Good question. Alkali metals (Li, Na, K, Rb, Cs,Fr) form simple salts (chlorides, nitrates, ...) that are soluble because alkali metals do not have a high charge (+1). Salts are held together through the attraction of opposite charges, so that salts with ions of low charges are less strongly held together (and can dissolve in water) than salts held together with high charges:
Na⁺ + Cl- --> NaCl (soluble)
Ba²+ + SO_4²- --> BaSO₄ (insoluble)

To makeinsoluble salts of alkali metals, one must use complex anions. Can you name a Na salt that does not easily dissolve? (Hint: You used it this morning in the shower).

Alkaline metals are easy to ionize by taking away an electron to give them a positive one charge. With salts that contain these types of ions the attraction between the positive ion (alkali metal ion) and the negatively charged ion is weak enough for water molecules to be able to separate, therefore allowing the compound to be soluble.

Some very good questions. ALkali metals are distinguished by having a single electron in its outermost (most reactive) ground state. Alkali salts are all ionic which refers to the bonding mechanism of the crystalline solid. Since water has a large dipole moment (the oxygen tends to be negative and the hydrogen atoms (which are located on one side of the oxygen tend to be positive) it is a good solvent of nearly all alkali salts. One exception is LiF which is insoluble because both Lithium and Florine have very small ions --making solvation energetically infeasible at room temperatures and pressures.

On the other hand -- all the elemental nitrate salts I know of are non-soluble.

The reason why water dissolves salts is because it is polar; the hydrogen atoms are on one side of the oxygen, and acquire a partial positive charge, while the oxygen acquires a partial negative charge. In essence, water is almost a quasi-salt itself.

The stronger two ions in a salt are held together, the more difficult it is for the water to pull them apart, surround them, and dissolve them. This strength is the strength of the electrostatic attraction between the two charges in the salt, which is proportional to the product of the charges times the distance between them (Coulomb's law). Thus, the greater the charges, or the smaller the ion containing the charges, the stronger the separation.

Alkali metals, while being small atoms, have only a +1charge, and even if paired with, say, oxygen (which multiplies to two charges squared), they don't exert enough attractive force to hold them together. Generally, you need four squared charges (e.g. bariumis +2, oxygen is -2, 2x2 = 4) in order to make something really insoluble. Nitrate is similar - it has only a -1 charge, so even with something like Aluminum (+3), it is still pretty soluble. Nitrate is also a huge ion, which increases the distance, and weakens the force.