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440 BCE
Democritus
Democritus was a Greek philosopher who was one of the first to theorize about the existence of one particle of which everything else is made of. He termed this the atom. He also suggested different types of atoms made up different types of material. -
440 BCE
Democritus's Atom
Democritus's model of the atom was simply the smallest particle something could be. He thought it looked like a sphere. This model remained uncontested for many hundreds of years. -
Dalton's Atom
John Dalton didn't make any discoveries about subatomic particles, so his image of the atom remained unchanged from Democritus. However, he did figure out there was a way for atoms to connect to each other. -
John Dalton
English Chemist John Dalton contributed immensely to the basis of atomic thinking. He came up with postulates that still define how we think of the atom, even if some were later refuted. He determined that atoms cannot be created, destroyed, reduced, or changed into another atom. Dalton also realized that atoms of different molecules can form compounds and that all atoms of the same molecule have the same characteristics. -
Dalton's Experiments
Dalton measured the weight of various gases and discovered that the mass of the gasses components added together was equal to the mass of their mixture. This led him to create his postulates on atomic theory. -
JJ Thomson
JJ Thompson, an English physicist, conducted experiments with cathode rays that led to his discovery of the first subatomic particle: the electron. -
Thomson's Experiments
Thomson experimented with a cathode ray on magnetic fields and electric currents and found that these deflected the ray. By measuring the mass to charge ratio of the cathode ray particles he was able to figure out the mass of each particle, which turned out to be smaller than a Hydrogen atom. He termed these particles electrons. -
Thomson's Atom
Thomson determined that the negatively charged electrons had to be balanced out by a positive charge in the atom. He theorized a "plum pudding model", where the electrons were dispersed randomly throughout the positive charge of the atom. -
Ernest Rutherford
Ernest Rutherford was a New Zealand physicist who worked in England. His experiments with gold foil and alpha particles led him to understand the nucleus of the atom. -
Rutherford's Experiments
Rutherford's most significant experiment was the gold foil experiment. He fired alpha particles at a sheet of gold foil and watch the path of the particles. Based on Thomson's plum pudding model, he expected all the alpha particles to pass through the atoms undisturbed. Instead, he found that some of the particles were deflected, while a few were reflected. This led to his discoveries about subatomic particles. -
Rutherford's Atom
Rutherford theorized that the atom was mostly empty space with a small, dense, positively charged center. He called this center the nucleus. Because the positively charged alpha particles were deflected by the nucleus, Rutherford determined that the nucleus must contain a positive charge, with positive particles that made up the mass of the atom. He term these particles protons. He also determined that the electrons orbited around the nucleus. -
Niels Bohr
Niels Bohr was a Danish physicist who created the Bohr Atomic Model. As Rutherford's model was unable to explain why different elements had different properties or why some metals gave off colors when heated, Bohr made observations that led him to focus on the negatively charged electrons that orbited the nucleus. Although we now know his model is partially incorrect, it is still utilized as a simple and understandable drawing of the atom. -
Bohr's Atom
The Bohr model has several rules about electrons that clear up the inconsistencies in the Rutherford Model. He theorized that electrons traveled on fixed paths around the nucleus, called energy levels. Based on this rule, he proposed several other rules. Electrons can quantum jump from one energy level to another, but not exist between levels. Furthermore, electrons have more energy on higher levels, and these levels get closer to each other as energy increases. -
Schrödinger's Atom
The quantum mechanical model shows where electrons have a higher probability of being. Schrödinger kept the basis of Bohr's idea of an energy level, but changed its characteristics significantly. He hypothesized that energy levels were made up of sub-levels, each of which was composed of atomic orbitals. An atomic orbital is a 3-D area with a high probability of finding an electron. There are 4 types of atomic orbitals: s orbitals, p orbitals, d orbitals, and f orbitals. -
Erwin Schrödinger
Austrian physicist Erwin Schrödinger developed the foundation of quantum mechanics, which allowed him to create the widely accepted atomic model of today: the quantum mechanical model.
