The orbital diagram shows threeunpaired electrons. The placement of the nextelectron must follow Hund's rule. Theorbital energy diagram for carbon is shown below. The number ofunpaired electrons is evident from the orbital diagram. The electron configuration for carbon is 1s 22s 22p 2.Notice the electron configuration does not clearly indicate thenumber of unpaired electrons in the element. According to Hund's rule the most stable arrangement in a setof degenerate orbitals is that with the most number of unpairedelectrons. The question becomes whether thenext electron should be pair the other electron or whether theelectron should be placed in an empty 2p orbital. The energy level diagram for boron is show below.įor the next element, carbon, the sixth electron must beplaced in the correct orbital. The electron configuration for boron is 1s 22s 22p 1. The electron can be placed in any of the three 2porbitals. The orbitaldiagram for boron as shown has the one electron in the 2porbital. The next element is boron with 5 electrons. The energy required to pairthe first 2s electron is less than the energy required to placethe electron into the 2p orbital. The fourthelectron is placed in the 2s orbital. Theorbital diagram for beryllium is shown here. The next element is beryllium which has four electrons. The energy level diagram, on the left shows the relativeenergy of the 2s and 2p orbitals based on the ability of thesublevels to penetrate to the nucleus. Theelectron configuration for lithium is 1s 22s 1. So the orbital diagram for lithium is shown below. It can be stated the 2s orbitalpenetrates closer to the nucleus than does a 2p orbital.
If the electron spends more time closer to the nucleusthe electron will experience a greater attraction to the nucleusand it is lower in energy. The essence of these calculations is that when anelectron is placed into the 2s orbital the electron is likely tospend more time closer to the nucleus than an electron in a 2porbital. So we must consider which orbital, whenthe electron is placed into it, has the lowest energy? This isanswered by considering some complicated mathematicalcalculations. When the electron is added to the secondlevel it can go into the 2s orbital or the 2p, the question iswhich orbital is the electron placed? The primary criteria, theAufbau principle, states the electrons are to be placed into theorbital of lowest energy. The first level is filled and can not accommodateany more electrons. When the third electron is to be placed it must go into thesecond level. In multi-elecron atoms the degeneracy of theenergy of the sublevels is lost. In the hydrogen atom the sublevels in each principle levelare degenerate. Notice that there hasbeen a change in the relative energies of the 2s and 2p orbitals.This is an important point that must be addressed at this point. Theenergy diagram for helium is shown as here. So while hydrogen has the electron configuration of 1s 1,helium has the electron configuration of 1s 2. It turns out that the energy required to accommodatetwo electrons in the 1s orbital is significantly less than theenergy required to place the second electron into the higherenergy n = 2 level. So the secondelectron could go into the 1s orbital with the opposite spin ofthe first electron or it could go into the next orbital in the n= 2 level. The next atom is helium with 2 electrons. The orbital diagram, the electron configuration andthe energy diagram. So we have three ways to represent the electron arrangementin an atom. A portionof the energy level diagram is shown, We can alsodisplay the energy level diagram for the hydrogen atom. Theelectronic configuration for hydrogen can be written as 1s 1.This is a short-hand notation which identifies the level, thesublevel and the number of electrons in the sublevel. This notation uses a box to represent the orbital, the labelfor the orbital and an arrow to represent the electron. The orbital diagram for hydrogencan be represented in the following way. Writing the electronconfiguration requires that we recall how many orbitals arecontained in each type of sublevel For example hydrogen with one electron has an electronconfiguration of 1s 1. To designatethe electron configuration we use the level number and the letterof the sublevel and a superscript number to represent the numberof electrons contained in the sublevel. And we use the Pauli exclusionprinciple and other rules to arrange the electrons. Arrangements of electrons in the orbitals of an atom iscalled its electron configuration. Arrangements of electrons in the orbitals of an atom is called its electron configuration.
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