Thus, this model introduced the concept of sub-energy levels. Until , the atom was believed to be composed of a positively charged nucleus surrounded by negatively charged electrons. In , James Chadwick bombarded beryllium atoms with alpha particles. An unknown radiation was produced. Chadwick interpreted this radiation as being composed of particles with a neutral electrical charge and the approximate mass of a proton.
This particle became known as the neutron. With the discovery of the neutron, an adequate model of the atom became available to chemists. In real life, he probably had color. Democritus is credited with coming up with the atom. The question was, what would happen if you keep taking something like a tree and breaking into smaller and smaller pieces?
Would it always be a piece of a tree? Could you keep breaking it into smaller and smaller pieces? Democritus said that if you keep breaking it down, you would get to a size that could no longer be broken. This would be the indivisible piece. Thus, the atom. I know there is more to the Greeks, but I need a place to start. I am not going to go into the experimental evidence for Dalton's model of the atom, it's good stuff though. Let me just state what Dalton said:.
Basically, Dalton just expanded on the Greek idea of the atom. An atom is a small things, and there are different masses with different properties. Thomson played with cathode rays. These are just beams of electrons but cathode ray sounds cooler. By having the beam interact with electric and magnetic fields, Thomson was able to determine the mass to charge ratio for an electron.
So, from that he knew that the electron came from the atom, it had a negative charge and a small mass. Here is the model that he proposed. Thomson took the idea of the atom and tried to incorporate the evidence for the electron. In this model, the electrons are the small things and the rest of the stuff is some positive matter.
This is commonly called the plumb pudding model because the electrons are like things in positive pudding. Ernest Rutherford said one day "hey, I think I will shoot some stuff at atoms. I know how much you like it. He shot some alpha particles which are really just the nucleus of a helium atom at some really thin gold foil. Here is a diagram of his experiment.
If you shoot these positive alpha particles at this positive pudding atom, they should mostly bounce off, right? Well, that is not what happened. Rutherford found that most of them went right through the foil. Some of them did bounce back. How could that be if the plumb pudding model was correct? Rutherford's experiment prompted a change in the atomic model. If the positive alpha particles mostly passed through the foil, but some bounced back.
In July of , Danish physicist Niels Bohr published the first of a series of three papers introducing this model of the atom, which became known simply as the Bohr atom.
Bohr, one of the pioneers of quantum theory, had taken the atomic model presented a few years earlier by physicist Ernest Rutherford and given it a quantum twist.
Rutherford had made the startling discovery that most of the atom is empty space. The vast majority of its mass is located in a positively charged central nucleus, which is 10, times smaller than the atom itself. The dense nucleus is surrounded by a swarm of tiny, negatively charged electrons.
According to the rules of classical physics, the electrons would eventually spiral down into the nucleus, causing the atom to collapse. Bohr suggested that instead of buzzing randomly around the nucleus, electrons inhabit orbits situated at a fixed distance away from the nucleus.
In this picture, each orbit is associated with a particular energy, and the electron can change orbit by emitting or absorbing energy in discrete chunks called quanta. In this way, Bohr was able to explain the spectrum of light emitted or absorbed by hydrogen, the simplest of all atoms.
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