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6.9: Hydrogen and Alkali Metals

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     Large Sodium Explosion.
    Figure \(\PageIndex{1}\) (Credit: User:Ajhalls/Wikimedia Commons; Source: Commons Wikimedia, Large Sodium Explosion (opens in new window) []; License: Public Domain)

    Can you guess what kind of reaction is taking place in this picture?

    Some chemistry students just enjoy learning about the science, while others are intrigued by the violent reactions that sometimes can occur. Many chemistry classes have been enlivened by the demonstration of how reactive sodium is with water. In some instances, the demonstration has gone off safely. Unfortunately, in other situations students and instructors have incurred serious injury due to their failure to observe proper safety precautions.

    One value of the periodic table is the ability to make predictions about the behavior of individual elements. By knowing which group an element is in, we can determine the number of reactive electrons and say something about how that element will behave.

    Hydrogen and Alkali Metals

    The periodic table is arranged on the basis of atomic numbers (number of protons in the nucleus). One of the valuable consequences of this arrangement is that we can learn a lot about the electron distribution in these atoms. The colors in the table below indicate the different groupings of atoms based on the location and number of electrons in the atom.

    Alkali metals on the periodic table
    Figure \(\PageIndex{2}\) (Credit: User:Cepheus/Wikimedia Commons, modified by CK-12 Foundation; Source: Commons Wikimedia, Periodic Table(opens in new window) []; License: Public Domain)

    If we look at Group I (red column), we see that it is labeled alkali metals. Also note the green H above the alkali metals. All of these elements have a similar configuration of outer-shell electrons (see table below). In each case, there is one electron in the outer orbital and that is an s-orbital electron. Hydrogen is not an alkali metal itself, but has some similar properties due to its simple one proton (located in the nucleus), one electron arrangement. The lone electron exists in a s-orbital around the nucleus. For lithium, there are two 1s electrons in an inner orbit and one 2s electron in the outer orbit. The same pattern holds for sodium and potassium.

    Element, Symbol, Electron Configuration



    Electron Configuration






    \(\left[ \ce{He} \right] 2s^1\)



    \(\left[ \ce{Ne} \right] 3s^1\)



    \(\left[ \ce{Ar} \right] 4s^1\)



    \(\left[ \ce{Kr} \right] 5s^1\)



    \(\left[ \ce{Xe} \right] 6s^1\)



    \(\left[ \ce{Rn} \right] 7s^1\)

    Even an atom with a very complex electron composition such as cesium still has the single \(s\) electron in its outer orbital (see figure below).

    Electron orbitals in cesium
    Figure \(\PageIndex{3}\): Celsium Orbitals. (Credit: Joy Sheng; Source: CK-12 Foundation; License: CC BY-NC-SA 3.0(opens in new window))

    This one electron is very easily removed during chemical reactions. The group I elements react rapidly with oxygen to produce metal oxides. They are very soft metals, which become liquid just above room temperature.

    Li reacts with water to produce hydrogen gas. Sodium also reacts the same way, just more rapidly. Potassium reacts rapidly with water producing hydrogen gas and heat which ignites the hydrogen gas. Rubidium and cesium react yet more vigorously and explode on contact with water.


    • Group I (alkali metals and hydrogen) elements all have one electron in their outer shell. This electron is in a \(s\) orbital.
    • The Group I metals are all very reactive with water.


    1. What group are the alkali metals and hydrogen in?
    2. What is the outer shell electron configuration in this group?
    3. How reactive are the alkali metals with oxygen?
    4. How reactive are these metals with water?

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