Answer the following questions. 1. By using the past models of Ernest Rutherford, J. J. Thomson, John Dalton, and Niels Bohr, how did we acquire the current model? 2. What were the models of these four scientists of the past who created

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### 1. Evolution of the Atomic Model Leading to the Current Understanding

The development of the atomic model has been a progressive journey, with each scientist building upon or revising the ideas of their predecessors. The contributions of John Dalton, J. J. Thomson, Ernest Rutherford, and Niels Bohr were pivotal in shaping our current understanding of the atom. Here's how their models collectively led to the modern atomic theory:

- **John Dalton (Early 19th Century):** Dalton proposed the **Billiard Ball Model**, which depicted atoms as indivisible, hard spheres. This model emphasized that each element consists of unique atoms differing in size and mass, and that chemical reactions involve the rearrangement of these atoms. While simple, Dalton's model laid the foundation for atomic theory by introducing the concept of atoms as fundamental building blocks of matter.

- **J. J. Thomson (Late 19th Century):** Thomson discovered the electron through his experiments with cathode rays, leading to the **Plum Pudding Model** of the atom. In this model, the atom is envisioned as a positively charged "soup" in which negatively charged electrons are embedded, similar to plums in a pudding. This model introduced the idea that atoms are divisible and contain smaller subatomic particles, challenging Dalton's notion of indivisible atoms.

- **Ernest Rutherford (Early 20th Century):** Through the famous **Gold Foil Experiment**, Rutherford found that atoms have a small, dense, positively charged nucleus surrounded by electrons. This led to the **Nuclear Model** of the atom, which proposed that most of an atom's mass is concentrated in the nucleus, with electrons orbiting around it. This model refined the understanding of atomic structure by highlighting the central role of the nucleus.

- **Niels Bohr (1913):** Bohr expanded upon Rutherford's model by introducing quantized electron orbits in his **Bohr Model**. He proposed that electrons occupy specific energy levels or shells and can transition between these levels by absorbing or emitting energy. This model successfully explained the spectral lines of hydrogen and marked the incorporation of quantum theory into atomic structure.

- **Development to the Current Quantum Mechanical Model:** Building upon Bohr's foundation, the modern **Quantum Mechanical Model** (also known as the Schrödinger or electron cloud model) incorporates principles from quantum mechanics. It describes electrons not as particles in fixed orbits but as existing in probabilistic regions called orbitals. This model accounts for the complex behavior of electrons and the probabilistic nature of their positions and energies, providing a more accurate and comprehensive understanding of atomic structure.

In summary, each successive model addressed the limitations of its predecessors, integrating new experimental findings and theoretical advancements. This cumulative progression has culminated in the sophisticated quantum mechanical model that effectively explains a wide range of chemical and physical phenomena.

### 2. Historical Atomic Models Proposed by Dalton, Thomson, Rutherford, and Bohr

Understanding the contributions of these four pioneering scientists helps illuminate the evolution of atomic theory:

1. **John Dalton – Billiard Ball Model (Early 1800s):**
   - **Description:** Dalton envisioned atoms as solid, indivisible spheres (like billiard balls) with no internal structure.
   - **Key Features:**
     - Each chemical element is composed of unique atoms differing in size and mass.
     - Atoms of different elements combine in simple whole-number ratios to form compounds.
     - Physical and chemical properties are determined by the arrangement and interactions of atoms.
   - **Significance:** Provided the first scientific theory to explain chemical reactions through atomic interactions.

2. **J. J. Thomson – Plum Pudding Model (1897):**
   - **Discovery:** Identified electrons as negatively charged subatomic particles.
   - **Description:** Proposed that atoms consist of a positive "matrix" or "soup" with embedded electrons, resembling plums within a pudding.
   - **Key Features:**
     - Atoms are divisible into smaller particles (electrons).
     - The positive charge is spread out to balance the negative charge of electrons, maintaining overall neutrality.
   - **Significance:** Introduced the concept of internal structure within atoms and the existence of subatomic particles.

3. **Ernest Rutherford – Nuclear Model (1911):**
   - **Discovery:** Conducted the gold foil experiment, revealing that atoms have a small, dense, positively charged nucleus.
   - **Description:** Depicted the atom as having a central nucleus surrounded by electrons.
   - **Key Features:**
     - The nucleus contains most of the atom's mass and all its positive charge.
     - Electrons orbit the nucleus, similar to planets around the sun.
     - The atom is mostly empty space.
   - **Significance:** Shifted the focus to the nucleus as the core component of the atom and laid the groundwork for understanding atomic structure and nuclear physics.

4. **Niels Bohr – Bohr Model (1913):**
   - **Description:** Enhanced Rutherford's model by introducing quantized electron orbits.
   - **Key Features:**
     - Electrons travel in specific, fixed energy levels or shells around the nucleus.
     - Electrons can jump between energy levels by absorbing or emitting discrete amounts of energy (quanta).
     - The model could explain the spectral lines of hydrogen, aligning with observed atomic spectra.
   - **Significance:** Incorporated quantum theory into atomic structure, providing a more accurate description of electron behavior and energy distribution within atoms.

Each of these models addressed the questions and inconsistencies left by the previous ones, progressively refining our understanding of atomic structure. Their collective work paved the way for the sophisticated quantum mechanical model we use today, which accounts for the complex and probabilistic nature of electron behavior and interactions within atoms.
John Dalton proposed atoms as indivisible spheres, J. J. Thomson discovered electrons and introduced the plum pudding model, Ernest Rutherford found the nucleus in his gold foil experiment, and Niels Bohr developed the Bohr model with quantized electron orbits. These models collectively led to the current quantum mechanical model of the atom, which describes electrons in probabilistic orbitals around the nucleus.

Answer the following questions. 1. By using the past models of Ernest Rutherford, J. J. Thomson, John Dalton, and Niels Bohr, how did we acquire the current model? 2. What were the models of these four scientists of the past who created

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