Figure 2.1.1 tells us that the next lowest energy orbital is 2 s, so the orbital diagram for lithium isĪs we continue through the periodic table in this way, writing the electron configurations of larger and larger atoms, it becomes tedious to keep copying the configurations of the filled inner subshells.
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. Unless there is a reason to show the empty higher energy orbitals, these are often omitted in an orbital diagram: Figure 2.1.1), 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. Some authors express the orbital diagram horizontally (removing the implicit energy axis and the colon symbol): Here is a schematic orbital diagram for a hydrogen atom in its ground state: A filled orbital is indicated by ↑↓, in which the electron spins are said to be paired. We use the orbital energy diagram of Figure 2.1.1, recognizing that each orbital can hold two electrons, one with spin up ↑, corresponding to m s = +½, which is arbitrarily written first, and one with spin down ↓, corresponding to m s = −½.
First we determine the number of electrons in the atom then we add electrons one at a time to the lowest-energy orbital available without violating the Pauli principle. We construct the periodic table by following the aufbau principle (from German, meaning “building up”).
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