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Democritus

He was a philosopher that speculated that matter is made of finite units, atoms. He said that if you continually divide matter in half, you would get to a point where you could no longer divide matter. This point was called “atomos” where we get our word atom. His ideas were speculation; not all based on experiment.

His ideas include that atoms are indivisible pieces of matter that are too small to see. Atoms are small particles that make up all matter. He also said that matter’s particles like atoms are always moving to some degree. This started this idea called the kinetic theory of matter.

John Dalton

He did experiments with gases, gas pressures, and compounds. Through these experiments, he was able to give evidence to Democritus’s idea of the atom.

He stated that the atom is solid like a billiard ball. Dalton came up with the atomic theory. 1.) all matter are made up of indivisible units called atoms. Atom’s can’t be created or destroyed. 2.) all atoms of the same element are exactly the same including weight. Atoms of different elements have different weights, masses, and properties. Atoms come together to make compounds. The compounds made always consist of the same kind of atoms and the same proportion.

Joseph John Thompson

He did experiments with the Cathode ray tube. The cathode ray tube was a glass tube with metal ends on them. Most of the gas had been pumped out of this tube. The tube was hooked to an electrical current. This produced a beam of particles. This beam of particles bent when put next to positively and negatively charged plates. Thomson varied the voltage to see how that affected the bonding of the beam of particles. During this, he was able to determine the mass of the particles that made up the beam.

He knew that this beam was not light; it was a beam of particles that were attracted to a positive charge and repelled by a negative. Therefore, the beam of particles had a negative charge. He called each negative particle a “corpuscle,” which was later renamed an electron. He also determined the mass of an electron, which was 9.10 × 10^-31 kg.

Ernest Rutherford

He did the famous gold foil experiment. In this experiment, he “shot” alpha particles (helium nucleus +2 charge) at a thin sheet of gold foil. He expected them all to go straight through, but some were deflected above 90 degrees. True most did go straight through, but the ones deflected caused him to reimagine the atom’s structure.

Atoms have tiny, dense place for its positive charge and mass. He called this dense place for the atom’s positive charge and mass, the nucleus. It was the nucleus of the gold atoms of those alpha particles that caused some of the alpha particles to deflect. So the atom is mostly empty space small positive charged center with electrons orbiting around like planets.

Niels Bohr

He was a student of Rutherford’s. He focused his work on the role of electrons.

He stated that electrons are in fixed distances from the nucleus called energy levels. Electrons tend to fill energy levels closer to the nucleus first. High energy levels are further away from the nucleus. Electrons can absorb energy to move to a higher energy level and will release energy “light”” who moving back to the lower energy level.

To move to the higher energy levels, a set amount of energy must be minimally absorbed like rungs on a ladder.

Milkan-oil drop Experiment (Robert Andrews Millikan)

His experiment confirmed the mass of an electron and told the charge of an electron. In his experiment, he sprayed a fine mint of oil droplets into a chamber. By how fast they fell to the bottom, he was able to determine the mass of the oil droplets. Then he charged the oil droplets and the air by exposing them to x-rays. Now he timed and watched the oil drops’ movement through a microscope.

Through this, he determined the charge of an electron to be 1.59 × 10^-19. (it also determines the mass of an electron to be 9.10 × 10^-21 kg.)

Chadwick-neutron (Sir James Chadwick)

He repeated an experiment done by previous scientists. In this experiment, he bombarded beryllium atoms with alpha particles. This caused the Beryllium to release “something” that caused paraffin wax, which was behind Beryllium, to release protons. This “something” was neutrally charged because it wasn’t attracted or repelled by the charged plates. This particle caused the paraffin wax to release protons.

This neutral particle we call neutrons. These particles are located in the nucleus and have about the same mass as a proton.

Schrodinger-Electron Cloud Model (Erwin Schrodinger)

The Bohr model said electrons were in fixed energy levels, but the problem with this model is that it didn’t explain how electrons fully behaved. For example, none of the models explained why certain atoms were magnetic. Electrons behaved similar to light in that it acts like a particle and a wave. He came up with equations to determine the likely place of the electron. These likely places where electrons would be found are called orbitals.

The exact location could not be determined according to the Heisenberg Uncertainty Principle. These orbitals had various shapes: round, ring-like, dumbbell-like. This model is called the electron cloud model of the quantum mechanics model