The Discovery of the Atom

What would happen if we took a piece of paper and repeatedly cut it in half? Eventually, it would become impossible to continue with regular tools such as scissors, as the piece would simply be too small. However, even then, we'd be far from reaching the final split, as this definition of "small" refers to a macroscopic scale. Just to put it in perspective, a sheet of paper is roughly a million atoms thick. Over time, scientists found ways to continue the process of splitting things in half, far beyond our visible eye, and at last reached the fundamental building blocks of matter. Or so they thought, as beyond atoms there are electrons, fermions and bosons, with quarks remaining the smallest things as far as we know. Ancient Greeks had thought of this same principle and prompted the discovery and study of atoms. In this article, we'll look at the history of discovering atoms and understanding their structure.

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    The Timeline of Discovering the Atom

    The full timeline of discoveries made in connection to the atom is compiled in the table below. A more in-depth breakdown of some of the major discoveries can be found later in the article.

    YearScientistsDiscovery
    430 BCEDemocritus and LeucippusTheory about indivisible fragments "atomos" making up all matter.
    1804John DaltonAtomic theory, stating that all matter is made up of indivisible atoms, which differ in size and mass depending on the chemical element.
    1886Eugen GoldsteinDiscovery of positively charged particles - protons.
    1897Joseph John ThomsonThe plum pudding theory. Discovery of negatively charged particles - electrons.
    1898Marie and Pierre CurieDiscovery of polonium and radium, which are strongly radioactive elements. Introduction to radioactivity.
    1911Ernest RutherfordPerformed the gold foil experiment. The structure of the atom - small, positively charged nucleus in the middle surrounded by mostly empty space.
    1913Niels BohrThe Bohr model, describing the atom as a small nucleus surrounded by orbiting electrons.
    1914Henry MoseleyDiscovery of the atomic number.
    1932James ChadwickDiscovery of the neutral particle - neutron.
    1949Maria Goeppert MayerThe nuclear shell model, describing the distribution of nucleons in shells with different energy levels.

    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 19th century a British chemist named John Dalton further developed the atomic theory.

    The law of conservation of mass states that in chemical reactions, the mass is neither created nor destroyed.

    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 19th 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.

    A fundamental particle is a particle that isn't composed of smaller elements.

    The atom was once thought of as a fundamental particle, however, soon after the discovery of the electron, new types of particles kept emerging. Around the 1930s, the final set of particles composing all matter in classical physics were the electrons, protons, and neutrons. Now, we know that the electron is the only actual fundamental particle of the bunch, while protons and neutrons are made up of quarks. The current list of fundamental particles consists of 17 particles: 12 fermions and 5 bosons. Fermions are known as the matter/antimatter particles, while bosons are the force-carrying particles.

    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.

    The plum pudding model is an atomic model, in which negatively charged particles are evenly distributed in a positively charged spherical cloud.

    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.

    Discovery of Atom A positively charged sphere with negatively charged electrons randomly scattered throughout StudySmarterFig. 1 - The plum pudding model proposed by J.J. Thompson, where negatively charged electrons are placed in a positively charged space.

    Plum pudding is a British dessert. An analogous example would be a muffin with blueberries, for example, where the berries are the negatively charged particles and the muffin is the 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.

    Discovery of the Atom The Ancient Greek philosopher Democritus StudySmarterFig. 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.

    Discovery of the Atom A diagram of the experimental setup of Rutherford scattering experiment. It consists of an evacuated chamber, source of alpha particles, aimed towards a gold foil and a particle detector which can be rotated StudySmarterFig. 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 8000th 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 Rutherford model states that an atom consists of a densely packed positive nucleus with negatively charged electrons orbiting around it in set orbits.

    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.

    Maxwell's electromagnetic theory states that charged particles, which are accelerating, will emit electromagnetic radiation. This means that an orbiting charged particle will lose energy over time, falling into the nucleus! Electrons spinning around a nucleus aren't an exception, so based on Rutherford's theory, if there was a set orbit, the atom would collapse.

    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.

    Discovery of the Atom Marie and Pierre Curie working in their lab StudySmarterFig. 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.

    The Discovery of the Atom - Key takeaways

    • Ancient Greek philosopher Democritus hypothesized that repeatedly splitting material will eventually lead to a final indivisible fragment called "atomos".
    • Dalton elaborated on Democritus' idea and developed the atomic theory, stating that all matter is made up of indivisible atoms, which differ in size and mass depending on the chemical element
    • Thomson discovered electrons and proposed a new atomic structure - the plum pudding model.
    • The plum pudding model is an atomic model in which negatively charged particles are evenly distributed throughout a positively charged cloud.
    • Ernest Rutherford disproved the plum pudding model by performing the Rutherford scattering experiment, also called the gold foil experiment.
    • The Rutherford model states that an atom consists of a densely packed positive nucleus with negatively charged electrons orbiting around it in set orbits. Electromagnetism quickly disproved the possibility of charged electrons orbiting the nucleus, though.
    • Marie and Pierre Curie discovered that the radiation energy comes from the inside of an element, in the form of tiny particles, rather than coming directly from the surface of the material. This confirmed the divisibility of an atom.

    References

    1. Fig. 1 - The plum pudding model diagram, StudySmarter Originals.
    2. Fig. 2 - Democritus, ancient Greek philosopher (https://commons.wikimedia.org/wiki/File:Democritus_in_Thomas_Stanley_History_of_Philosophy.jpg) by Thomas Stanley is licensed by Public Domain.
    3. Fig. 3 - The experimental setup for Rutherford's gold foil experiment, StudySmarter Originals.
    4. Fig. 4 - Pierre and Marie Curie at work in a laboratory (https://commons.wikimedia.org/wiki/File:Pierre_and_Marie_Curie_at_work.jpg) by an unknown author is licensed by CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/deed.en).
    Frequently Asked Questions about The Discovery of the Atom

    How did the discovery of radioactivity contradict Dalton's atomic theory?

    The discovery of radioactivity contradicts Dalton's atomic theory because he proposed that atoms are indivisible, while an example of radioactivity is when nucleons of an unstable atom are ejected from it. 

    Who is credited with the discovery of the atomic number?

    The atomic number was discovered by Henry Moseley. 

    How does the discovery of atomic structure impact life?

    The discovery of atomic structure impacts life by explaining the properties of matter, and providing better understanding of physical and chemical processes.

    What key discoveries shaped the development of the atomic model?

    Key discoveries that shaped the development of the atomic model are the discovery of the subatomic particles and their properties. 

    When did Aristotle make his discovery of the atomic theory?

    Aristotle did not make any discoveries regarding the atomic theory, as he believed that all the matter was made up of four elements: water, fire, air and earth.

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