4.1.3 The Development of the Model of the Atom

In this lesson, we will explore how scientific models of the atom have evolved over time based on new experimental evidence. We will discuss the pre-electron model, the plum pudding model, the nuclear model, Bohr's model, and the discovery of protons and neutrons.

Pre-Electron Model

Before the discovery of the electron, atoms were believed to be tiny spheres that could not be divided. At that time, the internal structure of atoms was not well understood. Scientists thought that atoms were the fundamental building blocks of matter and were indivisible.

Plum Pudding Model

In the late 19th century, the discovery of the electron by J.J. Thomson revolutionised our understanding of the atom. The plum pudding model, proposed by Thomson, suggested that the atom is a ball of positive charge with negatively charged electrons embedded within it.

According to this model, the electrons were scattered throughout the atom, like plums in a pudding, within a positively charged matrixUsed in mathematics, this is an arrangement of numbers, symbols, or expressions arranged into rows and columns..

Nuclear Model

In 1909, Ernest Rutherford conducted the famous alpha particle scattering experiment. Alpha particles were directed at a thin gold foil. Most alpha particles passed through, but some were deflected at large angles, and a few even bounced back.

The unexpected deflections and rebounds of alpha particles indicated that the mass of an atom was concentrated in a small, dense, and positively charged region called the nucleusA membrane-bound organelle in eukaryotic cells that contains DNA.. Rutherford's experiment provided evidence that the nucleus carried a positive charge.

Bohr's Model

In 1913, Niels Bohr proposed a model that added more structure to the atom.

• Quantum Theory: Bohr incorporated the principles of quantum theory and suggested that electrons orbit the nucleus at specific distances or energy levels.
• Energy Transitions: According to Bohr's model, electrons can move between these energy levels by absorbing or emitting energy in discrete packets or quanta.
• Agreement with Observations: Bohr's theoretical calculations agreed with experimental observations, providing further support for the model.

Protons and Neutrons

In the 1910s, further experiments revealed that the positive charge of any nucleus could be subdivided into smaller particles, each carrying the same amount of positive charge. These positively charged particles were named protons.

In 1932, James Chadwick conducted experiments that led to the discovery of neutrons. The discovery of neutrons provided additional evidence supporting the nuclear model and the existence of a dense, positively charged nucleus.

Difference between the Plum Pudding Model and the Nuclear Model

Plum Pudding Model:

• Structure: The plum pudding model proposed that the atom is a ball of positive charge with negatively charged electrons embedded within it.
• Distribution of Charge: According to this model, the positive charge and the negative charge of the electrons are spread uniformly throughout the atom.
• No Concentrated Nucleus: The plum pudding model suggests that there is no concentrated centre or nucleus within the atom.
• Lack of Subatomic Particles: This model did not account for the presence of protons and neutrons in the atomic structure.

Nuclear Model:

• Structure: The nuclear model, proposed as a result of the alpha particle scattering experiment by Ernest Rutherford, suggests a different structure for the atom.
• Concentrated Nucleus: In the nuclear model, the atom is depicted as having a small, dense, and positively charged nucleus at the centre.
• Electrons in Orbit: Electrons are shown to orbit the nucleus in specific energy levels or orbits.
• Majority of Mass in Nucleus: The nuclear model acknowledges that most of the mass of an atom is concentrated in the nucleus.
• Protons and Neutrons: The presence of protons, which carry a positive charge, and neutrons, which are electrically neutral, is accounted for within the nucleus.

Conclusion

We explored the development of the model of the atom based on new experimental evidence. The pre-electron model was replaced by the plum pudding model, which was later replaced by the nuclear model based on the results of the alpha particle scattering experiment. Niels Bohr's model introduced electron orbits and quantum transitions. Subsequent experiments led to the discovery of protons and neutrons within the nucleus.