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The chemical properties of the elements reflect their electron configurations. The highest occupied electron shell is called the valence shell, and the electrons occupying this shell are called valence electrons. In the third period of the table, the atoms all have a neon-like core of 10 electrons, and shell #3 is occupied progressively with eight electrons, starting with the 3s-orbital. As we progress from lithium (atomic number=3) to neon (atomic number=10) across the second row or period of the table, all these atoms start with a filled 1s-orbital, and the 2s-orbital is occupied with an electron pair before the 2p-orbitals are filled. Shell #2 has four higher energy orbitals, the 2s-orbital being lower in energy than the three 2p-orbitals. According to the Aufbau principle, the electrons of an atom occupy quantum levels or orbitals starting from the lowest energy level, and proceeding to the highest, with each orbital holding a maximum of two paired electrons (opposite spins).Įlectron shell #1 has the lowest energy and its s-orbital is the first to be filled. The truncated periodic table shown above provides the orbital electronic structure for the first eighteen elements (hydrogen through argon). Consequently, our understanding of organic chemistry must have, as a foundation, an appreciation of the electronic structure and properties of these elements. Other interactive periodic tables provide comprehensive data for each element, including nuclide properties, environmental and health factors, presentation in different languages and much more.įor comic relief you may wish to examine a periodic table linked to element references in comic books.įour elements, hydrogen, carbon, oxygen and nitrogen, are the major components of most organic compounds. There are, of course, over eighty other elements.Ī complete periodic table, having very useful interactive links has been created by Mark Winter. The periodic table shown here is severely truncated.
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This module introduces some basic facts and principles that are needed for a discussion of organic molecules.Įlectron Configurations in the Periodic Table 1A The study of organic chemistry must at some point extend to the molecular level, for the physical and chemical properties of a substance are ultimately explained in terms of the structure and bonding of molecules. By extrapolation, we expect all the group 2 elements to have an ns 2 electron configuration.Electron Configurations & The Periodic Table The next element down, magnesium, is expected to have exactly the same arrangement of electrons in the n = 3 principal shell: s 2. Beginning with beryllium, we see that its nearest preceding noble gas is helium and that the principal quantum number of its valence shell is n = 2.ī Thus beryllium has an s 2 electron configuration. Write the valence electron configuration of each element by first indicating the filled inner shells using the symbol for the nearest preceding noble gas and then listing the principal quantum number of its valence shell, its valence orbitals, and the number of valence electrons in each orbital as superscripts.Ī The group 2 elements are in the s block of the periodic table, and as group 2 elements, they all have two valence electrons.Locate the nearest noble gas preceding each element and identify the principal quantum number of the valence shell of each element. Identify the block in the periodic table to which the group 2 elements belong.Use the periodic table to predict the valence electron configuration of all the elements of group 2 (beryllium, magnesium, calcium, strontium, barium, and radium).Īsked for: valence electron configurations For elements after No, the electron configurations are tentative. The electron configurations of the elements indicated in blue are also anomalous, but the reasons for the observed configurations are more complex. The electron configurations of elements indicated in red are exceptions due to the added stability associated with half-filled and filled subshells.
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As a result, the periodic table can be divided into “blocks” corresponding to the type of subshell that is being filled, as illustrated in Figure \(\PageIndex\): Electron Configurations of the Elements. Although the table was originally organized on the basis of physical and chemical similarities between the elements within groups, these similarities are ultimately attributable to orbital energy levels and the Pauli principle, which cause the individual subshells to be filled in a particular order. To correlate the arrangement of atoms in the periodic table results in blocks corresponding to filling of the ns, np, nd, and nf orbitalsĪs you have learned, the electron configurations of the elements explain the otherwise peculiar shape of the periodic table.