Atomic Theory: A Historical Timeline of Discoveries
Evolution of the Atomic Theory
5th Century BC: Leucippus and Democritus
Leucippus and his pupil Democritus proposed the idea that matter consisted of tiny, indivisible particles called atoms. These ideas were opposed by one of the greatest thinkers of that time, Aristotle. It was only after two millennia that the idea of the atom as a fundamental component of matter was taken up again.
1808: John Dalton’s Atomic Theory
John Dalton proposed the Atomic Theory, which revived and gave scientific value to the term “atom.” According to Dalton, the atom was a solid sphere. The basic tenets of this theory are:
- Matter consists of indivisible and indestructible particles: atoms.
- All atoms of an element are identical.
- Atoms of different elements have different masses and properties.
- Atoms are neither created nor destroyed; in a chemical reaction, only a redistribution of atoms occurs.
- Atoms in compound substances are formed by a union of atoms of different elements: they are “compound atoms.”
The acceptance and impact of this theory were linked to its ability to explain experimental facts, which allowed the formulation of the law of conservation of mass.
1811: Avogadro’s Hypothesis
Avogadro published an article that complemented Dalton’s Atomic Theory. Avogadro assumed that the constituent particles of substances were not individual atoms but a combination of a certain number of them. He called these clusters of atoms molecules. This discovery led to the birth of the atomic-molecular theory.
1897: Thomson’s Discovery of the Electron
Thomson discovered the electron. His atomic model was called the “Plum Pudding Model.”
1900: Rutherford’s Nuclear Atom
Rutherford proposed that matter was not distributed uniformly inside atoms. Instead, most of the mass and all the positive charge were concentrated in a central area, which he called the nucleus. He proposed the first model of the nuclear atom, in which he argued for the existence of a positively charged nucleus of very small size. Outside the nucleus, he placed the spinning electrons describing circular orbits. However, this model was unconvincing because it contradicted the laws of classical physics.
1913: Bohr’s Atomic Model
Bohr developed an atomic model that solved the problem of Rutherford’s model while retaining the idea that nuclei are positively charged. Bohr proposed that electrons revolve in orbits, but not all orbits are stable, only a certain number of them. Each electron spins at a given distance from the nucleus related to the energy it possesses.
Subatomic Particles
Experiments revealed that the atom was not the most elementary constituent but, in turn, was comprised of smaller particles called subatomic particles.
- The Electron: This was the first particle to be identified as a subatomic component. In the late 19th century, Thomson confirmed the existence of particles with a negative electric charge, which he called electrons.
- The Proton: Rutherford, in 1919, confirmed the existence of this positively charged particle as a constituent of all atoms and called it the proton.
- The Neutron: In 1932, Chadwick proved the nature of this radiation, describing it as a beam of neutral particles, i.e., having no electric charge, with a mass similar to that of the proton, which he called neutrons.
Atomic Number
Represented by the letter Z, it is placed at the bottom left of the element’s symbol. It indicates the number of protons.
Mass Number
Represented by the letter A, it is placed at the top left of the element’s symbol. It indicates the number of protons and neutrons.
Isotopes
These are atoms of the same element that have different numbers of neutrons; therefore, they have the same atomic number and different mass numbers.
Ions
These are atoms that are electrically charged and are formed by the loss or gain of electrons.
- Loss of electrons: cation (+)
- Gain of electrons: anion (-)
Particles
Found in a specific region of space, they can move from one place to another. They can be described in terms of position and speed.
Waves
These also move from one place to another but are not found at any specific point.
Current Atomic Model
In 1927, Heisenberg formulated one of the most famous laws of quantum mechanics: the uncertainty principle. According to his ideas, while electrons move at high speed, they do not emit or absorb energy, and one cannot speak of orbits. One can only establish areas where there is a high probability that they are moving around the nucleus. As one can speak in probabilistic terms about the position of an electron in the extranuclear area, it is usually represented with a dot plot or a blur.