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Democritus / 460 BC
Democritus formed the idea of atoms. He would pound up materials in his Pestle & Mortar until he had reduced them to smaller and smaller particles which could not be divided anymore. He then called these, Atoma. -
What is an Atom?
An Atom is the smallest part of an element that retains its identity in a chemical reaction. -
Aristotle / 342 BC
Aristotle believed that there was no limit to the amount of times a certain substance could be broken down. -
John Dalton / History of the Atom
John Dalton suggested that all matter was made up of tiny spheres that were able to bounce around with perfect elasticity and called them, Atoms. -
Dalton's Atomic Theory (Continued)
Sadly, most of John Dalton's statements are known to be flawed now. -
Dalton's Atomic Theory
Dalton's Atomic Theory consists of 4 different ideas:
1. All elements are composed of atoms (which can’t be divided).
2. Atoms of the same element have the same mass and atoms of different elements have different masses.
3. Compounds contain atoms of more than one element.
4. In a compound, atoms of different elements always combine in the same way. -
Sir William Crookes
Crookes discovered the element of Thallium while conducting an experiment, and also he developed the Crookes tubes, investigating cathode rays. -
Sir William Crookes
He also devised one of the first instruments for the study of nuclear radioactivity, the spinthariscope. -
Marie Curie
Marie Curie's achievements include a theory of radioactivity techniques for isolating radioactive isotopes, and the discovery of two elements, polonium and radium. -
Marie Curie
She founded the Curie Institutes in Paris and in Warsaw, which remain major centres of medical research today. -
Joseph John Thomson
Joseph John Thomson found that atoms could sometimes eject a much smaller negative particle which he called an Electron. -
Joseph John Thomson (Continued)
Joseph John Thomson discovered the Electron, the subatomic particle which happens to contain a negative charge.
His experiments involved the use of a Cathode Ray Tube. -
Joseph John Thomson
Thompson discovered the idea that an atom was made up of electrons, scattered unevenly within an elastic sphere surrounded by a gel of positive charge to balance the Electron's negative charge. -
Thomson's Atomic Model
Jospeh John Thomson ended up developing the Plum Pudding Model. This allowed Electrons to be evenly distributed throughout a positively charged material. -
Robert Andrew Millikan
Robert Andrew Millikan began by measuring the course of charged water droplets in an electrical field. The results suggested that the charge on the droplets is a result of the electric charge, but the experiment was not accurate enough to be convincing. He acquired more accurate results in 1910 with his famous oil-drop experiment in which he replaced water (which tended to evaporate too quickly) with oil. -
Robert Andrew Millikan
Millikan worked on an oil-drop experiment in which he measured the charge on a single electron. J.J. Thomson had already discovered the charge-to-mass ratio of the electron. -
Ernest Rutherford
Ernest Rutherford was an overseer to Geiger and Marsden carrying out his famous experiment, which was firing helium nuclei at a piece of gold foil, which was only a few atoms thick. -
Ernest Rutherford
They found that while most of the helium nuclei passed through the foil, a small number were deflected and, to their surprise, some helium nuclei bounced straight back. -
Ernest Rutherford
Rutherford tested his theory that electrons were evenly distributed throughout the atom within positively charged material.
He called this the Gold-Foil Experiment -
Ernest Rutherford's Model
Rutherford proposed that atoms contain a nucleus, a small, dense, positively-charged sphere in the center of the atom.
He also said that an atom contains mostly empty space, and that the nucleus is tiny compared with the atom as a whole. -
History of the Atom
Rutherford’s new evidence allowed him to propose a more detailed model with a central nucleus. -
History of the Atom (Continued)
Rutherford suggested that the positive charge was all in a central nucleus. With this holding the electrons in place by electrical attraction -
History of the Atom (Continued)
- Dalton’s Theory - all matter is made up of atoms, which can’t be divided
- Thomson’s Model - discovered atoms were made up of smaller particles (these smaller particles are charged)
- Rutherford’s Theory - discovered the positively charged nucleus
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Niels Bohr
Niels Bohr studied under Rutherford at the Victoria University in Manchester.
Bohr refined Rutherford's idea by adding that the electrons were in orbits. Rather like planets orbiting the sun. With each orbit only able to contain a set number of electrons. -
Parts of an Atom
- Atom = nucleus surrounded by one or more electrons
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Parts of an Atom (Continued)
Subatomic Particles
- Protons (+)
- Neutrons (0)
- Electrons (-) -
Niels Bohr
From a study of Hydrogen, Bohr concluded that an electron travels around the nucleus in circular paths called orbits
Each pathway represents an energy level.
Only orbits of certain energies are allowed. Electrons can’t exist between orbits. -
Niels Bohr
Bohr stated that energy must be added for an electron to move up a level, also claiming that energy is released when an electron moves down. -
Niels Bohr
Bohr’s “orbits” are now “principal energy levels”.
Bohr was incorrect in assuming that electrons moved like planets in a solar system.
The Electron Cloud model shows the most likely location of electrons in an atom! -
Atomic Structure
Electrons are arranged in different Energy Levels or Shells around the nucleus of an atom. -
Atomic Structure (Continued)
First Shell = a maximum of 2 electrons
Second Shell = a maximum of 8 electrons
Third Shell = a maximum of 18 electrons -
Henry Moseley
Moseley's law helped propel atomic physics by providing the first experimental evidence in favor of Niels Bohr's theory, aside from the hydrogen atom spectrum which the Bohr theory was designed to represent. That theory refined Ernest Rutherford's model. -
Henry Moseley
Henry Moseley helped project the idea of Atomic Numbers and how every element has one. -
Parts of an Atom (Continued)
Electron configuration – the arrangement of electrons in the orbitals of an atom -
Parts of an Atom
Orbital – a region of space around the nucleus where an electron is most likely to be found
