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490 BCE
Democritus
His experiment: Democritus broke a seashell into halves multiple times. It was then crushed to a powder and couldn't break up anymore. He thought there was an indivisible unit that makes up everything, which he named as the atom.
He believed that it was indestructible, always in motion, and in an infinite number. Significance: The first to acknowledge the atomic existence, he is credited for writing and making popular these descriptions of atoms which influenced discoveries that followed his. -
Period: to
Laws (Definite & Multiple Proportions, Conservation of Mass)
The law of conservation of mass states that mass can't be destroyed or created during chemical reactions or physical changes. The law of definite proportions states that a compound always contains exactly the same proportion of elements by mass. The law of multiple proportions states that elements combine in small whole number ratios to form compounds. Significance: these laws allowed scientists to examine chemical reactions and they acknowledge the ratios between elements and compounds. -
John Dalton
Dalton proposed a theory which stated that atoms:
1. made up all matter
2. of an element have identical properties, of different elements differed in properties
3. cannot subdivide, be created, or destroyed
4. combine in whole-number ratios to form chemical compounds
5. combine, separate, or rearrange in chemical reactions Significance: Dalton provided a framework for the modern atomic theory and introduced ideas on chemical reactions and compounds. In the theory, statements 1 and 2 remained. -
Period: to
Cathode Ray Experiment
Experiments were performed in which electric current passed through gases at low pressure. The experiments were carried out through cathode-ray tubes. Opposite to the cathode, the surface of the tube glowed when a current passed. It was a hypothesis that the glow was caused by a cathode ray, which was found to be made up of negatively charged particles. Significance: This investigation resulted in the first discovery of a subatomic particle, the electron. -
Dmitri Mendeleev
Mendeleev created his periodic table in 1869. He ordered elements into a table by increasing atomic weight. The elements' properties seemed to repeat in a pattern. Three gaps were on his table, which he predicted will be filled in by three new elements. Future scientists have expanded on his table and used it for what is now modern chemistry. Significance: The completed table is universally used to this day by chemists and students, greatly increasing our understanding of atoms within elements. -
Henri Becquerel
Becquerel experimented by soaking a uranium crystal in sunlight. Then that crystal was placed on a photographic plate, which it left its imprint on. Becquerel concluded that the uranium was giving off the absorbed energy of the sun in the form of an x-ray, but it resulted the same with the absence of light. It was then that radioactivity was discovered. Significance: He discovered radioactivity, which this contribution led to find different types of radiation that emit naturally from atoms. -
J.J. Thomson
Thomson discovered the electron, a subatomic particle, by experimenting with a cathode ray tube. He demonstrated that the atom wasn't the basic and indivisible unit of matter it was thought to be. He also discovered the negative charge on electrons. Significance: The discovery of the electron was made by Thomson, whose observations and ideas were used and confirmed. His discovery revised Dalton's Atomic Theory statement about atoms being indivisible. -
Marie Curie
Curie followed up on Becquerel's radioactive discovery by experimenting with uranium and radioactivity herself. In her work, she discovered radium and polonium. Significance: She studied chemical compounds and revealed that the strength of radiation didn't depend on the compound, but instead on the amount of uranium or thorium.. therefore the ability to radiate was linked to the interior of the atom. Her discovery helped develop physics. -
Plum Pudding Atomic Model
The Plum Pudding model was developed by J.J. Thomson. He stated that the electrons were free floating in an empty, positively charged space in the atom. This was to balance out the charge. The model was later disproved when the nucleus was discovered. Significance: The model was an important step in developing the atomic theory. It introduced discoveries such as the electron and acknowledged that the atom was made up of smaller units of matter. -
Robert Millikan
Millikan measured the size of an electron's charge through his oil drop experiment, revealing that it had a huge charge for its tiny mass. He also confirmed the electron's negativity. Significance: his famous oil drop experiment helped to first quantify the charge of an electron and gave more depth to our knowledge of this subatomic particle and its internal nature. -
Oil Drop Experiment
Droplets of oil were ionized as they fell through the air, with a balance in an electric force pulling the drops up and gravity pulling the drops down. The drops lost electrons as they were pulled downwards and gained a positive charge. When the forces stopped, the mass, voltage, and electron charge was recorded. From the results, Millikan concluded that 1.5924(17)×10−19 C was the value of an electron's charge. -
Ernest Rutherford
Rutherford was famous for conducting the gold foil experiment and for developing Rutherford's model, which both introduces a part of the atom that hadn't been discovered before, the nucleus. -
Gold Foil Experiment
Rutherford conducted the gold foil experiment, where he and his associates aimed a beam of alpha particles at a thin piece of gold foil. It seemed like every particle passed through, except for a few that redirected to the source. He believed that those were hitting a solid mass at the center of the atom, which was the existence of the nucleus. Significance: Discovering the nucleus provided a more accurate idea of the atom's internal structure, and expanded to more clearer theories. -
Rutherford Model
Ernest Rutherford revised Thomson's plum pudding model because it incorrectly portrayed the atom. The improved model included the nucleus, which was a bundle of neutrons and protons. The electrons were said to orbit the nucleus in Rutherford's version. Significance: his model was believed to accurately depict the atom's structure for a long time, but at the time his model was groundbreaking. It was the basis for future models of the atom. -
Niels Bohr
Bohr developed an atomic model that combined Rutherford's nucleus discovery and Max Planck's quantum ideas. His atomic theory suggested that atoms have a positively charged nucleus that electrons travel in circular orbits around. Significance: His model was the first model to consist of energy levels. He disproved the early idea that electrons floated in empty space. -
Bohr Planetary Model
Bohr's model shows that electrons surround and orbit the nucleus just like planets would the sun. Each energy level required a minimum amount of energy for the electron to jump to it. Electrons emitted light when they dropped levels. The levels kept them colliding into the nucleus. Significance: Bohr's planetary model is thought to be the most accurate and modern representation of the complete atom. -
Erwin Schrodinger
Erwin Schrodinger was the developer of the Quantum Mechanical model. He discovered that electrons exhibited wave-like properties instead of like particles. He used math equations to describe the probability of finding an electron in a certain position. -
Quantum Mechanical Model
The Quantum Mechanical Model predicted the odds of the location of an electron. In the model, a nucleus is surrounded by an electron cloud. Finding an electron was more likely in the most dense cloud than in the less dense area.
Significance: The model introduced sub-energy levels. -
James Chadwick
Chadwick bombarded beryllium atoms with alpha particles, producing an unknown radiation. He thought that the radiation was made up of neutrally charged particles, that had a proton's mass. Significance: he discovered the neutron, which was the neutrally charged particle in his beryllium experiment. The neutron was the final subatomic particle to be discovered, to complete the atomic structure's picture. -
Murray Gell-Mann
Murray Gell-Mann showed scientists what makes up protons and neutrons, quarks. A quark is an elementary particle with an electric charge. A force called the strong nuclear force holds the quarks together. Significance: He found out what gave atoms their electrical charge. As of now, quarks were a last discovery to the atomic theory.
