Plum Pudding Model — Overview & Importance

The Plum Pudding Model

What is the Plum Pudding Model?

How Did This Model Come to Be?

(Video) Thomson's Plum Pudding Model of the Atom

by Veritasium

This video will teach you about the plum pudding model. In this atomic model, electrons were viewed as being small particles embedded in the atom, much like a plum in a bowl of pudding. Since the overall atom is neutral, the plums (electrons) would be negatively charged and the pudding (protons) would be positively charged. However, the electrons could still be removed by a strong positive force.

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Various Atomic Models

Throughout the early 1900's there were various models developed by scientists which described the atom and it's structure. These models were described as looking like a plum pudding. A plum pudding is a solid pudding or bread which has plums (today we call these raisins) dispersed throughout. The whole pudding is analogous to an atom and the plums are the parts inside the atom. Another way to picture it is a chocolate chip muffin. Where the muffin part is the atom and the chocolate chips are the parts of an atom.

J.J. Thompson's Model

The earliest model was described by J.J. Thompson. At the time he created his model, electrons or negatively charged parts of an atom had already been discovered. It was also known that the atom had no net charge. However the presence of protons (positive parts of an atom) in the nucleus was not yet known. Thompson's plum pudding model described a positive medium (pudding) with negatively charged particles dispersed throughout (plums). This model we know now is not correct, but it was used as the basis for other scientists' models of atoms.

Ernest Rutherford's Model

Ernest Rutherford was a student of J.J. Thompson. Some years after Thompson's pudding model, Rutherford disproved the model and developed his own. The Rutherford model was developed using information from the gold foil experiment. The model showed a high amount of charge in the center of the atom. This central area of the atom contains positive charges and most of the atom's mass. The area around the charged center contains the electrons. The central part of the atom would later be called a nucleus. This model showed negatively charged particles or electrons dispersed around the positive center. This model is closer to what future scientists would discover about the structure of the atom.

The Start of Atomic Theory

The word atom comes from the Greek "atomos." Its root word is indivisible. The philosopher Democritus formulated the concept around 430 B.C.E.! He believed that matter consisted of unbreakable spheres. How did he come to such a belief? He performed a thought experiment. He wondered what would happen if he continually divided something in half. He realized that one would eventually reach the smallest possible unit. At that point, you could no longer divide it. What remained? The foundational building block. That's the atom.

He also had another fascinating insight! He concluded that matter's properties get determined by the type of atom. Unfortunately, he thought every object had its own unique atom. But, his perception of the nature of chemistry was incredible.

Dalton's Atomic Theory

Two thousand years after Democritus, John Dalton proposed his atomic theory. Do you remember its basic tenets?

• All matter is made of atoms.

• Atoms are indestructible.

• The atoms within a pure element are the same.

• Compounds are created from atoms of different elements in fixed ratios.

• Different compounds have different element ratios.

• In chemical reactions, atoms unite, shuffle, and disconnect to form new compounds.

• This was the earliest formulation of chemical bonds breaking and reforming. Now, we know of synthesis, decomposition, single replacement, and double replacement reactions. So, Dalton's work anticipated each of these reactions.

It's a very insightful proposition. But notice its indebtedness to Democritus. That highlights an important point about the scientific method. Scientists continually build on others' work. Sometimes, they disprove a previous proposal. Other times, they confirm and expand an earlier concept. Periodically, a theory will survive unchallenged for thousands of years. Then suddenly, a new discovery prompts us to reevaluate everything. That's what happened when JJ Thomson discovered the electron.

Cathode Ray Tube Experiment

For over 2000 years, we thought atoms were indivisible. But, in the early 1890s, JJ Thomson transformed chemistry. He was working with cathode rays. He wondered if electric and magnetic fields could deflect the cathode rays. They could! That indicated that cathode rays must consist of charged particles. Based on the deflection angle, he calculated the particles' mass-to-charge ratio. What did he discover? It was 2000 times smaller than the hydrogen atom! At the time, that was unthinkable. He created a whole new area of exploration within chemistry!

Plum Pudding Model

Before the cathode ray tube experiment, we assumed atoms were unbreakable spheres. But suddenly, realized they have smaller components. So, how do we model the atom? That's a question that would take years to work out. But JJ Thomson was the first to attempt an answer. He proposed the plum pudding model of the atom.

Before taking chemistry, I had never heard of plum pudding. So, it wasn't a helpful analogy for me. Plum pudding is an English dessert. In the United States, our pudding is a dessert with a custard-like constancy. But, in most of the world, pudding refers to a process. The ingredients are surrounded by a casing. Then, they get boiled. The end result is similar to a cake. But because of the case, the pudding forms a dome shape. Within the domed cake resides fruit. In the case of plum pudding, the fruit is actually raisins. Why? In traditional English cooking, they called raisins plums.

So, how does this relate to the atom? JJ Thomson envisioned it as a ball of plum pudding. In it, the electrons get embedded within a positively charged sphere. He imagined the sphere as a thick jelly. The electrons distribute within the jelly to minimize their electrostatic repulsion. But the jelly isn't matter! Instead, it's just a charge. The model successfully accounts for two facts. Atoms are neutral. But, they have small negatively charged components. That's electrons. It even creates a fascinating geometry problem. What's the best configuration for the electrons? You have to distribute the electrons to minimize the repulsion. But, they need to reside on the surface of a sphere. It's very similar to molecular geometry problems. For example, four electrons would form a tetrahedron. That's the same as a central atom bonded to four external atoms. But it predates the VSEPR model.

Unfortunately, there were some problems with Thomson's model. It's interesting that he discovered a negatively charged particle. But he never considered a positively charged counterpart. But, even before the proton's discovery, he couldn't explain some concepts. For example, radioactive elements caused him problems. He couldn't explain the positively charged alpha particle. The model also struggled to explain atomic spectra.

Future Atomic Discoveries

Today, we know the plum pudding model wasn't quite right. But that highlights a consequential facet of the scientific method. When we gain further data, we reevaluate. Sometimes, the new information confirms our previous understanding. Other times, it forces us to come up with entirely new models. Models of the atom underwent many iterations. After the plum-pudding model, Rutherford proposed the nuclear model. It was an improvement. But it couldn't explain everything. So, Neils Bohr suggested the Bohr Model. It was a step in the right direction and could explain more. But it still didn't explain everything. Next, Erwin Schrodinger proposed the quantum mechanical model. That was a massive leap forward. And in general chemistry, that's where we'll stop. But, if you take advanced physical chemistry, you'll discover it was missing something too. Paul Dirac managed to incorporate relativity into quantum mechanics. That initiated quantum field theory. Even today, physicists are exploring new areas of research based on Dirac's work. Who knows, maybe you'll be the person to propose the next prominent atomic theory.