Determine the type of crystalline solid, the force holding it together, and rank by...

1. Identify the type of solid and bonding for Niobium (Nb): Niobium is a transition metal. Metals form metallic solids where metal atoms are held together by metallic bonds (a "sea" of delocalized electrons). Metallic solids generally have high melting points.
2. Identify the type of solid and bonding for Hydrogen Fluoride (HF): HF is a polar covalent molecule. In the solid state, it forms a molecular solid. Due to the high electronegativity difference between H and F, HF exhibits hydrogen bonding, which is a strong type of intermolecular force. Molecular solids with hydrogen bonding have relatively high melting points compared to other molecular solids.
3. Identify the type of solid and bonding for Oxygen (O2): Oxygen is a diatomic molecule. In the solid state, it forms a molecular solid. The O2 molecule is nonpolar, so the intermolecular forces are London dispersion forces, which are weak. Molecular solids with only London dispersion forces have low melting points.
4. Compare melting points:
- Metallic solids (like Nb) generally have very high melting points due to strong metallic bonding.
- Molecular solids with hydrogen bonding (like HF) have intermediate to high melting points.
- Molecular solids with only London dispersion forces (like O2) have low melting points.
Therefore, the order of decreasing melting point is Nb > HF > O2.
5. Rank the solids:
- Nb: Metallic solid, metallic bonding, rank 1 (highest melting point).
- HF: Molecular solid, hydrogen bonding, rank 2 (intermediate melting point).
- O2: Molecular solid, London dispersion forces, rank 3 (lowest melting point).