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2.51: Hydrogen and Alkali Metals

Difficulty Level: At Grade Created by: CK-12
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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 instructor have incurred serious injury due to their failure to observe proper safety precautions.

One of the values 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 the 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.

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 these elements have a similar configuration of outer-shell electrons.  In each case, there is one electron in the outer orbital and that is an \begin{align*}s\end{align*}-orbital electron.  In the case of hydrogen (one proton in the nucleus and one electron), the arrangement is very simple.  The lone electron exists in a \begin{align*}s\end{align*}-orbital around the nucleus.  For lithium, there are two \begin{align*}1s\end{align*} electrons in an inner orbit and one \begin{align*}2s\end{align*} electron in the outer orbit.  The same pattern holds for sodium and potassium.



Electron Configuration






\begin{align*}[He] 2s^1\end{align*}



\begin{align*}[Ne] 3s^1\end{align*}



\begin{align*}[Ar] 4s^1\end{align*}



\begin{align*}[Kr] 5s^1\end{align*}



\begin{align*}[Xe] 6s^1\end{align*}



\begin{align*}[Rn] 7s^1\end{align*}  

Even an atom with a very complex electron composition such as cesium still has the single \begin{align*}s\end{align*} electron in its outer orbital.

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, with the exception of cesium which is a liquid at room temperature. The alkali metals also react readily with water to produce hydrogen gas and metal hydroxides in the following video.


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.


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


Use the link below to answer the following questions:


  1. How is hydrogen similar to the alkali metals? How is it different?
  2. Why don’t we know much about francium (atomic number 87)?
  3. Describe the physical properties of the alkali metals.


  1. What group are the alkali metals and hydrogen in?
  2. What is the outer shell electron configuration in this group?
  3. Which alkali metal is a liquid at room temperature?
  4. How reactive are the alkali metals with oxygen?
  5. How reactive are these metals with water?

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alkali metal Metal in group 1 of the periodic table that has one valence electron and is highly reactive.

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Difficulty Level:
At Grade
7 , 8
Date Created:
Oct 31, 2012
Last Modified:
Oct 22, 2016
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