BOHR MODEL OF THE ATOM The Planetary Model of the Atom “ ”

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WHO IS NIELS BOHR? What does his atomic model looks like?

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NIELS BOHR (October 1885 – November 1962) Danish physi c ist who made fundamental contributions to the field of understanding atomic structure and quantum mechanics. Born in Copenhagen, Denmark in 1885. Graduated in the University of Copenhagen. Influenced by Ernest Rutherford. Known for his atomic model, the theory in which electrons travel in discrete orbits around the atom’s nucleus.

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THE BOHR MODEL

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BOHR’S MODEL OF THE ATOM In 1913, Niels Bohr proposed an improvement for Rutherford’s atomic model. In his model he placed each electron in a specific energy level.

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While Rutherford’s model focused on describing the nucleus, Niels Bohr turned his attention to describing the electron. It depicted the electron as an orbiting planet.

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In the Bohr Model the neutrons and protons (symbolized by red and green balls in the adjacent image) occupy a dense central region called the nucleus, and the electrons orbit the nucleus much like planets orbiting the Sun.

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He reasoned that larger shells could hold more electrons and proposed that each shell could hold 2 n squared electrons where n is the shell number. So the first shell could hold 2x(1 squared)=2 electrons, the second shell 2x(2 squared) electrons and so on. This gives the following sequence of numbers of electrons in shells: 2, 8, 18, 32, 50 etc.

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2(n) 2 Where n is the shell number.

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The basic feature of quantum mechanics that is incorporated in the Bohr Model and that is completely different from the analogous planetary model is that the energy of the particles in the Bohr atom is restricted to certain discrete values. One says that the energy is quantized . This means that only certain orbits with certain radii are allowed; orbits in between simply don't exist . BUT THE ORBITS ARE QUANTIZED

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The adjacent figure shows such quantized energy levels for the hydrogen atom. These levels are labeled by an integer n that is called a quantum number . The lowest energy state is generally termed the ground state . The states with successively more energy than the ground state are called the first excited state , the second excited state , and so on. Beyond an energy called the ionization potential the single electron of the hydrogen atom is no longer bound to the atom. Then the energy levels form a continuum. In the case of hydrogen, this continuum starts at 13.6 eV above the ground state (" eV " stands for "electron-Volt", a common unit of energy in atomic physics ).

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Bohr proposed that electrons are restricted to certain (quantized) orbits. An electron can jump, suddenly, between these orbits by absorbing or emitting a photon with the appropriate precise wavelength.

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His theory is a great leap forward. It worked well for Hydrogen and reasonably well for Helium, but did not fit the emission spectra of larger elements.

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