NaF solid and Hf gas...in same conditions..why?

Doji

17

Q: Why is NaF a solid in rtp whereas HF is gas in the same conditions?

Reply 1

YorkshireTerrier

NaF is an ionic compound. It is a giant ionic lattice and the Na+ and F+ ions are held together by strong electrostatic interactions. Therefore at room temperature this will be a solid.

HF, however, is a simple molecular compound. The HF molecules consist of a single covalent bond between the two atoms. The attraction between molecules is by hydrogen bonding (yes, you usually think of water for this, but HF does too). This is because the fluorine atom is so electronegative that it attracts the bonding pair of electrons so close to itself that it effectively leaves a bare proton in place of the hydrogen. This can then form a "reasonably" strong hydrogen bond with a lone pair of electrons from a fluorine on an adjacent HF molecule. Hydrogen bonds might be reasonably strong as far as simple interactions, but they don't match anywhere near as strong as the ionic bonds in NaF. This is why HF is a gas.

Matt (Chemistry teacher)