Early Concepts and Developments
Structure of An Atom
Models of an Atom
J.J Thomson’s Plum Pudding Model
Ernest Rutherford’s Nuclear Model
Erwin Schrödinger Quantum Model
Neil Bohr’s Planetary Model
John Dalton’s Billiard Ball Model
Experiments with gases prompted John Dalton to develop a modern theory of the atom in 1803
He proposed that all matter is composed of small indivisible particles known as atoms, which he envisioned as "solid, massy, hard, impenetrable, moving particle(s)." He imagined atoms as little spheres, therefore drawing atoms would be like drawing circles
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J.J. Thomson discovered that all atoms contain small negatively charged subatomic particles or electrons as a result of his research with cathode ray tubes.
in 1904, he proposed a model of the atom that he compared to plum pudding
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Rutherford bombarded a piece of gold foil with positively charged alpha particles, expecting the particles to pass directly through it. Instead, many alpha particles ricocheted off the foil, indicating that these particles were colliding with something positive.
Rutherford imagined the atom as a miniature solar system, with electrons orbiting a massive nucleus and mostly empty space, with the nucleus filling just a very small portion of the atom
Bohr modified Rutherford's model of the atom by stating that electrons moved around the nucleus in orbits of fixed sizes and energies. Electron energy in this model was quantized; electrons could not occupy values of energy between the fixed energy levels.
Bohr viewed the atom as a small, positively charged nucleus surrounded by orbiting electrons
Erwin Schrödinger advanced the Bohr atom model in 1926. He observed that electrons flow in waves rather than in straight lines around the nucleus. It is difficult to determine the precise location of the electrons; instead, we have 'clouds of probability' known as orbitals, in which we are more likely to locate an electron.
This concept can be portrayed as a nucleus encircled by an electron cloud
Electron (1897)
An electron is a negatively charged subatomic particle
Experimented with a Crookes, or cathode ray, tube.
Nucleus (1911)
Gold Foil Expermient
Proton (1919)
Neutron (1932)
Bombarded Beryllium with alpha particles from the natural radioactive decay of Polonium
Ancient Greek Concepts of the Atom
Leucippus Concept of the Atom (5th Century BCE)
Leucippus, a Greek philosopher, is deemed the originator of the theory of atomism, which states that everything in the physical world is made up of an infinite number of indivisible particles that are not visible to the human eye because of their small size
Democritus' Concept of the Atom (430 BCE)
Plato's Concept of the Atom (427 BCE)
Aristotle's Concept of the Atom (300s BCE)
Democritus, a student of leucippus, claimed that atoms were uniform, solid, hard, incompressible, and indestructible. He also believed that they moved in infinite numbers through an empty space called the void until stopped.
Aristotle did not believe in the atomic theory and did not teach it. He believed that all materials on Earth were made out of the four elements, Earth, Fire, Water, and Air, rather than atoms. He believed that all substances were made up of trace amounts of these four matter elements.
There are only five solid shapes whose sides are made up of regular polygons, according to Plato (triangles, squares, pentagons, hexagons, etc). Plato was so taken aback by this discovery that he believed all matter atoms descended from these five fundamental solids. He proposed the atomic theory, in which ideal geometric forms are used to represent atoms.