The Journey of Atomic Theory: From Ancient Greeks to Quantum Mechanics
Discover the captivating progression of atomic theory through centuries of scientific inquiry. Explore key contributions from Dalton, Thomson, Rutherford, and Bohr that shaped our understanding of the atom.

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Now Playing:History and development of atomic theory – Example 0a
Intros
  1. History of the atom
  2. i) Ancient Greece – properties of matter.
    ii) John Dalton: elements and atoms.
  3. JJ Thomson: electrons and the plum-pudding model.
Examples
  1. Recall the scientists involved in the development of atomic theory and their contributions.
    Which scientist's theory was the first to use empirical evidence in forming their ideas?
    a) JJ Thomson
    b) Ernest Rutherford
    c) John Dalton
    d) Niels Bohr
    Atomic structure
    Notes
    In this lesson, we will learn:
    • The key scientists involved in developing atomic theory.
    • The steps in the breakthroughs to lead to our modern understanding of the atom.
    • How a coherent scientific theory is developed by observation, hypothesis and experiment.
    Notes:

    • Today, the words ‘atom’ and ‘element’ are learned very early in chemistry and are well understood. We have a detailed periodic table showing all the elements and a detailed model of the atom and its features.
      But the idea of 'atoms' is thousands of years old. The word comes from the ancient Greek “atomos” meaning indivisible. However, without microscopes and other technology, in ancient Greece the best way of describing matter was by the properties that could be felt by human senses, such as hot or cold and dry or wet.
      These properties were held by the classical elements:
      • Fire was hot and dry while water was cold and wet.
      • Earth was cold and dry while air is hot and wet.
      • There was also “ether”, the substance that fills empty space.
      Many cultures around the world had somewhat similar ideas – the idea that ultimately, all the complex matter in the universe is made up of much smaller, simpler substances or energies that interact with one another.

      Because the technology to measure and see atoms did not exist, not many serious scientific experiments could be done to investigate them. Little progress was made in atomic theory until the 19th century, when a number of experiments were done and their results could be explained by atomic theory.

    • In the early 19th Century, John Dalton proposed his atomic theory: matter came in a variety of elements, and all the atoms of a given element were identical in mass and their other properties.
      These atoms couldn't be destroyed or created, only rearranged and combined in different ways. This became the conservation of mass, which is part of our current understanding of a chemical reaction.
      Dalton also made major contributions to our knowledge of chemical compounds and formulae, measuring the relative masses of elements which he found reacted together to make new chemical substances.

    • JJ Thomson is credited with discovering the electron, as a small electrically charged part of an atom. He took the correct idea that atoms are neutral overall and devised the plum-pudding model: electrons were negatively charged ‘plum’ chunks sitting dispersed through the rest of the atom – the ‘pudding’, which must be a positively charged cloud to balance out the electrons and give the overall neutral atom.

    • Ernest Rutherford’s gold foil experiment was a massive breakthrough which tested Thomson’s ‘plum pudding’ model. Positively-charged alpha particles were fired at high-speed at a thin gold foil sheet and the way they deflected was recorded.
      • Rutherford’s experiment is a classic example of how the scientific method works in an observation-hypothesis-experiment cycle:
        • Scientists take an observation from an existing topic: according to the plum pudding model, atoms are neutral overall, electrons are small negatively charged particles inside the atom so the rest of the atom must be sparse, dispersed cloud of positive charge.
        • Scientists create a hypothesis or prediction to test the observation. They predict a result that almost no deflection of alpha particles will take place, because if the observation was true then the fast-moving charged alpha particles will not be deflected by the sparse, ‘cloudy’ positive charge of the gold atoms.
        • Scientists design an experiment which applies the hypothesis, where a measurable result will tell you if it is true or not. The hypothesis is about deflection of particles, so the scientists measure deflection of particles. If the hypothesis is true, there would be almost no deflection. If it is not true, there will be significant deflection

      Instead of virtually no deflection in all the alpha particles, while most particles passed through unaffected, some had huge deflection angles. Some particles even scattered back towards the source.
      • To add to the above, the observation (made from the plum pudding model) did not make sense anymore. How could the densely charged high velocity alpha particles get knocked back and away by the sparse charge of the gold atoms? The emprical evidence did not back up the hypothesis made before the test.
        When this happens, the earlier observation is incorrect. In its place, we have the new observation, the results of this experiment. Scientists then need to develop a new theory or revise the current one so that it accounts for the new observation/evidence.

      From this failed hypothesis, Rutherford developed his own atomic theory. What did some of the alpha particles collide with that caused such a huge deflection in their path? Rutherford explained that the atom’s positive charge is entirely concentrated in a tiny core of the atom called the nucleus. This is also where most of the atomic mass is found.
      • Positive alpha particles colliding with a positive nucleus would cause strong charge repulsion and radically deflect the particles from their path. As the nucleus’s size is a tiny fraction of the whole atom, only a small fraction of particles would deflect like this. This is exactly what the experimental evidence showed – the revised theory now correctly explains the new experimental evidence. We are now back at a better observation that is explained by a better theory.
        This is how the scientific method, using an observation-hypothesis-experiment loop, self-corrects and improves understanding. See our lesson CAP.1.1: Using the scientific method for more on this.

      The electrons, relatively tiny, form a ‘cloud’ surrounding the nucleus. This is the planetary model of the atom. Later experiments on nuclear mass led him to conclude the existence of neutrons.

    • Niels Bohr dealt with Rutherford’s planetary model’s weaknesses. If it was true, the cloud of negative electrons orbiting a nucleus would continuously lose energy and spiral into the nucleus. It also didn’t explain why atoms released light of specific energy when heated, known as atomic emission spectra (AES). There was also important new evidence in quantum physics at the time, which Bohr applied to his model.
      The Bohr Model showed electrons being held in discrete shells or energy levels. In the same way, electrons moved up an energy level if a quantized amount of energy was absorbed, and emitted the same quantized amount to move back down to its ordinary (ground) state. This was the first time quantum physics had been used to explain atomic structure. The Bohr model is now obsolete, but electron energy levels in the Bohr model set a foundation for the current understanding of atoms and electron structure.
    Concept

    Introduction: The Journey of Atomic Theory

    The history and development of atomic theory is a fascinating journey that spans centuries of scientific inquiry. From ancient Greek philosophers to modern quantum mechanics, our understanding of the atom has evolved dramatically. The introduction video provides a captivating overview of this progression, highlighting key milestones in the evolution of atomic models. It showcases how scientists like Dalton, Thomson, Rutherford, and Bohr contributed to shaping our current atomic theory. This topic is crucial in chemistry and science education, forming the foundation for understanding chemical reactions, molecular structures, and the behavior of matter. By exploring the history of atomic theory, students gain insight into the scientific process and how our knowledge of the universe has expanded over time. The video serves as an excellent starting point for delving deeper into this fundamental concept, inspiring curiosity and encouraging further exploration of atomic structure and its implications in various scientific fields.

    FAQs

    Here are some frequently asked questions about the history and development of atomic theory:

    1. What is the history of atomic theory?

    The history of atomic theory spans over 2,400 years, beginning with ancient Greek philosophers like Democritus who proposed the concept of indivisible atoms. Major milestones include John Dalton's modern atomic theory in the early 19th century, J.J. Thomson's discovery of electrons, Ernest Rutherford's nuclear model, Niels Bohr's quantum model, and the development of modern quantum mechanical models in the 20th century.

    2. What are the five main atomic theories?

    The five main atomic theories are: 1) Dalton's atomic theory, 2) Thomson's "plum pudding" model, 3) Rutherford's nuclear model, 4) Bohr's quantum model, and 5) the modern quantum mechanical model. Each theory built upon and refined the previous ones, leading to our current understanding of atomic structure.

    3. Who is considered the father of atomic theory?

    John Dalton is often considered the father of modern atomic theory. His work in the early 19th century laid the foundation for scientific understanding of atoms, proposing that all matter is composed of indivisible particles called atoms, which was a significant advancement from earlier philosophical concepts.

    4. How has atomic theory changed over time?

    Atomic theory has evolved significantly over time. It began with simple concepts of indivisible particles, progressed to models of atoms with internal structures, and now includes complex quantum mechanical descriptions. Each new discovery and theory has refined our understanding, moving from a simple billiard ball model to the current view of atoms as complex systems of subatomic particles with wave-like properties.

    5. What is the significance of the modern quantum mechanical model of the atom?

    The modern quantum mechanical model is our most accurate representation of atomic structure. It describes electrons as probability clouds rather than particles in fixed orbits, incorporates principles like the uncertainty principle and wave-particle duality, and uses quantum numbers to describe electron states. This model explains atomic spectra, chemical bonding, and various atomic properties with unprecedented accuracy, forming the basis for much of modern physics and chemistry.

    Prerequisites

    Understanding the history and development of atomic theory is a fascinating journey through scientific discovery and human ingenuity. While there are no specific prerequisite topics listed for this subject, it's important to recognize that a solid foundation in basic scientific principles and historical context can greatly enhance your comprehension of this complex topic.

    A general understanding of scientific methods and the evolution of scientific thought over time can provide valuable context for exploring atomic theory. Familiarity with basic chemistry concepts, such as the structure of matter and chemical reactions, can also be beneficial. Additionally, knowledge of key historical periods and scientific revolutions can help you appreciate the societal and technological factors that influenced the development of atomic theory.

    Although not strictly prerequisites, topics such as the philosophy of science, the history of chemistry, and basic physics concepts can enrich your understanding of atomic theory's development. These areas of study can provide a broader perspective on how scientific ideas evolve and how different branches of science intersect.

    As you delve into the history and development of atomic theory, you'll encounter various models and theories proposed by scientists throughout history. Each of these models built upon previous knowledge and addressed limitations of earlier theories. Understanding this progression is crucial to grasping the full scope of atomic theory's evolution.

    You'll explore key figures like John Dalton, J.J. Thomson, Ernest Rutherford, and Niels Bohr, each of whom made significant contributions to our understanding of atomic structure. Their work spans from the early 19th century to the early 20th century, encompassing a period of rapid scientific advancement and changing worldviews.

    The development of atomic theory also intersects with other scientific discoveries, such as the discovery of radioactivity and the emergence of quantum mechanics. These connections highlight the interconnected nature of scientific progress and the importance of cross-disciplinary knowledge.

    As you study the history and development of atomic theory, you'll gain insight into how scientific knowledge is built, tested, and refined over time. This process of continuous improvement and reevaluation is a fundamental aspect of scientific inquiry and applies to all areas of scientific study.

    While specific prerequisites may not be mandatory, approaching this topic with curiosity, critical thinking skills, and a willingness to engage with complex ideas will greatly enhance your learning experience. The history and development of atomic theory offer a compelling narrative of human perseverance in unraveling the mysteries of the universe, and understanding this journey can provide valuable insights into the nature of scientific discovery and the atomic world that surrounds us.