Helium valence electrons

In the sodium atom pairs of 3/2 states result from the promotion of the 3s valence electron to any np orbital with n > 2. It is convenient to label the states with this value of n, as n P 1/2 and n f 3/2, the n label being helpful for states that arise when only one electron is promoted and the unpromoted electrons are either in filled orbitals or in an x orbital. The n label can be used, therefore, for hydrogen, the alkali metals, helium and the alkaline earths. In other atoms it is usual to precede the state symbols by the configuration of the electrons in unfilled orbitals, as in the 2p3p state of carbon. 

The number of covalent bonds an atom forms depends on how many additional valence electrons it needs to reach a noble-gas configuration. Hydrogen has one valence electron (Is) and needs one more to reach the helium configuration (Is2), so it forms one bond. Carbon has four valence electrons (2s2 2p2) and needs four more to reach the neon configuration (2s2 2p6), so it forms four bonds. Nitrogen has five valence electrons (2s2 2p3), needs three more, and forms three bonds oxygen has six valence electrons (2s2 2p4), needs two more, and forms two bonds and the halogens have seven valence electrons, need one more, and form one bond. 

Figure 16-3D shows the simplified representation of the interaction of two helium atoms. This time each helium atom is crosshatched before the two atoms approach. This is to indicate there are already two electrons in the Is orbital. Our rule of orbital occupancy tells us that the Is orbital can contain only two electrons. Consequently, when the second helium atom approaches, its valence orbitals cannot overlap significantly. The helium atom valence electrons fill its valence orbitals, preventing it from approaching a second atom close enough to share electrons. The helium atom forms no chemical bonds. ... 

For example, nitrogen ( N ) has five valence electrons and needs three more electrons to complete its octet. Chlorine (-CL) has seven valence electrons and needs one more electron to complete its octet. Argon OArO already has a complete octet and has no tendency to share any more electrons. Hydrogen (H-) needs one more electron to reach its helium-like duplet. Because hydrogen completes its duplet by sharing one pair of electrons, we say that it has a valence of 1 in all its compounds. In general, the valence of an element is the number of bonds that its atoms can form. 

In an atom of the second column of the periodic system, such as mercury, the two valence electrons are in the normal state s-electroiis, and form a completed sub-group. Two such atoms would hence interact in a way similar to two helium atoms the attractive forces would be at most very small. This is the case for Hg2, which in the normal state has an energy of dissociation of only 0.05 v.e. But if one or both of the atoms is excited strong attractive forces can arise and indeed the excited states of Hg2 are found to have energies of dissociation of about 1 v.e. 

Neon has eight valence electrons and all of them are paired, hence the valence orbitals of neon are completely filled. Therefore neon is very unreactive and does not bond with any other element. Similarly, the group 8A elements (noble gases) helium and argon are very unreactive. However, krypton and xenon may form bonds under certain conditions. 

For the representative elements, the valence electrons are all electrons in the outer s and p orbitals of an atom. A quick way of determining the number of valence electrons is to locate the element on the periodic table. There are eight columns of representative elements. The first column, headed by H and Li, has one valence electron, the second column has two, skip the transition elements, the next column, headed by B and Al, has three. This continues to the last (eighth) column where there are eight valence electrons. The only exception to this procedure is helium, which only has two valence electrons. 

The loss of these r-subshell valence electrons explains the common +1 and +2 charges on ions of these elements, except for helium, which is chemically inert. 

As mentioned previously, in 1916, Lewis noted that noble gases were particularly stable and did not form compounds. Lewis used these facts to formulate the octet rule. The noble gases have their outer electron shell filled with eight electrons. (Helium is an exception with only two electrons in its outer shell.) The octet rule says that the most stable electron configuration of an atom occurs when that atom acquires the valence electron configuration of a noble gas. That is, when an atom can acquire eight (octet) electrons in its valence shell (or two for hydrogen to become like helium). 

The role of subexcitation electrons is most important when the irradiated medium contains small amounts of impurity molecules the excitation energy ha) 0j (or the ionization potential I ) of which is below h(o0l. Such additive molecules can be excited or ionized by the subexcitation electrons the energy of which is between h(o 0j and fuom, and, consequently, the relative fraction of energy absorbed by an additive will be different from what it should be if the distribution of absorbed energy were solely determined by the relative fraction of valence electrons of each component of the mixture.213 214 According to estimates of Ref. 215, this effect is observed when the molar concentration of the additive is of the order of 0.1%. This selective absorption with ionization of additives has been first pointed out by Platzman as an explanation for the increase in the total ionization produced by alpha particles in helium after small amounts of Ar, C02, Kr, or Xe were added (the so-called Jesse effect).216... 

There are two ways that atoms can interact to attain noble-gas configurations. Sometimes atoms attain noble-gas configurations by transferring electrons from one atom to another. For example, lithium has one electron more than the helium configuration, and fluorine has one electron less than the neon configuration. Lithium easily loses its valence electron, and fluorine easily gains one ... 

There are exceptions to the octet rule. Helium, for example, is incredibly stable with just two valence electrons in its outermost principal energy level. The same holds true for lithium and beryllium as well. This indicates that it isn t so much having an octet that stabilizes the atom, as it is the issue of having a full outermost principal energy level. 

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