The hydration of the five ions (Li+, Na+, K+, F- and Cl-) in infinite dilute aqueous solutions is investigated by molecular dynamics (MD) simulations at 298 K. Clear microscopic configurations of ions in aqueous solutions are obtained. Cations are surrounded by water molecules with oxygen atoms approaching them, while anions are surrounded by water molecules with hydrogen atoms approaching them. A new hydration factor is proposed to quantitatively characterize the strength of ionic hydration. The order of hydration strength for cations is Li+ > Na+ > K+, whereas, for anions F- > Cl-. For Li+, although there is a second coordination shell, no second hydration shell exists. Pauling radius of an ion is the most important factor that determines the strength of hydration. When constructing a corresponding molecular thermodynamic model for electrolyte solutions, parameters related to ionic radii should be preferred. Ionic hydration may affect not only short-range structures of ions, but also diffusion properties of ions. The electrostatic interaction between an ion and a water molecule is the essential reason to cause the hydration structure.