The molecular world
The molecular world

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The molecular world

3.4 Outer electronic configurations and the Periodic Table

The essential message of Figure 22 is that the Groups of elements that appear in columns of the Periodic Table usually have atoms with similar outer electronic configurations. Figure 23 incorporates these configurations into our mini-Periodic Table of typical elements; they appear at the top of each Group. They imply that the typical elements have outer electronic configurations either of the type nsx, where x = 1 or 2, or of the type ns2npx, where x runs from 1 to 6. For any particular element, n is the principal quantum number of the outer occupied shell. This can easily be found from Figure 23, because it is equal to the number of the Period in which the element is to be found. The outer electrons are simply those in occupied sub-shells with this principal quantum number.

Figure 23
Figure 23 A mini-Periodic Table for the typical elements up to radium. Along the top are the Group numbers in roman numerals, and the outer electronic configurations of the elements of each Group. As in Figure 18, hydrogen has been omitted

Question 16

According to Figure 23, what are the principal quantum numbers of the outer occupied shells for the atoms of silicon and lead?

Answer

Three and six, respectively: silicon appears in Period 3 and lead in Period 6.

According to Figure 23 therefore, silicon and lead have outer electronic configurations of the type ns2np2, with n = 3 for silicon and n = 6 for lead. This is just what you got when you worked out the full electronic configuration of the silicon and lead atoms in Sections 3.2 and 3.3.1, respectively.

Question 17

According to Figure 23, what is the relationship between the Group number for silicon and lead, and the outer electronic configurations of their atoms?

Answer

In both cases, the Group number is four and there are four outer electrons: two s electrons and two p electrons.

Here is confirmation of the explanation of chemical periodicity mentioned at the beginning of Section 3. Elements in the same Group of the Periodic Table behave similarly because they usually have similar outer electronic configurations. It also demonstrates that, for the typical elements, the total number of outer electrons is equal to the Group number. It is to preserve this generalisation that, in this course, we take the Group number of the noble gases to be VIII rather than zero. Apart from helium (1s2), they have eight outer electrons (s2p6).

Finally, notice that Figures 17 and 22 imply that the atoms of highest known atomic number (113-118) at the outer limit of the Periodic Table are expected to be typical elements. This is only one of the reasons that makes them of special interest (see Box 2 The island of stability).

Box 2: The island of stability

The elements of highest atomic number are made through the collision of atoms and ions of lighter elements in particle accelerators. Success does not come easily, because the atoms that are formed are highly radioactive and very short lived. However, theory suggests that somewhere above atomic number 110 there is an 'island of stability', where the atoms will have longer lifetimes. This island is marked by favourable combinations of neutrons and protons, with its summit centred around an atom of atomic number 114 and mass number 298. So far [2007], the elements of highest atomic number for which isotopes have been identified are 114, 116 and 118. These may therefore supply evidence for the existence of the island.

In 1999, scientists at Dubna in Russia made the first atoms of element 114, to which we shall give the provisional name auditorium (Ad)! Ions of the isotope

were accelerated to 30 000 km h−1, and directed on to a target containing the plutonium isotope . Two nuclei fused, three neutrons were ejected, and an atom of the 289 isotope of element 114 was produced:

The half-life of proved to be 30 seconds. It underwent α-decay to the 285 isotope of element 112, whose half-life is 15.4 minutes. These half-lives may seem short, but you must go back to element 103 to find known isotopes which are as long lived.

Figure 24 shows the half-lives of the known isotopes of elements 112-118. and the two other isotopes of element 114 support the emergence of an island of stability, but we are still a long way (9 neutrons) from the predicted summit at 184 neutrons. There is a reason for this. The proportion of neutrons in the most stable isotope of an element increases with atomic number. So the lighter isotopes such as and , from which the new heavy elements are made, lack the neutrons needed to produce the most stable isotope of the heavier element that they create.

This is simultaneously encouraging and discouraging. It means that the expected summit of the island of stability will be hard to reach. But if we do get there, we may find very stable elements. It may even be possible to study their chemistry.

In the most recent piece of research [2007], collaboration between Californian and Russian scientists is believed to have produced the 294 isotope of element 118 (176 neutrons). This has a half-life of about one millisecond and decays to the 290 isotope of element 116 which has a half-life of 10 milliseconds.

Figure 24
Figure 24 As the number of neutrons in known isotopes of each of the elements 112-118 increases above 165, the half-lives increase (even-numbered elements only shown). This indicates the emergence of an island of stability whose summit is predicted to be at 114 protons and 184 neutrons. (1 μs = 10−6 s; 1 min = 1 minute)
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