How the Electrons in an Atom Are Distributed

Oxygen has three isotopes, and The atomic number of oxygen is 8. How many 

For a long time, electrons were perceived to be particles—infinitesimal planets that orbit the nucleus of an atom. In 1924, however, a French physicist named Louis de Broglie showed that electrons also have wavelike properties. He did this by combining a formula developed by Albert Einstein that relates mass and energy with a formula developed by Max Planck that relates frequency and energy. The realization that electrons have wavelike properties spurred physicists to propose a mathematical concept known as quantum mechanics. 

Quantum mechanics uses the same mathematical equations that describe the wave motion of a guitar string to characterize the motion of an electron around a nucleus. The version of quantum mechanics most useful to chemists was proposed by Erwin Schrodinger in 1926. 

According to Schrodinger, the electrons in an atom can be thought of as occupying a set of concentric shells that surround the nucleus. The first shell is the one closest to the nucleus. The second shell lies farther from the nucleus. The third and higher numbered shells lie even farther out. 

Each shell contains subshells known as atomic orbitals. Each atomic orbital has a characteristic shape and energy and occupies a characteristic volume of space. 

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