Krypton is actually not lighter than other noble gas atoms. In fact, krypton is heavier than some of the noble gases such as helium, neon, and argon. The weight of an atom is determined by its atomic mass, which is the combined mass of its protons, neutrons, and electrons.
Here are the atomic masses of some noble gases:
- Helium (He): 4 atomic mass units (amu)
- Neon (Ne): 20.18 amu
- Argon (Ar): 39.95 amu
- Krypton (Kr): 83.80 amu
- Xenon (Xe): 131.29 amu
- Radon (Rn): 222.0 amu
As you can see, krypton (Kr) has a higher atomic mass compared to helium, neon, and argon, but it is lighter than xenon and radon.
If you are referring to the density of krypton, which is the mass per unit volume, then it is true that krypton has a lower density compared to some other noble gases. This is because density depends not only on the atomic mass but also on the volume occupied by the atoms. The density of a gas is influenced by factors such as atomic size, intermolecular forces, and temperature.
In the case of noble gases, the density generally increases as you move down the group in the periodic table. This is because the atoms become larger and heavier, leading to a higher density. Krypton, being in the middle of the noble gases, has a moderate density compared to helium, neon, and argon, but it is lighter than xenon and radon, which have larger and heavier atoms.
It's worth noting that the differences in density among noble gases are relatively small, and they are all considered to be very low-density gases under normal conditions.