In this study guide, we will learn how our understanding of the atom has evolved through philosophical debates, experimental discoveries, and revolutionary theories.

🤔 Early Philosophical Concepts of Atoms

About 2500 years ago, early Greek philosophers believed that the universe was a single, big entity. Contrary to the main beliefs at the time, Democritus was the first philosopher that introduced atomos suggesting that indivisible, small objects make up matter.

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He questioned the infinite divisibility of matter. His theory suggested that, eventually the smallest possible piece would be obtained. In other words, the smallest piece of matter would be indivisible and indestructible.

Despite Democritus’ insights, his theory was met with resistance at the time. Aristotle, a prominent philosopher, proposed an alternative view that all matter was composed of four elements: earth, air, fire, and water, which dominated scientific thought for nearly 2000 years. 🌎

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💫 The Chemical Revolution

In the 18th-century, chemistry was considered a formal science. During this time, fundamental laws that hinted at the atomic nature of matter were introduced:

• ⚖️ Law of Conservation of Mass by Antoine Lavoisier demonstrated that mass remains constant in chemical reactions, suggesting the involvement of indestructible particles.
• ➗ Law of Definite Proportions by Joseph Proust asserted that chemical compounds are composed of elements in fixed ratios, further implying the existence of atoms.

We still use these laws today in our chemical calculations!

🎱 Dalton's Atomic Theory (1803)

This theory is sometimes known as billiard ball model, because Dalton’s atoms were ball-shaped. John Dalton's experiments led to the formulation of a few key postulates:

1. Matter is made up of atoms.
2. Atoms combine in whole number ratios to form chemical compounds.
   1. This confirmed Proust’s law of definite proportions.
3. In chemical reactions, atoms are combined, separated, or rearranged, but never created nor destroyed.
   • This confirmed Lavoisier’s law of conservation of mass.

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However, Dalton's theory needed refinement. It was soon discovered that atoms could be subdivided into smaller parts, and not all atoms of an element had identical masses.

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⚛️ Discoveries in Atomic Structure

The ideas surrounding atomic structure continued to evolve.

🍮 J.J. Thomson (1897)

Thomson's cathode ray experiments led to the discovery of the electron ⚡, challenging the notion of indivisible atoms. His "plum pudding" model envisioned atoms as spheres of positive charge with embedded electrons, like a pudding with plums dotted throughout.

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Thomson performed three major experiments with the cathode ray, which was later termed an electron gun. First, he showed that the rays emitted by tube were negative because they were deflected by the negative pole of a magnet. Then, he further proved that the rays were negative by deflecting them with an electrical field. Finally, he used his existing data to calculate the mass-to-charge ratio. He found that these particles from the ray tube would’ve had to be a thousand times smaller than a hydrogen molecule, so he made the bold claim that they were subatomic particles, nested in the otherwise positively charged atom.

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🥇 Ernest Rutherford (1911)

Another scientist expanded on Thomson’s model. Rutherford performed an experiment using gold foil, an alpha particle emitter, and a screen that could detect where particles hit it. According to Thomson’s plum pudding model, the alpha particles should’ve all passed through the foil. Instead, some of the particles were deflected off at large angles, suggesting that they had a dense, positively charged center with electrons orbiting around it. This gave rise to the Rutherford model.

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To sum it up, Rutherford’s gold foil experiment lead to us understanding the following about an atom:

1. There is a small, dense, positively charged nucleus at the center of an atom.
2. Most of an atom is made up of empty space.
3. Electrons occupy only a very small part of the atom’s volume around the nucleus.

Here’s Rutherford’s atomic model:

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♾️ Niels Bohr (1913)

Bohr introduced the concept of quantized energy levels for electrons. He proposed that electrons encircled the nucleus in different orbitals, with different charges. Each element was given a specific electron configuration that helped define its properties. While his theory was later disproven, it began to explain the atomic spectra and laid the groundwork for quantum mechanical understanding of the atom.

Image courtesy of Enoch Lau

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🌐 From Bohr to Quantum Mechanics

Following Bohr, the quantum mechanical model evolved. Some of the key contributions include:

• Werner Heisenberg formulated the Uncertainty Principle—position and momentum cannot both be precisely known.

• Erwin Schrödinger developed wave equations showing probabilistic electron locations—thus shaping our current “electron cloud” model where density dictates likelihoods for finding an electron.

  

  Image courtesy of Wikimedia Commons

• James Chadwick discovered neutrons in 1932, completing our picture of atomic structure with neutral particles alongside protons in nuclei.

These ideas continue to evolve today!

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✏️ Practice Questions

1. The word for atom comes from the Greek word atomos, meaning…
   1. Invisible
   2. Undivided
   3. Indivisible
   4. Indestructible
2. What is the positively charged subatomic particle?
   1. The neutron
   2. The positron
   3. The proton
   4. The electron
3. What is the negatively charged subatomic particle?
   1. The neutron
   2. The positron
   3. The proton
   4. The electron
4. What is the neutrally charged subatomic particle?
   1. The neutron
   2. The positron
   3. The proton
   4. The electron
5. Who used gold foil to discover the electron?
   1. Ernest Rutherford
   2. Niels Bohr
   3. Democritus
   4. Schrödinger
6. Compare and contrast the plum pudding model, the Bohr model, and the electron cloud model.

📌 Solutions

1. A, indivisible
2. C, the proton
3. D, the electron
4. A, the neutron
5. A, Ernest Rutherford
6. The plum pudding model describes electrons as being spotted throughout a positively charged atom. The Bohr model relies on Rutherford’s finding that atoms have a dense, positively charged center, and states that electrons orbit it, like planets. This dense, charged center is in contrast to the electrons mixed throughout the positive charge from Thomson’s model. The electron cloud model is different yet—it rejects Bohr’s idea that electrons remain in clear orbits, rather stating that they float in a cloud around the nucleus. Nonetheless, all three models recognize the existence of subatomic particles.

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⚛️ Wrapping Up Atomic Theory

Today's atomic theory is grounded in the Standard Model of particle physics, describing the fundamental particles (quarks and leptons) and forces governing their interactions ✨. Recent discoveries, like the Higgs boson, continue to expand our understanding of the universe at the smallest scales.

This guide covered the history of how atomic theory came to be, and having this knowledge will be fundamental in moving to the next topic on atom structures and subatomic particles!