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Transcript of Atomic Structure
He came up with his atomic theory as a result of his research into gases.
He discovered that certain gases only could be combined in certain proportions even if two different compounds shared the same common element or group of elements.
Through deductive reasoning and experimentation, he made an interesting discovery: elements combine at the atomic level in fixed ratios.
In 1897, he discovered the electron - first subatomic particle.
He was the first to attempt to incorporate the electron into a structure for the atom.
He proposed the "plum pudding" model of the atom in 1904.
In 1909, he conducted the first nuclear scattering experiments.
He tested Thomson's hypothesis by devising his gold foil experiment.
He reasoned that if Thomson's model was correct, then the mass of the atom was spread throughout the atom. Then, if he shot high velocity alpha particles at an atom, then there would be very little to deflect the alpha particles.
As expected, most alpha particles when right through the gold foil but few alpha particles rebounded almost directly backwards.
He reasoned that the only way the alpha particles could be deflected backwards was if most of the mass in an atom was concentrated in a nucleus.
Dalton's atomic theory was based on:
1. Matter is made up of atoms that are indivisible and indestructible.
2. All atoms of an element are identical.
3. Atoms of different elements have different weights and different chemical properties.
4. Atoms of different elements combine in simple whole numbers to form compounds.
5. Atoms cannot be created or destroyed. When a compound decomposes the atoms are recovered unchanged.
John Dalton also discovered color blindness, which is still -sometimes- referred to as "Daltonism"
He was afflicted with color blindness.
Plum Pudding Model
The atom is composed of electrons, surrounded by a soup of positive charge to balance the electrons' negative charges. (Negatively charged "raisins" surrounded by positively charged "pudding")
Atom has a 'cloud' of positive charge.
mass of an atom when the electron was only about 1/1000 the mass of a hydrogen atom
creating a neutral atom when the only particle available was negatively charged.
Most of the alpha particles pass straight through the gold foil.
Some of the alpha particles get deflected by very small amounts.
A very few get deflected greatly.
Even fewer get bounced of the foil and back to the left.
The atom is 99.99% empty space
The nucleus contains a positive charge and most of the mass of the atom.
The nucleus is approximately 100,000 times smaller than the atom.
Rutherford envisioned the electrons orbiting the nucleus in a matter analogous to the planets orbiting the Sun.
The orbiting electrons should emit electromagnetic waves as predicted by Maxwell's equation for electromagnetic phenomena. This loss of energy should cause the electrons' orbits to decay until they crashed into the nucleus.
Rutherford's model was unable to explain atomic emission and absorption line spectra.
Neils Bohr's Atomic Model
Electrons orbit the nucleus at set distances.
When an electron changes orbits, it does so in a sudden quantum leap.
The energy difference between initial and final orbit is emitted by the atom in bundles of electromagnetic radiation called photons.
The Bohr model was based on his observations of the atomic emissions spectrum of the hydrogen atom. When white light is diffracted with a prism, all the colors of the visible spectrum can be seen. Each color corresponds to a specific amount of energy; however when the light given off by the hydrogen atom was passed through a prism, only certain colors of light could be seen. This led Bohr to theorize that electrons only have certain energies in an atom and they had to be in energy levels. Bohr found the energy of the colors of light that the hydrogen atom released. He used these energies to find the energies that the single electron in the hydrogen atom could have. Bohr said that the electron had to release energy to change its energy so the differences between the energies of light seen in the atomic spectrum should correspond to the differences in energies of the energy levels.
Failures of the Bohr Model
It fails to provide any understanding of why certain spectral lines are brighter than others.
The Bohr model treats the electron as if it were a miniature planet, with definite radius and momentum. This is a violation of the uncertainty principle which states that position and momentum cannot be simultaneously determined.
He developed a powerful model of the atom in 1926.
He combined the equations for the behavior of the waves with the de Broglie equation to generate a mathematical model for the distribution of electrons in an atom.
His model allowed the electron to occupy 3D space.
His model describes the probability that an electron can be found in a given region of space at a given time.