The refractive indices of gases such as ammonia or nitrogen are, at ambient temperature and pressure and at visible wavelength, very close to one. It means that they have a very small electric polarization compared to solids. Why is it so? Are there gases with higher refractive indices (for example 1.2 or above) ?
There is a direct relationship between the amount of light that a material absorbs and its refractive index (the Kramers-Kronig relations). Essentially, every absorption feature in a material causes the refractive index to increase at all wavelengths larger than the wavelength at which that absorption feature occurs. Given this, it is easy to see why gases tend to have small refractive indices: their density is so small that they don't absorb much light.