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Democritus' Idea of the Atom (492 BCE)
One of the creators of the first basis for the Atomic Theory who stated that everything is composed of "atoms", which are physically, but not geometrically, indivisible.
He also declared that atoms are indestructible and have always been, and will always be, in motion.
His theory was often questioned because he could not explain the chemical side of his atomic theory. -
Dalton's Atomic Theory
5 Main Ideas of Dalton's Atomic Theory:- Elements are made of extremely small particles called atoms.
- Atoms of a given element are identical in size, mass, and other properties; atoms of different elements differ in size, mass, and other properties.
- Atoms cannot be subdivided, created, or destroyed.
- Atoms of different elements combine in simple whole-number ratios to form chemical compound.
- In chemical reactions, atoms are combined, separated, or rearranged.
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Sir William Crookes' Discharge Tube Experiments
Crookes’ tube consists of a sealed glass tube from which air has been removed. Through the walls 2 electrodes are passed. When a high voltage is applied between the 2 electrodes, cathode rays are emitted from the cathode and are accelerated toward the anode. Many of these cathode rays miss the anode and strike the glass wall of the tube, causing fluorescence.
He showed that cathode rays are negatively charged by studying the direction in which they are deflected by a magnetic field. -
J. J. Thomson's Experiments (Charge-to-Mass Ratio of the Electron)
Thomson first demonstrated that cathode rays were composed of a previously unknown negatively charged particle, and thus he is credited with the discovery and identification of the electron; and, in a broader sense, with the discovery of the first subatomic particle.
From Maxwell's theory, he knew that charged particles could be deflected in a magnetic field. So, using the cathode ray tube, he determined the charge-to-mass ratio of the electron, e/m. -
J. J. Thomson's Experiments (Positive Rays)
In Thomson's "plum pudding" model, he stated that the atom is composed of electrons surrounded by a soup (or cloud) of positive charge to balance the electrons' negative charges, like negatively charged "raisins" surrounded by positively charged "pudding".
The electrons were thought to be positioned throughout the atom, but with many structures possible for positioning multiple electrons, particularly rotating rings of electrons. -
J. J. Thomson's Experiment (charge/mass ratio of positive ions)
Thomson observed the hydrogen ion. It looked like ions of hydrogen were carrying positive charge in the same way that electrons carried negative charge.
By using a very large discharge tube, he managed to identify this ion as an ionized hydrogen molecule. Two atoms of hydrogen bond together to form a molecule. When it becomes ionized one electron is lost, producing one positive charge on the ion, but its mass is twice that of the hydrogen atom. -
Gold Foil Experiment by Rutherford et al
Rutherford's model suggested that most of the mass of the atom was contained in the small nucleus, and that the rest of the atom was mostly empty space.
His experiment involved the firing of radioactive particles through thin metal foils and detecting them using screens coated with zinc sulfide. He found that although the vast majority of particles passed straight through the foil approximately 1/8000 were deflected, leading him to his theory. -
Millikan’s Oil Drop Experiment
Millikan determined the size of the charge on a single electron.
He put a charge on a tiny drop of oil, and measure how strong an applied electric field had to be in order to stop the oil drop from falling. Since he was able to work out the mass of the oil drop, and he could calculate the force of gravity on one drop, he could then determine the electric charge that the drop must have. -
Henry Moseley's Experiment (Atomic Numbers)
Moseley used an electron gun to fire a stream of electrons at samples of different elements. He found that the elements gave off X-rays.
He measured the frequency of the X-rays given off by different elements. Each element gave a different frequency and he found that this frequency was mathematically related to the position of the element in the Periodic Table – he could actually measure atomic number. -
James Chadwick’s discovery of the Neutron
Chadwick repeated Rutherford's experiments, using many different elements as radiation targets. By comparing the energies of particles ejected from all these targets, he was able to prove that the radiation causing the ejected particles was much more energetic than could be accounted for by photons.
What really occurred when one bombarded beryllium with alpha particles, Chadwick explained, was the formation of a carbon-12 nucleus and the emission of a neutron.
