The simplified notation allows us to see the valence-electron configuration more easily. This means that the chemistry of an atom depends mostly on the electrons in its outermost shell, which are called the valence electrons. For example, represents the 1 s 22 s 22 p 6 electron configuration of neon ( Z = 10), so the electron configuration of sodium, with Z = 11, which is 1 s 22 s 22 p 63 s 1, is written as 3 s 1: Electron Configuration of Neon and Sodium Neonīecause electrons in filled inner orbitals are closer to the nucleus and more tightly bound to it, they are rarely involved in chemical reactions. In practice, chemists simplify the notation by using a bracketed noble gas symbol to represent the configuration of the noble gas from the preceding row because all the orbitals in a noble gas are filled. 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. When we reach neon, with Z = 10, we have filled the 2 p subshell, giving a 1 s 22 s 22 p 6 electron configuration:Īs 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. The Aufbau Principle We construct the periodic table by following the aufbau principle (from German, meaning building up). Similarly, fluorine has the electron configuration 1 s 22 s 22 p 5: Fluorine has one electron pair in the 1 s orbital, one electron pair in the 2 s orbital, and 2 electrons pairs with one unpaired electron in the 2 p orbital. Oxygen has one electron pair in the 1 s orbital, one electron pair in the 2 s pair, and one electron pair, and 2 unpaired electrons in the 2 p orbitals. Because all the 2 p orbitals are degenerate, it doesn’t matter which one has the pair of electrons. One electron must be paired with another in one of the 2 p orbitals, which gives us two unpaired electrons and a 1 s 22 s 22 p 4 electron configuration. The electron configuration of nitrogen is thus 1 s 22 s 22 p 3.Īt oxygen, with Z = 8 and eight electrons, we have no choice.
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