Hund, 1896–1997), which today says that the lowest-energy electron configuration for an atom is the one that has the maximum number of electrons with parallel spins in degenerate orbitals. Choice c illustrates Hund’s rule (named after the German physicist Friedrich H. Similarly, experiments have shown that choice b is slightly higher in energy (less stable) than choice c because electrons in degenerate orbitals prefer to line up with their spins parallel thus, we can eliminate choice b. Figure 6.29 tells us that the next lowest energy orbital is 2 s, so the orbital diagram for lithium isīecause of electron-electron interactions, it is more favorable energetically for an electron to be in an unoccupied orbital than in one that is already occupied hence we can eliminate choice a. We know that the 1 s orbital can hold two of the electrons with their spins paired. The next element is lithium, with Z = 3 and three electrons in the neutral atom. Otherwise, our configuration would violate the Pauli principle. Written as 1 s 2, where the superscript 2 implies the pairing of spins. The orbital diagram for the helium atom is therefore From the Pauli exclusion principle, we know that an orbital can contain two electrons with opposite spin, so we place the second electron in the same orbital as the first but pointing down, so that the electrons are paired. We place one electron in the orbital that is lowest in energy, the 1 s orbital. \)), and the electron configuration is written as 1 s 1 and read as “one-s-one.”Ī neutral helium atom, with an atomic number of 2 ( Z = 2), has two electrons.
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