http://www.ck12.org Chapter 6. The Periodic Table
FIGURE 6.6
The alkaline earth metals include beryl-
lium, magnesium, calcium, strontium, and
barium. Strontium and barium react with
air and must be stored in oil.Helium has a configuration of 1s^2 , which would seem to place it with the alkaline earth metals. However, it is instead
placed in Group 18 at the far right of the periodic table. The elements in this group, called the noble gases, are very
unreactive because their outermost s and p sublevels are completely filled. Since it is part of the first period, helium
does not have a p sublevel. Its filled 1s sublevel makes it very similar to the other members of Group 18.
The p-block
The p-block consists of the elements in groups 13-18. The p sublevel always fills after the s sublevel of a given
principal energy level. Therefore, the general electron configuration for an element in thep-block isns^2 np^1 −^6. For
example, the electron configuration of elements in Group 13 isns^2 np^1 , the configuration of elements in Group 15 is
ns^2 np^3 , and so on. The elements of Group 18 (helium, neon, argon, krypton, xenon, and radon) are called thenoble
gases. They are an especially important group of the periodic table because they are almost completely unreactive,
due to their completely filled outermost s and p sublevels. As noted above, helium might at first seem to be out of
place, because it has a configuration of 1s^2 instead of thens^2 np^6 configuration that is characteristic of the other noble
gases. However, because there are no 1p orbitals, helium also has a completely filled outermost energy level, which
leads to the various chemical properties exhibited by the other noble gases.
Note that the noble gases were not a part of Mendeleev’s periodic table because they had not yet been discovered.
In 1894, English physicist Lord Rayleigh and Scottish chemist Sir William Ramsay detected argon as a small
percentage of the atmosphere. Discovery of the other noble gases soon followed. The group was originally called
theinert gasesbecause they were believed to be completely unreactive and unable form compounds. However,
beginning in the early 1960s, several compounds of xenon were synthesized by treating it with highly reactive
fluorine gas. The name of the group was later changed to noble gases.
The number of valence electrons in elements of the p-block is equal to the group number minus 10. As an example,
sulfur is located in Group 16, so it has 16 –10 = 6 valence electrons. Since sulfur is located in period 3, its outer
electron configuration is 3s^2 3p^4. In the older system of labeling groups, the representative elements are designated
IA through VIIIA. Using this system, the number of valence electrons is equal to the number preceding the letter A.
Using the same example, sulfur is a member of Group VIA, so it has 6 valence electrons.
The elements of Group 17 (fluorine, chlorine, bromine, iodine, and astatine) are called thehalogens. The halogens
all have the general electron configurationns^2 np^5 , giving them seven valence electrons. They are one electron short
of having full outer s and p sublevels, which makes them very reactive. They undergo especially vigorous reactions
with the reactive alkali metals. In their pure elemental forms, chlorine and fluorine are gases at room temperature,