Subatomic Structure of an Atom

4 min readJun 30


The discovery of the atomic nucleus revolutionized our understanding of matter and paved the way for significant advancements in nuclear physics. In this article, we will explore the key experiment that established the existence of the atomic nucleus, delve into the impact of increasing the energy of alpha particles used in the scattering experiment, discuss an alternative method to measure the sizes of nuclei, and shed light on the relationship between the radius of a nucleus and its mass number. Furthermore, we will also uncover the density of nuclear matter and compare it to the density of ordinary matter.

Photo by Photoholgic on Unsplash

Who established the existence of the atomic nucleus and what experiment was performed to support this?

In the early 20th century, Ernest Rutherford, along with his collaborators, conducted the famous gold foil experiment that led to the discovery of the atomic nucleus. In this experiment, a beam of alpha particles (helium nuclei) was directed toward a thin sheet of gold foil. According to the prevailing “plum pudding” model proposed by J.J. Thomson, it was believed that atoms were uniformly distributed spheres of positive charge with negatively charged electrons scattered throughout.

However, Rutherford’s experiment yielded surprising results. While most alpha particles passed through the gold foil with minimal deflection, a small fraction of them scattered at large angles, and a few even bounced back directly. These observations were inconsistent with the prevailing model and required a new explanation.

Rutherford interpreted the scattering pattern as evidence for a small, dense, and positively charged region within the atom that he called the atomic nucleus. He proposed that the vast majority of the atom’s mass was concentrated in this tiny nucleus, while the electrons orbited around it at a significant distance.

How does increasing the energy of alpha particles used in the scattering experiment impact the results?

By increasing the energy of the alpha particles used in the scattering experiment, we can observe changes in the scattering pattern. If alpha particles with energies higher than 5.5 MeV (million electron volts) are employed, the distance of the closest approach to the gold nucleus becomes smaller.

As the energy increases, the alpha particles possess greater kinetic energy, allowing them to penetrate deeper into the gold foil. At some point, when the alpha particle energy surpasses a certain threshold, the scattering behavior begins to be influenced by the short-range nuclear forces present within the atom’s nucleus. These forces, which are different from the long-range Coulomb repulsion between positive charges, cause deviations from Rutherford’s predictions.

This deviation point, where the scattering pattern starts to deviate significantly from what would be expected based solely on Coulomb repulsion, provides valuable insights into the sizes of nuclei.

What alternative method has been used to accurately measure the sizes of nuclei of various elements?

In addition to the gold foil experiment, scientists have employed scattering experiments involving fast electrons as projectiles bombarding targets made of various elements to accurately measure the sizes of nuclei. By analyzing the scattering patterns produced when electrons interact with atomic nuclei, researchers can extract valuable information about the size and structure of the nucleus.

These experiments allow scientists to investigate different elements and compare their nuclear sizes, providing a broader understanding of the subatomic world.

How is the radius of a nucleus related to its mass number?

The radius of a nucleus is related to its mass number (A) through the formula R = R0 * (A)¹/3, where R0 is a constant equal to 1.2 × 10^–15 m or 1.2 femtometers (1 fm = 10^–15 m).

According to this formula, as the mass number of a nucleus increases, the radius of the nucleus also increases. The relationship between the two is cubic, meaning that

a small change in the mass number leads to a more significant change in the radius. Consequently, larger nuclei have larger radii compared to smaller nuclei.

What is the density of nuclear matter and how does it compare to the density of ordinary matter?

The density of nuclear matter, the matter found within atomic nuclei, is exceptionally high. It is approximately 2.3 × 10¹⁷ kg m^–3. This value is orders of magnitude greater than the density of ordinary matter, such as water, which has a density of around 10³ kg m^–3.

The density of nuclear matter is significantly larger because the majority of the atom is composed of empty space, with the nucleus containing nearly all of its mass. The immense density arises from the fact that nuclear matter consists of densely packed protons and neutrons, whereas ordinary matter contains mostly empty space surrounding the atomic nucleus.

The establishment of the atomic nucleus through Rutherford’s gold foil experiment marked a turning point in our understanding of the atom’s structure. Increasing the energy of alpha particles in scattering experiments provides insights into the influence of short-range nuclear forces. Furthermore, alternative methods utilizing fast electrons have been employed to accurately measure the sizes of nuclei. The radius of a nucleus is related to its mass number, and the density of nuclear matter far exceeds that of ordinary matter. Unveiling the secrets of the atomic nucleus continues to unravel the mysteries of the subatomic world, paving the way for further scientific discoveries.

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