In each science we often talk about atoms. If the teacher asks, we remember that it is made out of neutrons, protons, and electrons. Yet, rarely do we ponder how the atomic model looked throughout different time periods and what actually happened that we have managed to end up at the Quantum model.
Greek atomic model
If we want to understand the idea of an atom, we have to travel back to the 5th century BCE. Leucippus of Miletus wrote that the building blocks of matter are atomos (in Greek meaning indivisible). He also claimed that atoms of the four elements (water, earth, fire and, air) are spheres of different sizes. This theory, however, was later ignored for the next 2000 years due to the influence of Aristotle, who did not agree with it.
Dalton’s atomic model
The next key observations in the historical model of an atom were made in the 1800s by an English chemist and physicist John Dalton. His book A New System of Chemical Philosophy was the first ever application of the atomic theory to chemistry. His work with Joseph-Luis Gay-Lussac and Amadeo Avogadro, provided the experimental foundation for atomic chemistry. Through several experiments and scientific processes, he concluded that:
1. All matter consists of very small particles called atoms.
2. Atoms are indestructible and resist changes.
3. Elements are characterised by the mass of their atoms.
4. When atoms are involved in chemical reactions, they combine to form molecules.
This model was generally accepted as Dalton incorporated in it some well-established ideas, such as the law of conservation of mass, the law of definite proportions and the law of multiple proportions. However, two major limitations of his theory soon became apparent. Specifically, it was shown that atoms of the same element are not identical (they can have different masses) and that they are composed of subatomic particles.
Thomson’s atomic model
Another large shift in the understanding of the atom happened in 1897. J.J. Thompson discovered a negatively charged particle, the electron. It was a ground-breaking discovery as the atom was viewed as a homogenous particle. He noticed that electrons must be within the atoms of the elements. Thus, he also supported Lord Kelvin “plum-pudding” model shown in Figure 1.
Figure 1. Thomson’s atomic model from Britannica
Rutherford’s atomic model
It didn’t take long for this model to be questioned as in 1911 a former Thomson’s student Ernest Rutherford performed alpha particle experiments. He and several other scientists aimed positively charged particles at a thin sheet of gold foil and later observed the interactions. They found that some particles went through the foil, some were deflected, and some even bounced back. Rutherford concluded that there must be a positive core and electrons orbiting around it, as shown in Figure 2.
Figure 2. Rutherford’s model from Britannica
Bohr’s atomic model
Two years later, in 1913, Danish physicist Niels Bohr proposed his quantized shell model of the atom. Bohr modified Rutherford’s model by stating that the electrons move on orbits of fixed size and energy, shown in Figure 3. The energy of an electron depends on the size of the orbit, being lower the closer it is to the core. He also suggested that each orbit has different energy level. He found that electrons can either absorb or release energy when they move between these energy levels.
Figure 3. Bohr’s model from Britannica
Quantum atomic model
In 1926, Austrian physicist Erwin Schrödinger used mathematical equations to find the probability of finding electrons in specific positions. He devised the idea of an electron cloud, which is a region of space outside the nucleus where an electron is likely to be, as shown in Figure 4.
In 1932, English physicist James Chadwick discovered a neutral particle which has the same mass as the proton and is located in the nucleus of the atom. This particle is now known as the neutron.
Figure 4. Quantum atomic model from Britannica
Modern atomic model
In the 1960s, subatomic particles called quarks were discovered. A proton is made out of two up quarks and one down quark. The neutron however is made up of one up quark and two down quarks, which leads to the conclusion that the nucleus of the atom has no charge.
In summary
Throughout this article we can clearly see the amazing nature of humanity which aims to expand the extent of our collective knowledge. The collaborative work on each of these models has allowed us to reach to the pinnacle of understanding about the atomic model.
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