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. 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. The coefficient multiplies that by #4#, so we get #8xx4=32# atoms of hydrogen.Electron Configurations & The Periodic Table #H:# Ignoring the coefficient, we know there's #5+3=8# atoms of hydrogen. #C#: It only appears once and without a subscript, so #1xx4=4#. Again, counting the coefficient, there's #4xx2=8# atoms of nitrogen. #N#: The bracket around #NH_2# has a subscript of #2#, so that means there's #2# nitrogen atoms if we're not counting the coefficient. #H#: There's #3+1=4# atoms of hydrogen without the coefficient (it appears once as #H_3# and once as #H#), so there's #4xx5=20# atoms of hydrogen with the coefficient. After looking at the coefficient, though, there's #2xx5=10# atoms of carbon. That means that there are #2# carbon atoms if we don't look at the coefficient. In both cases, they don't have subscripts. #C#: Carbon appears twice in this formula. It also means that there's #2xx2=4# atoms of oxygen. When we put a coefficient in front of something, we're multiplying the entire thing by that number.įor example, in #2CO_2#, there are #2#CO_2# molecules. The coefficient, which is where the #x# is in #xA#.It also means that there's #2xx3=6# atoms of oxygen. So, for example, in #H_2O#, there are #2# hydrogen atoms. This tells us how many atoms of that specific element or group are in the compound. The subscript, which is where the #x# is in #A_x#.When determining how many atoms of an element are in a compound from the chemical formula, we need to keep in mind two things:
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