Quantum Theory and the Atom

​Niels Bohr, A Danish physicist working in Rutherford's laboratory in 1913 proposed a quantum model for the hydrogen atom that seemed to answer the question about why the emission spectrum generated by hydrogen was discontinuous and only made up of certain frequencies.

​Bohr proposed the hydrogen atom has only certain allowable energy states. The lowest allowable state of an atom is called its ground state. When an atom gains energy, it is said to be in an excited state.

​Bohr related the hydrogen atom's energy to the electron in the atom. He suggested that the electron moves around the nucleus in only certain allowable circular orbits. The smaller the orbit, the lower the atom's energy state. Conversely, the larger the orbit, the higher the energy state.

​Bohr assigned a number, n, called a quantum number, to each orbit. For the first orbit, the one closest to the nucleus, he referred to it as n=1. For the second, n=2, the third n=3, etc. When energy is added to the atom from an outside force, the electron moves to a higher energy orbit.

​When energy is added to an atom, it moves from a ground state to an excited state. The excited state is not the natural state and an atom will release energy in the form of a photon of light to go back to the ground state.

​Bohr's model works for the hydrogen atom, but failed to explain the emission spectrum of other elements. It also did not fully account for the chemical behavior of atoms. It did lay the groundwork for atomic models that would come later.

​In 1924, a French graduate student in physics named Louis de Broglie (1892-1987) proposed an idea that eventually accounted for the fixed energy levels of Bohr's model.

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De Broglie had been thinking that Bohr's quantitized electron orbits had characteristics similar to those of waves.​ He reflected on the fact that light has both wave and particle characteristics. He proposed that particles, including electrons, could also behave like waves.

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​The de Broglie equation predicts that all moving particles have wave characteristics. The wavelength is too small to be seen or detected.

​German theoretical physicist Werner Heisenberg (1901-1976) showed that it is impossible to take any measurements without disturbing the object.

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The Heisenberg uncertainty ​principle states that it is fundamentally impossible to know precisely both velocity and position of a particle at the same time.

​The wave function predicts a three-dimensional region around the nucleus, called an atomic orbital, which describes the electron's probable location. A atomic orbital is like a fuzzy cloud in which the density at a given point is proportional to the probability of finding the electron at that point. The cloud has no definite boundary and the electron can be found at a considerable distance from the nucleus.

​The quantum mechanical model assigns four quantum numbers to atomic orbitals. The first one is the principal quantum number (n) and indicates the size and energy of atomic orbitals. Each major energy level is called a principal energy level.

Principal energy levels contain energy sublevels. The first energy level has 1 sublevel, the second energy level contains 2 sublevels, the third has 3 sublevels, and the fourth has 4 sublevels.​

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​Orbitals and sublevels are areas where electrons can be found. Sublevels are labeled s, p. d, and f according to the shapes of the atom's orbitals. All orbitals are spherical, and all p orbitals are dumbbell-shaped.