What is the cause of hygroscopy?

Hygroscopy is the ability of a substance to attract and hold water molecules from the surrounding environment.

Why are some compounds hygroscopic and others are not? What is it about their structures that allows them to hold water? Is it due to the atoms being arranged in a certain structure due to bonding?

How can you tell if a compound is more hygroscopic than another?

• Welcome to chemistry.SE! If you had any questions about the policies of our community, please ‎visit the help center. – It's Over Aug 2 '15 at 16:18
• I was also wondering this as well. Can ions be hygroscopic, and if so, are ions like $\text{Na}^+$ and $\text{Cl}^-$ be considered hygroscopic? – phi2k Aug 2 '15 at 16:27
• – Mithoron Aug 2 '15 at 18:03
• Hygroscopy is macroscopic property - not applicable to single ions or molecules. – Mithoron Aug 2 '15 at 18:12

• Also, it is worth mentioning that formation of hydrates is a property of compound as a whole, rather than individual ions. See, for example: neither $\ce{NaCl}$ nor $\ce{CaCO_3}$ form hydrates, but both $\ce{Na_2CO_3}$ and $\ce{CaCl_2}$ do, the latter quite eagerly. – Ivan Neretin Sep 4 '15 at 14:58