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Electron Configuration. The Quantum Mechanical Model of the Atom. In 1924, French scientist Louis de Broglie showed that in order for Bohr’s electrons to behave the way they did, they must also have dual wave-particle nature.
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The Quantum Mechanical Model of the Atom • In 1924, French scientist Louis de Broglie showed that in order for Bohr’s electrons to behave the way they did, they must also have dual wave-particle nature. • Other scientists then showed that yes, electrons behaved like waves because their beams could be diffracted (bent as they pass a solid object) and interfered with (when waves overlap).
In 1927, German physicist Werner Heisenberg realized, that since electrons are detected through their interactions with photons, and photons have the same amount of energy as electrons, then any attempt to “find” an electron using a photon would knock that electron out of its path…so there is always an uncertainty in trying to find an electron.
Heisenberg uncertainty principle: It is impossible to determine both the position and velocity of an electron or any other particle. • In 1926, Austrian Erwin Schrödinger used quantum mechanics to solve equations that accounted for the wave-like properties of electrons. • The results of the Schrödinger equations give the probability of finding an electron in a given energy level.
Atomic Orbitals • The orbital designations are all results of solving Schrödinger’s mathematical equations, for four quantum numbers. • Every atom has different principal energy levels: n = 1,2,3,… The higher the energy level, the further the electron is from the nucleus. • There are sublevels in each energy level. • The number of sublevels is equal to n. For example, if n = 1, there is one sublevel. If n =3, there are three sublevels.
Sublevels each have letter names: s, p, d, and f. • Each has a different shape, which is defined by where the electron is most likely to be found in that level. • Each orbital can hold a specific number of electrons. S: 2; p: 6; d: 10; f: 14. • Every electron has a spin: either +1/2 or -1/2.
Electron Configuration • There are rules that explain how electrons are configured around and atom’s nucleus. • An atom’s electron configuration provides the lowest energy and therefore greatest stability for an atom.
Afbau principle: Electrons enter orbitals of lowest energy first. The energy levels follow the pattern shown below.
Pauli Exclusion Principle • Pauli Exclusion Principle: only two electrons per orbital are allowed; to occupy an orbital, the two electrons have to have opposite spins. • Spin is yet another result of Schrödinger’s equation: it is either clockwise or counterclockwise. • A spinning electron acts as a tiny magnet, with a north pole on one end and a south pole on the other. If electrons with opposite spin enter an orbital, their magnetic attraction counteracts the electric repulsion. They can share the space. But, if electrons with the same spin enter an orbital, it will always have the same spin as one of the electrons already there, and it will be repulsed.
Hund’s Rule • Hund’s Rule: When electrons occupy orbitals of equal energy, they enter with parallel spins until the orbital is full, then they begin pairing the spins. • The atomic orbitals determine the arrangement of the periodic table, and the elements are in order of increasing electron energy.
Noble gases provide exceptionally low energy, and what we call stability. • Noble gases are also very unreactive.
Exceptional Electron Configurations • Cr and Cu are unusual because they can benefit from extra stability from half-filled orbitals.
