Filled outermost shell

Why doesn t the sodium atom gain seven electrons so that its third shell becomes the filled outermost shell ... 

Why do elements in the same group in the periodic table have similar chemical behavior Why do metals and nonmetals have different properties G. N. Lewis was seeking answers to these questions during his development of the concept of valence electrons. He assumed that each noble gas atom had a completely filled outermost shell, which he regarded as a stable configuration because of the lack of reactivity of noble gases. He also assumed that the reactivity of other elements was influenced by their numbers of valence electrons. 

Since there are only about 100 electrons total in even the biggest atoms, it can easily be seen that the shells numbered 5 or higher never get filled with electrons. Another important limitation is that the outermost shell, called the valence shell, can never have more than eight electrons in it. The number of electrons in the valence shell is a periodic property. 

The first shell of any atom holds a maximum of two electrons, and the second shell holds a maximum of eight. Thus, the first two electrons of potassium fill the first shell, and the next eight fill the second shell. The outermost shell of any atom can hold at most eight electrons. In potassium, there are nine electrons left, which would fit into the third shell if it were not the outermost shell. However, if we put the nine electrons into the third shell, it would be the outermost shell. Therefore, we put 8 of the remaining electrons in that shell. That leaves the one electron left in the fourth shell. 

Ans. A state of great stability is a state in which the outermost s and p subshells are filled and no other subshell of the outermost shell has any electrons. 

In forming ions, the transition metals lose their valence (outermost) shell electrons first, followed by their outer d electrons. Note In order for transition metal ions to be colored, the d orbitals must be partially filled. In this case, the solution containing the Ni2+ ion would be colored (green). 

In recent years the chemistry of preparing dendrimers has become very successful although painfully cumbersome and time consuming [39-47]. This is not the only drawback. Because of space filling it has not been possible to prepare more than five generations. Either the reaction to a higher generation stops completely or, what happens in practice, the outermost shells will develop imperfections. As in the case of comb molecules, corrections have to be made to the properties of this idealized structure. 

Particularly stable electron arrangements arise when the outermost shell is fully occupied with eight electrons (the octet rule ). This applies, for example, to the noble gases, as well as to ions such as Cl (3s 3p ) and Na"" (2s 2p ). It is only in the cases of hydrogen and helium that two electrons are already suf dent to fill the outermost Is orbital. 

As one shell fills up with electrons, the Pauli principle rules that any further electrons have to move to shells more removed from the nucleus. (The electrons in an incomplete outermost shell are known as valence electrons, and those in filled inner shells are known as core electrons.) The location of the various electrons can be described by talking of an electron cloud the density of this cloud at any point is a measure of the probability of finding the electron at that point. 

Occupied shells in the group 18 elements helium through krypton. Each of these elements has a filled outermost occupied shell, and the number of electrons in each outermost occupied shell corresponds to the number of elements in the period to which a particular group 18 element belongs. 

As was shown in Figure 5-25, there are seven shells available to the electrons in any atom, and the electrons fill these shells in order, from innermost to outermost. Furthermore, the maximum number of electrons allowed in the first shell is 2, and for the second and third shells it is 8. The fourth and fifth shells can each hold 18 electrons, and the sixth and seventh shells can each hold 32 electrons. These numbers match the number of elements in each period (horizontal row) of the periodic table. Figure 6.1 shows how this model applies to the first four elements of group 18. 

The effect of the positive nuclear charge (represented by red shading) of a fluorine atom extends beyond the atom s outermost occupied shell. This positive charge can cause the fluorine atom to become attracted to the unpaired valence electron of a neighboring fluorine atom. Then the two atoms are held together in a fluorine molecule by the attraction they both have for the two shared electrons. Each fluorine atom achieves a filled valence shell. 

Neon s outermost shell is already filled to capacity with electrons. Any additional electrons would have to occupy the next shell out, which has an effective nuclear charge of zero. 

Atoms with their outermost shells filled, such as neon and all the other elements in the far right column of the periodic table, are very stable and can last for long periods of time without changing. Elements like lithium that have a mostly empty outermost shell 2 (only one electron) are likely to give that electron away to another element in a chemical reaction. This reaction leaves lithium more stable, with a completely filled shell 1 as its outermost shell. 

Three places after xenon there follows the remarkable group of the elements of the Rare Earths, because here, beginning with cerium, the Nf or 4f shell (/ — 3, m = —3, —2, —1, o, 1, 2, 3) is filled up. There is thus produced a group of 14 elements from cerium (58) to lutecium (71), which all possess the same electron configuration of the outermost shell as lanthanum and thus also show a great similarity in chemical properties [group of the lanthanides or lanthanons]. 

The rows of the periodic table are called the periods (hence periodic table). The position that an element occupies in a period is determined by the number of electrons in its outermost shell. Each time the atomic number increases by one, the number of protons in the nucleus increases by one, and the new element requires one more electron for a neutral atom. The electrons arrange themselves in shells around the nucleus. Each time a shell is filled, a row is finished. A filled shell is represented on the periodic table as a filled row, as shown in figure 1.2.4. 

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