The absence of electrons inside the nucleus of an atom was experimentally inferred rather than directly observed. The concept of the nucleus, consisting of positively charged protons and uncharged neutrons, was proposed by Ernest Rutherford in the early 20th century based on his famous gold foil experiment. The experiment provided indirect evidence for the existence of a small, dense, positively charged nucleus within the atom.
Here's a brief overview of the gold foil experiment and how it led to the understanding of the nucleus:
Rutherford and his colleagues fired alpha particles (helium nuclei) at a thin gold foil. They expected the alpha particles to pass through the foil with minimal deflection, as per the prevailing "plum pudding" model of the atom, where positive charge and electrons were assumed to be evenly distributed.
However, to their surprise, a small fraction of the alpha particles were deflected at large angles, and some even bounced back in the direction they came from. This result contradicted the expected behavior and indicated the presence of a concentrated positive charge in the atom.
Rutherford interpreted these unexpected deflections as evidence that the positively charged alpha particles were encountering a concentrated positive charge within the atom. He proposed that this positive charge was localized in a small region at the center of the atom, which he called the nucleus.
The deflections of the alpha particles could only be explained if the majority of the atom's mass and positive charge were concentrated in a tiny volume compared to the overall size of the atom. Since the deflections were caused by electrostatic repulsion, Rutherford inferred that the positive charge in the nucleus must be counterbalanced by negatively charged electrons located outside the nucleus.
This experimental evidence, along with subsequent studies, led to the development of the Rutherford model of the atom, where electrons orbit the nucleus in distinct energy levels.
It is worth noting that the direct observation of the absence of electrons within the nucleus is not possible due to the extremely small size of the nucleus compared to the size of an atom. However, subsequent experiments, such as scattering experiments and advancements in atomic spectroscopy, further supported the existence of a small, positively charged nucleus and provided more detailed insights into the structure of atoms.