This made him discover that within the atom, contains negatively charged particles, known as electrons. He also discovered that electrons have a mass of 1/2000 of a hydrogen atom. He then displayed his discovery by making the plum-pudding model, in which the electrons are surrounded by positively-charged particles.

This impacted the scientific world, in which before Thomson made his discovery, everyone thought that the atom was the smallest possible particle, until Thomson made the discovery on electrons, in which it made the world think that there are subatomic particles inside the atom.

Thomson's use of the cathode ray also influenced others to invent the mass spectrometer, a device that uses anode rays or positive charges to measure the mass-to-charge ratio of ions and molecules.

Sources:

“Discovery of the Electron and Nucleus (Article).” Khan Academy, Khan Academy, https://www.khanacademy.org/science/chemistry/electronic-structure-of-atoms/history-of-atomic-structure/a/discovery-of-the-electron-and-nucleus.

Development of the Atomic Theory. http://www.abcte.org/files/previews/chemistry/s1_p5.html#:~:text=Thomson%20discovered%20the%20electron%20by,charged%20particles%20in%20neon%20gas.

“Thomson Atomic Model.” Encyclopædia Britannica, Encyclopædia Britannica, Inc., https://www.britannica.com/science/Thomson-atomic-model.

Kozlowski, Rosann. “What Contributions Did J.J. Thomson Make to the Atom?” Sciencing, 25 Feb. 2020, https://sciencing.com/contributions-jj-thomson-make-atom-7996714.html. 

Furthermore, the second basis of his theory besides the law of conservation and mass is the law of constant composition which states that a pure compound will always have the same proportion of the same elements. 

The evidence of the theory:
Dalton conducted various experiments to prove the existence of atoms, which includes:

- His investigation of pressure and other properties of gases, from which he deduced that gases must consist of tiny, individual particles that are in constant and random motion.
- He conducted research on the properties of compounds,
- The Billiard Ball Model: He envisioned them as solid billiard balls and used wooden balls to model the atoms. Future scientists later proved that this model was too simple and inferred the existence of subatomic particles, which includes electrons, protons, and neutrons.

Its impact on science: 

His theory established a foundation for modern science as it is still studied to this day, namely the law of conservation of mass.

Resources:
- Academy , K. (2015). Dalton's atomic theory (article). Khan Academy. Retrieved August 3, 2022, from https://www.khanacademy.org/science/chemistry/electronic-structure-of-atoms/history-of-atomic-structure/a/daltons-atomic-theory-version-2 

Niels Bohr proposed a model of the atom in which the electron was able to occupy only certain orbits around the nucleus. This atomic model was the first to use quantum theory, in that the electrons were limited to specific orbits around the nucleus.

Evidence & Background (History) of the Theory

Whilst Bohr was working in England in 1913, he developed his nobel-prize winning atomic model. He developed the model after studying the way glowing, hot hydrogen gives off light. When an incandescent light bulb is lit, it gives off all the different wavelengths of light.

Bohr’s theory was a more fixed and flourished theory continuing from Rutherford’s findings and models, similarly proofing through the concepts of the gold-foil experiment to prove his basis and points out of his theory. His model specified how electrons are bound to the nucleus in different orbits

Furthermore, subsequent experiments by Antonius Van den Broek (Dutch physicist) and Niels Bohr refined the model further (based off of Rutherford’s model). While Van den Broek suggested that the atomic number of an element is very similar to its nuclear charge, the latter proposed a Solar-System-like model of the atom, where a nucleus contains the atomic number of positive charge and is surrounded by an equal number of electrons in orbital shells (known as the Bohr Model).

Bohr’s model refined certain elements of the Rutherford model that were problematic. These included the problems arising from classical mechanics, which predicted that electrons would release electromagnetic radiation while orbiting a nucleus. Because of the loss in energy, the electron then should have rapidly spiraled inwards and collapsed into the nucleus. In short, the atomic model implied that all atoms were unstable. 

The model also predicted that as electrons spiraled inward, their emission would rapidly increase in frequency when the orbit gets smaller and faster. However, experiments with electric discharges in the late 19th century showed that atoms only emit electromagnetic energy at certain discrete frequencies.

Bohr resolved this by proposing that electrons orbiting the nucleus in ways that were consistent with Planck’s quantum theory of radiation (Planck's quantum theory; different atoms and molecules can emit or absorb energy in discrete quantities only). 

The smallest amount of energy that can be emitted or absorbed in the form of electromagnetic radiation is known as quantum. In Bohr's model, electrons can occupy only certain allowed orbitals with a specific energy. He also specified that they can only gain and lose energy by jumping from one allowed orbit to another, absorbing or emitting electromagnetic radiation in the process.

These orbits were associated with definite energies, which he referred to as energy shells or energy levels. In other words, the energy of an electron inside an atom is not continuous, but quantized. 

These levels are thus labelled with the quantum number n (n = 1, 2, 3, etc.) or with letters (such as K, L, M, N, etc) which he claimed could be determined using the Rydberg formula (a rule formulated in 1888 by Swedish physicist Johannes Rydberg to describe the wavelengths of spectral lines of many chemical element).

How the Bohr Atomic Model Impacted Sciences Today

Bohr has contributed to changing the world of sciences (especially chemistry & physics) heavily by introducing the first atomic model that is based on quantum theory. He also introduced the theory of energy levels and how atoms have assigned energy levels that may be occupied by a certain number of electrons. This revolutionized the views on how atoms functions and work which ultimately drives the future discoveries to a fascinating path in the world of sciences.

Resources (APA Format)
- Britannica Staff. (n.d.). Niels Bohr. Encyclopedia Britannica. https://www.britannica.com/biography/Niels-Bohr

- Compound Interest. (2016, October 16). The history of the atom – Theories and models. https://www.compoundchem.com/2016/10/13/atomicmodels/

- Suthida Chamrat. (n.d.). Please wait... | Cloudflare. ResearchGate | Find and share research. https://www.researchgate.net/figure/The-atomic-models-timeline-sorted-by-the-year-they-were-proposed_fig12_318027503

- Bohr Research Document (Link, accompanied with resources) 
- https://www.youtube.com/watch?v=XBqHkraf8iE (Rutherford's Gold Foil Experiment)

The discovery of neutrons by Chadwick paved the way for the fission of uranium as neutrons had no electric charge which meant that they could penetrate undeflected into the atomic nucleus. Scientists realized that bombarding uranium with neutrons resulted in the fission of uranium which resulted in the release of a lot of energy. This paved the way for the discovery of atomic weaponry. Chadwick himself was an advocate for the construction of atomic bombs, being a member of the British MAUD committee, and later advocating for the UK to have its own stock of nuclear weapons.

Sources:
The Visual Learning. (2021). Discovery of neutron (3d animation) | Chadwick’s experiment | Chemistry,Class 9 and 11 [YouTube Video]. In YouTube. https://www.youtube.com/watch?v=tY7J1Iz0j2o

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James Chadwick | Biography, Model, Discovery, Experiment, Neutron, & Facts | Britannica. (2022). In Encyclopædia Britannica. https://www.britannica.com/biography/James-Chadwick

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Understanding the concept of wave-particle duality and the quantum theory that introduced packs of energy called quanta led to the modern model of the atom, the wave-mechanical model. The first tweak to the Bohr model would be redefining the position and momentum of electrons. The wave-mechanical model tells us that electrons are like standing waves with probability distributions for their positions. These probability distributions are known as wave functions.

Citations:
Lesson Explainer: Modern Atomic Theory | Nagwa. (2022). Nagwa.com; Nagwa. https://www.nagwa.com/en/explainers/624134386397/

The Nobel Prize in Physics 1929. (2022). NobelPrize.org. https://www.nobelprize.org/prizes/physics/1929/broglie/facts/

Schrödinger's equation, H^ψ=Eψ, can be solved to yield a series of the wave function ψ, each of which is associated with an electron binding energy, E. (H^ is a Hamiltonian operator). Solving this makes it possible to determine the allowed energy levels of a system and define the regions of space around the nucleus where an electron might be found, known as the electron cloud. This gave rise to atomic orbitals, which were much more complicated shapes than the circular, planar orbits of the Bohr model. Atomic orbitals are three-dimensional mathematical expressions that describe part of the electron cloud and are the most likely location of an electron in an atom. However, the Schrödinger equation can only be solved exactly for the hydrogen atom.

Note that it is impossible to know the position and energy of a given electron due to Heisenberg's uncertainty principle, which tells us that the more accurately we know the position of a particle, the less accurately we can understand its momentum and vice versa, which became the fundamental truth of the universe. Chemists generally accept that we can only approximate the electron's location with Schrödinger's probability equation.

Citations:
Nagwa. (2022). Lesson Explainer: Modern Atomic Theory | Nagwa. Nagwa.com; Nagwa. https://www.nagwa.com/en/explainers/624134386397/
The Nobel Foundation. (1926). The Nobel Prize in Physics 1933. NobelPrize.org. https://www.nobelprize.org/prizes/physics/1933/schrodinger/facts/
Khan Academy. (2022). Khanacademy.org. https://www.khanacademy.org/science/physics/quantum-physics/quantum-numbers-and-orbitals/a/the-quantum-mechanical-model-of-the-atom

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