Which of the following observations was not correct during Rutherford's scattering experiment? (a) Most of the \(\alpha\)-particles passed through the gold foil undeflected. (b) A small fraction of the \(\alpha\)-particles was deflected by small angles. (c) A large number of the \(\alpha\)-particles were bounced back. (d) A very few \(\alpha\)-particles \((-1\) in 20,000\()\) were bounced back.

Alpha particles, denoted as \(\alpha\)-particles, are a type of ionizing radiation and one of the most stable forms of atomic nuclei emitted during radioactive decay. They consist of two protons and two neutrons, which is essentially a helium-4 nucleus. Due to their mass, which is substantially more than that of other subatomic particles like electrons, \(\alpha\)-particles have a distinctive trait when involved in interactions. They have a relatively short range in air and can be stopped by a few centimeters of air or a piece of paper. However, when \(\alpha\)-particles hit a thin metal foil, they can cause significant effects, which were crucial to the understanding of atomic structure through Rutherford's scattering experiment.

When considering their interaction with matter, \(\alpha\)-particles typically cause ionization of the atoms they pass through, knocking out electrons and thereby leading to the formation of positive ions. This characteristic makes them an excellent probe to investigate the atomic structure, which was not well understood before the turn of the 20th century.

Ernest Rutherford's gold foil experiment was a revolutionary scientific procedure that aimed to understand the structure of the atom. In the experiment, \(\alpha\)-particles were directed at a piece of exceedingly thin gold foil in a vacuum. Rutherford and his colleagues then observed the patterns of scattering using a detection screen that fluoresced when struck by \(\alpha\)-particles.

The surprising outcome was that while most \(\alpha\)-particles passed through the gold foil with little to no deflection—a phenomenon reflecting the emptiness of most of the atom—some particles were deflected through small angles. These deflections implied the presence of a concentrated positive charge within the atom, leading to repulsion of the positively charged \(\alpha\)-particles. The most astounding observation was that a tiny fraction of \(\alpha\)-particles were deflected at angles greater than 90 degrees, suggesting that they were repelled by a massive core, which Rutherford later termed the nucleus. This contradicted the plum pudding model of the atom, which hypothesized a uniform distribution of positive charge.

The concept of atomic structure refers to the arrangement of subatomic particles—protons, neutrons, and electrons—within an atom. Prior to Rutherford's experiment, the prevalent model proposed by J.J. Thomson, known as the 'plum pudding model', posited that atoms consisted of electrons scattered within a positively charged 'soup'.

Rutherford's gold foil experiment fundamentally changed this view. The deflection patterns of \(\alpha\)-particles showed that an atom's positive charge and most of its mass were concentrated in a small area at the center, which came to be called the nucleus. Electrons were then understood to be orbiting this nucleus, leading to the nuclear model of the atom. This model laid the groundwork for the development of quantum mechanics and the modern understanding of atomic physics. It encapsulated the idea that the atom is mostly empty space, with a dense central nucleus that contains protons and, as later discovered by Rutherford's student James Chadwick, neutrons. Electrons occupy the vast empty spaces around the nucleus, defining the atom's size and playing a significant role in its chemical properties.