Summary
Highlights
The first answers to 'what things are made of' emerged in ancient Greece, with Leucippus and Democritus proposing the atomistic theory. The word 'atom' comes from Greek, meaning 'not divisible.' Today, the atom is defined as the smallest unit of matter, allowing for the identification of chemical elements and the development of technologies like nuclear energy. In 1808, John Dalton proposed an atomic model where the atom was like a billiard ball: all substances are made of atoms, atoms of different elements have different properties, atoms of the same element are identical, atoms do not change when forming chemical compounds, and atoms are permanent and indivisible. Chemical reactions are a reorganization of atoms.
By the late 19th century, electricity studies necessitated a new atomic model as Dalton's model couldn't explain electrical charges. In 1897, Joseph Thomson studied cathode rays, emitted from the negative part of a Crookes tube. Thomson deduced these rays were made of particles with mass and negative charge, which he called electrons. This discovery led him to propose a new atomic model where the atom was neutral but divisible, consisting of a central positive part with attached electrons, known as the 'plum pudding model'.
Scientists in the late 19th century were also interested in radioactivity, a phenomenon where elements spontaneously emit matter and energy, which Thomson's model couldn't explain. Ernest Rutherford, Thomson's student, conducted an experiment involving alpha radiation passing through a thin gold foil. Contrary to expectations (that alpha particles would pass through without deviation), many were deflected, and some even bounced back. Rutherford concluded that the atom has a positive, static nucleus at its center and electrons orbiting it, with a vast empty space between them. This model is known as the planetary model. The atom's large empty space was compared to a football stadium, where the nucleus is at the center and the closest electron is outside the stadium.
Rutherford's model was incomplete as it didn't explain why electrons don't spiral into the nucleus due to electrostatic attraction. In 1910, Niels Bohr, Rutherford's student, used quantum physics to address this. Bohr proposed that electrons don't orbit randomly but in specific energy levels. When an electron receives energy, it jumps to a higher orbit and then returns to its original orbit, emitting the same amount of energy as light. This explains why objects have different colors. Bohr was able to predict the energy levels for hydrogen atoms, leading to the Rutherford-Bohr model, also known as the quantum model, which introduced the concept of energetic layers within the atom.
In 1919, Rutherford discovered that the nucleus is made of positive subatomic particles called protons. In 1932, James Chadwick discovered the neutron, a particle with mass but no charge, also found in the nucleus. Our current understanding of the nucleus and electron cloud forms the basis of the atomic model used in chemistry. Further contributions include the discovery of quarks (basic units of protons and neutrons) and other subatomic particles like neutrinos. Atomic models are constructions that justify the atom's structure based on the science of the time, and future discoveries may continue to change our view.
A review of the discussed atomic models: Dalton's 'billiard ball' model (indestructible, indivisible), Thomson's 'plum pudding' model (discovery of the electron, divisible atom with positive center and negative electrons), Rutherford's nuclear model (large empty space, positive nucleus, orbiting electrons, based on gold foil experiment), and Rutherford-Bohr's model (quantized energy layers for electrons, explaining why electrons don't fall into the nucleus). The current model includes protons, neutrons, and other subatomic particles like quarks.
Atomic models are attempts to better understand the composition of matter using the atom as its smallest structure. Over history, various atomic models have been created, becoming increasingly complex and robust, leading to our current understanding of the atom, which explains daily phenomena. This video will explore the four most known models: Dalton, Thomson, Rutherford, and Rutherford-Bohr, developed according to the technology and scientific advancements of their respective eras.