You probably think of water as a substance that can put out fires. But some elements are so reactive that they burn in water. In fact, they virtually explode in water. An example is shownin the photo below. About 3 pounds of sodium were added to water, and the result was an explosive reaction. Why is sodium such a reactive element? In this section, you will find out.

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Figure $$\PageIndex{1}$$: Sodium reacting with water.

## The First Group

Sodium $$\left( \ce{Na} \right)$$ is an element in group 1 of the periodic table of the elements. This group (column) of the table is shown below. It includes the nonmetal hydrogen $$\left( \ce{H} \right)$$ and six metals called alkali metals. Elements in the same group of the periodic table have the same number of valence electrons. These are the electrons in the outer energy level of the atoms that can be involved in smashville247.netical reactions. Valence electrons determine many of the properties of an element, so elements in the same group have similar properties. All the elements in group 1 have just one valence electron. This makes them very reactive.

Figure $$\PageIndex{2}$$: Group 1 of the periodic table.

## Reactivity of Group 1 Elements

Hydrogen is a very reactive gas, and the alkali metals are even more reactive. In fact, they are the most reactive metals and, along with the elements in group 17, are the most reactive of all elements. The reactivity of alkali metals increases from the top to the bottom of the group, so lithium $$\left( \ce{Li} \right)$$ is the least reactive alkali metal and francium $$\left( \ce{Fr} \right)$$ is the most reactive. Because alkali metals are so reactive, they are found in nature only in combination with other elements. They often combine with group 17 elements, which are very "eager" to gain an electron.

## Other Properties of Alkali Metals

Besides being very reactive, alkali metals share a number of other properties:

Alkali metals are all solids at room temperature. Alkali metals are low in density, and some of them float on water. Alkali metals are relatively soft. Some are even soft enough to cut with a knife, like the sodium pictured in the figure below.

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api/deki/files/78133/CK12_Screenshot_6-9-4.png?revision=1&size=bestfit&width=406&height=238" />Figure $$\PageIndex{4}$$: Hydrogen has the smallest, lightest atoms of all elements. Pure hydrogen is a colorless, odorless, tasteless gas that is nontoxic but highly flammable. Hydrogen gas exists mainly as diatomic ("two-atom") molecules $$\left( \ce{H_2} \right)$$, as shown above. Hydrogen is the most abundant element in the universe and the third most abundant element on Earth, and occurs mainly in compounds such as water.