Rutherford Gold Foil Experiment: Videos & Practice Problems

In 1911, the groundbreaking gold foil experiment conducted by Ernest Rutherford, along with Hans Geiger and Ernest Marsden, revealed the structure of the atom, particularly the existence of a positively charged nucleus. This experiment is often referred to as both the Rutherford gold experiment and the Geiger-Marsden experiment, highlighting the collaborative effort behind this significant discovery.

The experimental setup involved bombarding a thin sheet of gold foil with alpha particles emitted from a radioactive source, typically iridium, which was encased in a lead box with an opening. Alpha particles are helium nuclei, consisting of 2 protons and 2 neutrons, giving them an atomic number of 2 and a mass number of 4. The elemental symbol for an alpha particle can be represented as ^{4}_{2}\text{He}^{2+}, indicating its positive charge due to the presence of protons.

As the alpha particles were directed at the gold foil, the detecting screen surrounding the foil recorded their trajectories. Surprisingly, while many alpha particles passed through the foil, some were deflected at various angles, and a few even bounced back toward the source. This unexpected behavior led Rutherford to formulate three key postulates about atomic structure:

1. The nucleus, which contains protons and neutrons, is located at the center of the atom.
2. Despite its small size, the nucleus accounts for most of the atom's mass.
3. Electrons surround the dense, positively charged nucleus, forming a cloud around it.

These postulates challenged existing theories of atomic structure at the time and laid the foundation for modern atomic theory. The insights gained from the gold foil experiment significantly advanced the understanding of atomic composition, illustrating the complex nature of matter and the arrangement of subatomic particles.

To determine how many gold atoms thick Rutherford's foil was, we start with the thickness of the foil, which is approximately \(6.0 \times 10^{-3}\) millimeters. We also know that a single gold atom has a diameter of \(2.9 \times 10^{-8}\) centimeters. The goal is to find the number of atoms that make up the thickness of the foil.

First, we need to convert the thickness of the foil from millimeters to centimeters, as the diameter of the gold atom is given in centimeters. The conversion factor for millimeters to centimeters is that \(1 \text{ mm} = 10^{-1} \text{ cm}\). Thus, we can convert the thickness:

\[6.0 \times 10^{-3} \text{ mm} \times \frac{1 \text{ cm}}{10^{-1} \text{ mm}} = 6.0 \times 10^{-2} \text{ cm}\]

Now that we have the thickness in centimeters, we can use the diameter of a single gold atom to find out how many atoms fit into this thickness. The diameter of one gold atom is \(2.9 \times 10^{-8}\) centimeters. To find the number of atoms, we divide the thickness of the foil by the diameter of a single atom:

\[\text{Number of atoms} = \frac{6.0 \times 10^{-2} \text{ cm}}{2.9 \times 10^{-8} \text{ cm}} \]

Calculating this gives:

\[\text{Number of atoms} \approx 2.1 \times 10^{6}\]

Thus, Rutherford's foil is approximately \(2.1 \times 10^{6}\) atoms thick. This calculation illustrates the incredibly small scale of atomic dimensions compared to everyday materials.

The Rutherford Gold Foil Experiment was a pivotal moment in the development of atomic theory, leading to the formulation of the nuclear model of the atom. Prior to this experiment, the prevailing theory was Thomson's plum pudding model, which proposed that atoms were composed of negatively charged electrons evenly distributed throughout a positively charged "soup." This model suggested that the overall charge of the atom was neutral due to the balance between the positive and negative charges.

However, Rutherford's experiment challenged this notion. By directing alpha particles at a thin foil of gold, he observed that while most particles passed through the foil, a small fraction were deflected at significant angles. This unexpected behavior indicated the presence of a dense, positively charged center within the atom, which we now understand as the nucleus. The deflections suggested that the nucleus contained most of the atom's mass and positive charge, leading to the conclusion that the atom is mostly empty space, with electrons orbiting around this central nucleus.

This groundbreaking discovery allowed Rutherford to discard the plum pudding model in favor of the nuclear model, which accurately describes the structure of the atom. In this model, the nucleus is surrounded by electrons, and the interactions observed during the experiment provided crucial insights into atomic structure and the forces at play within the atom.

Rutherford's gold foil experiment was pivotal in shaping our understanding of atomic structure. It demonstrated that atoms consist of a dense, positively charged nucleus, which contains protons, while electrons orbit this nucleus. Contrary to the misconception that electrons are positively charged, they are actually negatively charged subatomic particles. This experiment did not reveal the presence of neutrons, which are neutral particles found within the nucleus, as their existence was unknown at the time.

In terms of atomic composition, atoms are made up of protons, neutrons, and electrons. Protons, which are positively charged, are concentrated in the nucleus, disproving earlier models like Thomson's plum pudding model that suggested a more uniform distribution of charge. Additionally, it is important to note that protons are significantly heavier than electrons, with neutrons being the heaviest of the three subatomic particles. This hierarchy of mass is crucial for understanding atomic stability and behavior.

In summary, Rutherford's findings clarified that the nucleus is the core of the atom, housing protons and neutrons, while electrons orbit around it. This foundational knowledge laid the groundwork for modern atomic theory, emphasizing the distinct roles and properties of each subatomic particle.