Before Rutherford's experiment, the nucleus of an atom was not observed because it was not well understood that atoms had a central, dense region. The prevailing model of the atom at the time was the Thomson model, also known as the "plum pudding" model. According to this model, the atom was thought to be a uniform, positively charged sphere with negatively charged electrons scattered throughout it, much like plums in a pudding. In this model, there was no concept of a concentrated central nucleus.
Ernest Rutherford's gold foil experiment, conducted in 1909, provided evidence for the existence of the atomic nucleus. Rutherford bombarded a thin gold foil with alpha particles (positively charged particles) and observed their scattering patterns. Most of the alpha particles passed through the foil with slight deflections, but a small fraction underwent significant deflections and even bounced back.
Rutherford interpreted these unexpected results as evidence for a concentrated positive charge at the center of the atom, which he called the nucleus. He concluded that the atom had a tiny, dense, positively charged nucleus surrounded by mostly empty space, with electrons orbiting around it.
While Rutherford's experiment provided evidence for the existence of the nucleus, it did not directly observe it. The nucleus is extremely small in comparison to the overall size of the atom, making it difficult to observe directly even with advanced techniques. Instead, scientists infer the presence and properties of the nucleus through various indirect methods such as scattering experiments, nuclear reactions, and spectroscopy.
Modern experimental techniques, such as high-energy particle accelerators and advanced imaging methods, have allowed for more detailed studies of the nucleus. However, due to its small size and the challenges involved in probing it directly, the nucleus is still not directly observed in the traditional sense. Its existence and characteristics are inferred from the results of experiments and theoretical models based on the behavior of particles interacting with the nucleus.