaO | 3414 | |

NaBr | 732 | SrO | 3217 | |

NaI | 682 | |||

KF | 808 | ScN | 7547 | |

KCl | 701 | |||

KBr | 671 | |||

CsCl | 657 |

The lattice energies of NaCl and KF are given in the table in the introduction. Based on the lattice energies, would you expect the Na-Cl or the K-F distance to be longer?

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