6.2. Electron Configuration and the Periodic Table http://www.ck12.org
the second period. The s-block elements and the p-block elements are together called therepresentativeormain-
group elements.
The s-block
The s-block consists of the elements in Group 1 and Group 2, which are primarily composed of highly reactive
metals. The elements in Group 1 (lithium, sodium, potassium, rubidium, cesium, and francium) are called thealkali
metals. All of the alkali metals have a single s electron in their valence energy level. The general form for the
electron configuration of each alkali metal isns^1 , where thenrefers to the highest occupied principal energy level.
For example, the electron configuration of lithium (Li), the alkali metal of Period 2, is 1s^2 2s^1. This single valence
electron is what gives the alkali metals their extreme reactivity.Figure6.5 shows the alkali metal element sodium.
FIGURE 6.5
Like all alkali metals, sodium is very soft.
A fresh surface, which can be exposed by
cutting the sample, exhibits a luster that is
quickly lost as the sodium reacts with air.All alkali metals are very soft and can be cut easily with a knife. Due to their high reactivity, they must be stored
under oil to prevent them from reacting with oxygen or water vapor in the air. The reactions between alkali metals
and water are particularly vigorous and include the rapid production of large quantities of hydrogen gas. Alkali
metals also react easily with most nonmetals. All of the alkali metals are far too reactive to be found in nature
in their pure elemental form. For example, all naturally occurring sodium exists as a compound, such as sodium
chloride (table salt).
The elements in Group 2 (beryllium, magnesium, calcium, strontium, barium, and radium) are called thealkaline
earth metals(seeFigure6.6). These elements have two valence electrons, both of which reside in the outermost s
sublevel. The general electron configuration of all alkaline earth metals isns^2. The alkaline earth metals are still too
reactive to exist in nature as free elements, but they are less reactive than the alkali metals. They tend to be harder,
stronger, and denser than the alkali metals, and they also form numerous compounds with nonmetals.
Hydrogen and Helium
Looking at the block diagram (Figure6.4), you may be wondering why hydrogen and helium were not included
in our discussion of the alkali metal and alkaline earth metal groups. Though hydrogen, with its 1s^1 configuration,
appears as though it should be similar to the rest of Group 1, it does not share the properties of that group. Hydrogen
is a unique element that cannot be reasonably included in any single group of the periodic table. Some periodic
tables even separate hydrogen’s square from the rest of Group 1 to indicate its solitary status.