Which pair of scientists proposed the theory of radioactive disintegration to explain the spontaneous decay of unstable atomic nuclei?

Introduction / Context:
This question belongs to the history of atomic physics and nuclear chemistry. It asks about the scientists who developed the theory of radioactive disintegration, which explains how unstable nuclei spontaneously decay into more stable forms. Recognising the contribution of Rutherford and Soddy is important for understanding how early observations of radioactivity were converted into a coherent theoretical model.

Given Data / Assumptions:
- Four pairs of scientists are listed: Rutherford and Soddy, Soddy and Fajan, Thomson and Rutherford, and Hahn and Strassmann.
- The question asks which pair presented the theory of radioactive disintegration.
- We assume standard historical accounts used in chemistry and physics curricula.

Concept / Approach:
Radioactive disintegration theory explains that radioactive elements undergo spontaneous and random decay, transforming into other elements while emitting radiation such as alpha or beta particles. Ernest Rutherford and Frederick Soddy worked together at the start of the twentieth century to interpret the experimental observations of radioactivity and proposed that radioactive substances decay at characteristic rates. This work led to the idea of half life and the understanding that radioactivity involves nuclear transformations. Therefore, the correct pair is Rutherford and Soddy.

Step-by-Step Solution:
Step 1: Recall that Ernest Rutherford is widely known for his work on radioactivity, alpha scattering, and the nuclear model of the atom.
Step 2: Frederick Soddy collaborated with Rutherford on early studies of radioactive substances. Together they observed that radioactive elements transform into other elements over time.
Step 3: From their work, they proposed the theory of radioactive disintegration, which states that a radioactive atom has a certain probability of decaying per unit time and that such decays lead to transmutation of elements.
Step 4: Soddy and Fajan worked later on the radioactive displacement law, which describes how the position of an element in the periodic table changes during radioactive decay, but that is a separate concept from the original theory of disintegration.
Step 5: Thomson and Rutherford are both important figures in atomic physics, but J J Thomson is best known for discovering the electron, not for creating the theory of radioactive disintegration.
Step 6: Hahn and Strassmann discovered nuclear fission in uranium, a later development in nuclear physics, and did not propose the original theory of radioactive disintegration.

Verification / Alternative check:
Standard nuclear chemistry texts attribute the theory of radioactive decay and the concept of half life to the collaborative work of Rutherford and Soddy. Their papers describe how a radioactive element decays according to an exponential law and how daughter elements form. Historical timelines place their work before later contributions by Fajan and Hahn, which confirms that Rutherford and Soddy are the correct pair associated with the theory of radioactive disintegration.

Why Other Options Are Wrong:
- Soddy and Fajan: This pair is associated with the radioactive displacement law, not with the original disintegration theory itself.
- Thomson and Rutherford: Thomson discovered the electron and proposed the plum pudding model, while Rutherford later proposed the nuclear model, but the radioactivity theory was developed specifically by Rutherford and Soddy.

Why Other Options Are Wrong (continued):
- Hahn and Strassmann: They are famous for the discovery of nuclear fission, which is a different phenomenon from the general exponential decay described by the theory of radioactive disintegration.

Common Pitfalls:
Students may confuse different milestones in nuclear science and attribute the wrong discovery to a particular scientist. Because Rutherford appears in several important discoveries, learners might pair him with Thomson by habit. Others may recall Hahn and Strassmann from discussions of atomic energy and assume they were responsible for all radioactivity theories. To avoid such confusion, it is helpful to remember that Rutherford and Soddy dealt with natural radioactive decay and half life, whereas later scientists worked on fission and reactor physics.

Final Answer:
Rutherford and Soddy proposed the theory of radioactive disintegration to explain the spontaneous decay of unstable nuclei.