Хемија, Неметали и Метали

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Light and Matter

Atoms are made of smaller particles - Protons, Neurtrons, and Electrons. Oxygen is a common element. In its most most abundant isotopic form (O-16), it has:

8 Protons 8 Neutrons 8 Electrons

But, how are these subatomic particles arranged in the oxygen atom? Are they just thrown in like marbles in a bag? Or is there a unique order in the atom? Does the atom have structure?

Many scientists contributed to understanding the structure of atoms. They performed experiments and proposed models. A model is a theory or picture. In the nuclear model, the atom is pictured as a huge empty ball with a tiny nucleus at its center. The nucleus holds protons and is positively charged. In this model, the electrons circulate somewhere around the nucleus.

Niels Bohr proposed a new model (the Solar System Model) of the atom based on observations of light! Bohr explained how atoms produce "light", and gave us our first picture of how electrons are arranged in the atom.
Many scientists at the beginning of the 20th century were puzzled by how matter could emit light.
If we pass Ne light through a prism, we see a well defined pattern of bright lines. This series of bright lines is called an emission spectrum. Each element produces a unique emission spectrum. Here is a picture showing the emission spectrum of 3 elements, Ne, Hg, and Na. Sodium (Na) produces two closely spaced yellow lines in its emission spectrum.
The emission spectrum of an element is unique much like a fingerprint. And just like a fingerprint is used to identify an individual, an emission spectrum can be used to identify an element. Spectroscopy, the technique for studying the emission spectrum of elements, can help us identify the soucre of paint at a crime scene, or the composition of a star lightyears from Earth!
But...what causes an element to emit this bright line spectrum of colors? Bohr gave us the answer.

Bohr agreed with Rutherford that the atom was mostly empty space, and that protons were located in the nucleus at the center of the atom. But, Bohr went a step further and suggested that electrons circulated around the nucleus in definite orbits at well defined distances from the nucleus - like planets orbiting the sun.

Bohr based his suggestions on his observations of the simplest element...Hydrogen. Hydrogen has an Atomic Number of 1, which means it has 1 proton in the nucleus, and 1 electron circulating around the nucleus.

Bohr believed that the electron in an un-heated hydrogen atom circled the nucleus in an orbit not far from the center of the atom. According to Bohr, when the hydrogen atom is heated, that electron absorbs energy and jumps to higher orbit. How far it jumps depends on how much energy it absorbs. Let's say it absorbs only enough energy to jump to the next higher orbit. Now, what happens when the hydrogen atom cools down? Bohr said upon cooling, the electron would fall back to a lower orbit. Well, since it gained energy in jumping to the higher orbit....it only makes sense that it has to lose that energy when it falls back to the lower orbit. The lost energy is given off as light, and not just any light, but light of a certain frequency or COLOR! In other words, the color of light given off depends on how far the electron falls. If it falls a short distance then the color is probably closer to the red (lower energy) end of the spectrum, but if the electron falls a great distance, then the color will be at the blue (higher energy) end of the spectrum.
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