Electron Configuration of Helium

Which elements acquire the electron configuration of helium by covalent bonding ... 

By sharing two electrons in a covalent bond, each hydrogen effectively has one electron pair and the stable, filled-shell electron configuration of helium. As with ions (Section 6.12), a filled valence shell for each atom in a molecule leads to maximum stability. 

There is one more complication to the electron shells. Inside the shells themselves, electrons can be found in regions called orbitals. There are four types of orbitals—s, p, d, and/—and each has a specific shape. Blocks of the periodic table correspond to the different orbitals. The electrons in atoms of the first row of the table are found in the Is orbital. Helium, at the far right of the first row, consists of 2 electrons in the Is orbital. Neon, at the far right of the second row, has two electrons in the Is orbital, 2 electrons in the 2s orbital, and 6 electrons in the 2p orbital. These arrangements of electrons within orbitals are known as electron configurations. Chemists notate the electron configuration of helium as Is2 and neon as ls22s22p6. 

The simplest covalent bond is between the two single electrons of hydrogen atoms. Covalent bonds may be represented by an electron pair (a pair of dots) or a line as shown below. The shared pair of electrons provides each H atom with two electrons in its valence shell (the 1s orbital), so both have the stable electron configuration of helium. 

Because the maximum number of electrons in the first shell of an atom is 2, helium is stable with 2 electrons in its only occupied shell. The other very light elements—hydrogen, lithium, and beryllium—tend to form stable states by achieving the 2-electron configuration of helium. Having 2 electrons in the first shell, when that is the only shell and therefore the outermost shell, is a stable state, and the 2 electrons are sometimes called a duet. When there is only one shell, 2 electrons in that shell act like 8 electrons in any other outermost shell. Therefore, an atom with 2 electrons in its outermost first shell is often said to obey the octet rule, although duet rule would be more precise. 

Electron Configurations of Helium, Neon, Argon, and Krypton ... 

The two electrons in helium are both contained within the first principal quantum shell. Write down the electronic configuration of helium. 

Now, let s compare that electron configuration of hydrogen to the electron configuration of helium, shown in Figure 3-4b. 

Hydride has the electronic configuration of helium, but still is relatively reactive. Salts like sodium hydride are very basic because hydrogen, the conjugate acid, has a p/fa of 36. With that high a p TabHi hydride makes a very poor leaving group. However, a carbocation can abstract a hydride to form an equal or more stable carbocation. 

The ground state electronic configuration of helium, 2He, completely fills the 1 s-orbital with two electrons. The spins of the electrons are paired. The configuration for helium would be read as one-s-two (not one-s-squared). 

S2 and K+ both have the electronic configuration of argon, [Ar] Li" and Be2" both have the electronic configuration of helium, [He], (page 247)... 

The formation of these molecules illustrates the octet rule, formulated by Lewis An atom other than hydrogen tends to form bonds until it is surrounded by eight valence electrons. In other words, a covalent bond forms when there are not enongh electrons for each individnal atom to have a complete octet. By sharing electrons in a covalent bond, the individnal atoms can complete their octets. The requirement for hydrogen is that it attain the electron configuration of helium, or a total of two electrons. 

If a hydrogen atom were to gain a second electron, it would be isoelectronic with the stable electron configuration of helium. However, because two identical hydrogen atoms have an equal tendency to gain or lose electrons, an electron transfer from one atom to the other is unlikely to occur imder normal conditions. Each atom may attain a noble gas structure only by sharing its electron with the other, as shown with Lewis symbols ... 

Figure5.il Calculation of the energies of the S/ S states for the possible electronic configurations of helium I s 2s. The two-electron terms are as given in equation 5.67, with J and K components as given in cells J 2 and J 4.

The electronic structures of the atoms of these elements are listed in Table 4.1. With the exception of helium, the atoms have eight (or ns np , where n is a whole number greater than one) electrons in the outer shells of their atoms. The arrangement is called a stable octet of electrons. There seems to be a stability associated with this arrangement, and with the unique electron configuration of helium, which makes the gases unreactive. 

In forming the covalent bond, each hydrogen atom acquires a second electron, achieving the stable, two-electron, noble-gas electron configuration of helium. 

Put one pair of electrons between each bonded pair of atoms in the initial structure drawn in Step 2. Subtract the number of electrons used in this step from the total number determined in Step 3. Use the remaining electrons to complete the octets of all atoms in the structure, beginning with the atoms that are present in greatest number in the molecule. Remember, hydrogen atoms require only one pair to achieve the electronic configuration of helium. 

The monatomic hydride and lithium ions, H and Li", duplicate the electron configuration of helium with just two electrons 15. Unlike other elements in Groups 2A/2 and 3A/13, beryllium and boron tend to form covalent bonds by sharing electrons, rather than forming ions. We will look at covalent bonds later in this chapter. Figure 12.1 is a periodic table that summarizes the elements that form monatomic ions that are isoelectronic with noble-gas atoms. 

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