StellarReal Gases
The ideal gas law makes one incorrect assumption: gasses are ideal. In reality, particles in a gas do have some size and they do exert IMFs with other particles. While performing calculations on real gasses is hard, we can determine which gasses will deviate further from the ideal gas law.
One way gasses deviate from the ideal gas law is through exerting IMFs. Simply put, gasses that exert more IMFs on each other will deviate more. So for example, Helium gas will have very little IMFs between the particles, so it will not deviate much from the ideal gas law. However, a gas like ammonia is polar, so it will exert greater IMFs that make it deviate more from the ideal gas law.
Another way gasses deviate from the ideal gas law is through their shape, which affects the volume and pressure that a gas exerts on its container. The bigger a molecule is, the less “actual” volume is in the container since the molecule takes up some of that space. This means that a big molecule will have a lower actual volume, which will result in it having a higher pressure than an ideal gas should. You can also think about it intuitively. If a gas particle is bigger, it will hit the walls of the container more often, generating more pressure.