Question

Rutherford was surprised when some of the \(\alpha\) particles bounced back. He was surprised because he was thinking of Thompson's model of the atom. What if Rutherford believed atoms were as Dalton envisioned them? What do you suppose Rutherford would have expected, and what would have surprised him?

Short answer

If Rutherford had believed in Dalton's atomic theory instead of Thompson's model, he would have expected atoms to be solid, indivisible spheres, and he would not expect α particles to be deflected at all as they passed through the atoms. Instead, he observed some α particles bouncing back, which would have been even more surprising to him if he had based his expectations on Dalton's atomic theory, as this would directly contradict the core assumptions about atoms' impenetrability and indivisibility in Dalton's theory.

Step-by-step solution

Recall Thompson's model of the atom

Thompson's atomic model was the "plum pudding model," which described the atom as a positively-charged sphere with negatively-charged electrons embedded within it. In this model, the atom was thought to be uniformly dense, with electrons scattered like plums in a pudding.

Recall Dalton's atomic theory

Dalton’s atomic theory states that: 1. All matter is made up of tiny indivisible particles called atoms. 2. All atoms of a given element are identical in mass and properties. 3. Compounds are formed by a combination of two or more different kinds of atoms. 4. A chemical reaction is a rearrangement of atoms. According to Dalton's theory, atoms were considered as solid spheres. There was no information about the distribution of positive and negative charges within atoms in Dalton's atomic theory.

Rutherford's expectations based on Dalton's atomic theory

If Rutherford believed in Dalton's atomic theory, he would have considered atoms to be solid indivisible spheres. Thus, he would expect alpha particles, which are fast-moving and positively-charged particles, to easily pass through atoms without experiencing significant deflection, since atoms would be considered impenetrable and indivisible.

Explain what would have surprised Rutherford

Rutherford was surprised by the fact that some alpha particles were deflected back by a significant angle. If he believed in Dalton's atomic theory, he would not expect the alpha particles to be deflected at all, as atoms would be considered indivisible and impenetrable solid spheres. Thus, Rutherford's observation that some alpha particles were deflected with significant angles would have been even more surprising if it were based on Dalton's atomic theory, as it directly contradicted the assumptions about the impenetrability and indivisibility of atoms in this theory.

Key concepts

Thomson's Plum Pudding Model

To understand the early theories of atomic structure, let's start with the Thomson's plum pudding model. This theory, proposed by J.J. Thomson in 1904 after the discovery of the electron, was a pivotal step in atomic theory. Thomson postulated that the atom was a large, positively charged sphere with negatively charged electrons embedded throughout, similar to plums in a pudding or raisins in a loaf of bread.

Before this model, atoms were thought to be indivisible. Thomson's model integrated the presence of electrons within the atom, which were discovered as particles smaller than atoms, suggesting that atoms were indeed divisible. While the model was an improvement in that it acknowledged internal structure within atoms, it still could not explain certain experimental results, such as the outcomes of the alpha particle scattering experiment conducted by Ernest Rutherford, which ultimately led to the model's downfall.

The notion of 'plum pudding' might sound whimsical, but it was an attempt to describe how opposite charges could exist in a neutral atom. Even though it was eventually proven incorrect, it marked a significant stepping stone towards the current understanding of atomic structures.

Dalton's Atomic Theory

A century before Thomson's model, John Dalton proposed his atomic theory in the early 1800s, which was groundbreaking at the time. Dalton's theory laid the groundwork for our understanding of chemical reactions and matter. The primary postulates of Dalton's theory can be distilled into four main points:

• All matter is composed of extremely small particles called atoms.
• Atoms of the same element are identical in size, mass, and other properties, but differ from those of other elements.
• Atoms cannot be subdivided, created, or destroyed.
• Atoms of different elements combine in simple whole-number ratios to form chemical compounds.

Dalton's atomic theory treated atoms as solid, indivisible particles, akin to tiny billiard balls. The assumption that atoms were unbreakable and without internal structure is what would have influenced Rutherford's expectations during his alpha particle scattering experiment. The observation of alpha particles being deflected would have shocked Rutherford if he had solely adhered to Dalton's model, as it suggested a structure within the atom and the presence of substantial forces at play.

Alpha Particle Scattering Experiment

The alpha particle scattering experiment, sometimes referred to as the Rutherford gold foil experiment, was a critical turning point in the understanding of atomic structure. In this well-known experiment conducted by Ernest Rutherford in 1909, a stream of alpha particles, which are positively charged and have substantial mass for subatomic particles, was directed at a thin sheet of gold foil.

Rutherford anticipated that most of the alpha particles would pass straight through the foil or be slightly deflected, as predicted by the plum pudding model and Dalton's view of the atom. Instead, he found that while most particles passed through, some were deflected at large angles, with a very few reflecting straight back towards the source. This was unexpected and led Rutherford to conclude that the positive charge and most of the atom's mass were concentrated in a tiny central region, which he later termed the 'nucleus'.

The alpha particle scattering experiment not only disproved Thomson's plum pudding model and challenged Dalton's solid-sphere view of the atom but also led to the planetary model of the atom, where electrons orbit a dense, positively charged nucleus. This profound insight laid the foundation for the quantum mechanical models that would follow and ushered in a new era of atomic physics.