The Discovery of the Atom

Discovery of the Atom

The earliest records of humans speculating about the fundamental makeup of the world come from Ancient Greece, where the philosopher Democritus developed the basis for the atomic theory around 430 BCE. He hypothesized that repeatedly cutting an object in half will eventually lead to a fundamental, indivisible fragment of said object. This final piece was given the name of atomos, which translates to “indivisible”, and later down the line was adapted into the “atom” we know today.

Based on Democritus' ideas and Antoine Lavoisier's law of conservation of mass, at the beginning of the 19^th century a British chemist named John Dalton further developed the atomic theory.

Dalton claimed that atoms differ in mass and size, depending on the chemical element. His main research focus became determining the relative masses of different atoms.

Discovery of the Atomic Structure

Once the general idea about atoms and their behavior had been established, understanding the underlying cause of these characteristics became the main topic of interest. So by the end of the 19^th century, various scientific discoveries such as cathode rays, X-rays, and ionizing radiation, led to the discovery that atoms can be split further, revealing the fundamental particles, the first one being the electron.

The presence of electrons in particular was proven by J. J. Thompson, stemming from his experiments with cathode ray tubes. When measuring the mass of the cathode ray, he realized that it weighed 1000 times less than the lightest particle known to exist - a hydrogen atom. As a result, he came up with the plum pudding model.

Considering the overall charge of the atom was supposed to be neutral, and the electrons were known to have a negative charge, Thompson concluded that they must be embedded in a positively charged volume. Kind of like plums randomly scattered in a pudding, visible in Figure 1 below.

Fig. 1 - The plum pudding model proposed by J.J. Thompson, where negatively charged electrons are placed in a positively charged space.

This model was later tested by Ernest Rutherford, who performed the gold foil experiment. In this experiment, Rutherford aimed a ray of radioactive alpha particles at a thin gold foil. Most of the particles went straight through the foil, as predicted by the plum pudding model. However, some particles were reflected or scattered, which goes directly against Thompson's theory, resulting in a new atomic theory - the Rutherford model. More in-depth explanation of the experimental procedure and the model itself can be found later in the article.

Democritus' Discovery of the Atom

Although we have mentioned Democritus as the father of the initial atomic theory, he wasn't the only ancient philosopher who contributed to the discovery of the atom. Turns out, his less famous teacher - Greek Philosopher Leucippus - may have been the initial author of the theory. That being said, Democritus did come up with the name for the indivisible particles.

Fig. 2 - Ancient Greek philosopher Democritus prompted the discovery of the atom.

Democritus' description of the atom was very specific, emphasizing its constant, stable structure. He believed atoms to be unchanging, solid, and indivisible. In addition, the atom possessed the same properties as the material it was creating. For instance, sour things were made up of spiky atoms, while sweet things were associated with smooth atoms.

Although in many respects his theory was abstract and based on pure speculation, it turned out to be partially correct, and eventually lead to the discovery of the real atom.

Ernest Rutherford's Discovery of the Atom

Ernest Rutherford managed to disprove the plum pudding model by performing the Rutherford scattering experiment (also known as the gold foil experiment). The apparatus used in this experiment is visible in Figure 3 below.

Fig. 3 - The experimental apparatus used by Rutherford, in which \(\alpha\) particles are aimed at a gold foil, and detected using an adjustable particle detector.

The main components of this experimental setup are the source of alpha particles aimed at a gold foil. Once passed through, they're located using an alpha particle detector, all of which is happening in a vacuum chamber. During the experimental procedure, roughly every 8000^th particle was reflected backwards or scattered through angles larger than \(90^{\circ}\), so the detector was adjusted accordingly to catch these. The deflection and scattering of the particles was an astonishing observation, as Rutherford even compared it to a gun being shot at toilet paper and the bullet bouncing back.

The main conclusions drawn from Rutherford's scattering experiment were as follows.

1. The majority of the atom's mass must be located in the nucleus, which takes up a tiny volume.

2. The nucleus has to have positive charge, as it repels the positively charged alpha particles.

3. The majority of the atom is made up of empty space, as only a few particles get deflected.

4. In order for the atom to remain neutral, it must contain negatively charged electrons far from the positively charged nucleus.

These directly contradict the plum pudding model, meaning that a new atomic structure had to be established. As a result, Rutherford came up with a new atomic model known as the Rutherford model.

The later part of his model, proposing that electrons have set orbits around the nucleus, similar to those of planets orbiting around the Sun, was incorrect. Based on Maxwell's electromagnetic theory, the atom wouldn't be stable if that were the case.

Marie and Pierre Curie's Discovery of the Atom

Although Marie and Pierre Curie's discoveries aren't directly connected with atoms, their findings helped with the overall understanding of atoms and their structure.

Fig. 4 - Marie and Pierre Curie discovered polonium and radium, further confirming the divisibility of atoms. [1], CC BY 4.0 , via Wikimedia Commons

Just one year after the discovery of X-rays 1995, french physicist Henri Becquerel noticed that uranium emits similar rays on its own. Becquerel's student, Marie Curie, took his observations further and, together with her husband Pierre, studied radioactive materials and their properties, which led to the discovery of whole new radioactive elements: polonium and radium.

An additional observation they made after their discovery was that the radiation energy came from the inside of the element, in the form of tiny particles, rather than coming directly from the surface of the material. Marie called this property of matter radioactivity, coining the term. This confirmed the divisibility and changing nature of atoms, and played an important role in further understanding the structure of an atom.