When we have two or more electrons to place in the

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When we have two or more electrons to place in the orbitals in various ways, the different l values (the shapes) have slightly but significantly different energies for the same n. Still, we can discern a pattern in the periodic table. In this case, we want all electrons to be in their ground states, that is with the lowest (biggest negative) energies possible. Only two electrons (of opposite spin) can be in the same orbital. We just “fill up” the orbitals. There are one or two funny things. Remember this table? 1 st level: 1s 2 1=2 2 nd level: 2s 2p 2 (1+3)=8 3 rd level: 3s 3p 3d 2 (1+3+5)=18 4 th level: 4s 4p 4d 4f 2 (1+3+5+7)=32
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6 Copyright: 2010 Prof. Magde Chapter 7: Electrons in Atoms Using the lowest energy orbital, 1s, we can put in 1 e - and make H. We can put in two e - in the 1s and make He. To make bigger atoms, we fill up the 1s pair and then think about adding more e - . The 2s and 2p orbitals can accommodate 8 more e - . So if these are all fairly close in energy, we can use them to make elements with 3 to 10 electrons (atomic number). The 2s orbital is slightly lower in energy than 2p, if we already have 2 e - in the 1s orbital. So we expect Li has 2 e - in its 1s orbital and one more in the 2s orbital. Be uses 2s for both its third and fourth e - . Then we use the three 2p orbitals. This will give us room for 6 more e - and get us to Ne. We put them in different p orbitals, if possible. Next we use the 3s orbital to make Na and Mg. And six more elements use the 3p orbitals to make Al to Ar.
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6 Copyright: 2010 Prof. Magde Chapter 7: Electrons in Atoms Then we have out first problem. Using the n = 3 levels, we might expect to use the 3d orbitals after the 3p orbitals. No!! The d orbitals are shielded too much. Their e - do not feel the positive nucleus as much as they “should.” So 4s sneaks in first. Then 3d. Then 4p. So we have two periods (rows) of 8 elements before we get to a row of 18 elements. The extra 10 are transition metals with 1 to 10 d electrons. Similarly, we get two rows of 18 before we get around to using the f orbitals to make the really big periods that have 32 elements. The long rows have not only transition metals, but also lanthanides and actinides. If you think it is hard to remember these names, look at the periodic table. It tells you the names of these extra pieces that use f electrons! Ain’t she sweet?
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If you memorize the order of orbital energies, and know how many electrons can be accommodated in each, after counting all the m l sub-sub-levels, then you can build up the periodic table using the proper orbitals. Actually, looking at the periodic table, you can figure out the orbitals. Ain’t she sweet? Using the correct orbitals matters if we are to explain observable, macroscopic chemical properties. This is the order of the last e - if we are filling them all with e - . It is not the order for a single e - or the order for the low levels when all are filled.
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