Answer: A solution forms when the attractive _forces_ between the solute and solvent are _similar/higher_ in strength to the solvent-solvent and solute-solute attractions and are therefore able to replace them.
Explanation: Intermolecular forces between solute and solvent must be able to match or exceed the natural intramolecular forces between the solvent-solvent atoms/molecules and solute-solute atoms/molecules.
This is where the solubility law of 'like dissolves like (polar dissolves polar and likewise)' comes into play. Only polar compounds can form such attractive forces with fellow polar compounds and the same story goes for non-polar compounds.
The atomic size decreases; this is because as you go across the period, more electrons are found in the outer shell. As more electrons are added, the more attraction there is between the protons, in the nucleus, and the electrons, making the atom smaller.
Hope this helps!
