In the real world, it is extremely rare for atoms to exist in their absolute lowest-energy bound state, also known as the ground state. The ground state of an atom corresponds to the configuration where all its electrons are in their lowest possible energy levels.
According to the Pauli exclusion principle, which is a fundamental principle in quantum mechanics, no two electrons can occupy the exact same quantum state simultaneously. This means that as electrons occupy the available energy levels in an atom, they must occupy higher energy levels if the lower ones are already filled.
In most cases, atoms have multiple electrons, and these electrons occupy various energy levels based on the atomic structure. Electrons can absorb or emit energy through interactions with other particles or fields, causing them to transition between different energy levels. These transitions can occur due to external factors such as temperature, electromagnetic radiation, or collisions with other particles.
However, it's important to note that certain systems, such as isolated atoms in vacuum or in highly controlled environments, can be prepared in states that are very close to the ground state. Techniques such as laser cooling and trapping have been developed to cool atoms to extremely low temperatures, reducing their internal energy and bringing them close to their ground state. These techniques are used in fields like atomic physics, quantum optics, and quantum information science.
So while it is challenging to achieve a perfectly ground state in practice, it is possible to get atoms in highly cooled states that are very close to their lowest-energy configurations.