Chemical Reactivity

Hydrogen

Unlike the rest of the Group 1A elements, which exist as metals, elemental hydrogen exists as gaseous H₂ molecules. Compounds formed between hydrogen and non-metals are molecular rather than ionic. (i.e., hydrogen forms covalent bonds with non-metals). For example, hydrogen reacts with halogens (Group VIIA) according to:

where X can be any halogen, such as F, Cl, Br, or I. Hydrogen in these compounds has an oxidation state of +1 while the halogens are -1. Similarly, hydrogen reacts to other elemental non-metals in a predictable fashion:

Hydrogen can also form compunds with more active metals to form ionic hydrides. For example, lithium hydride is formed according to:

The metal (Li in this case) loses an electron to become a cation and H gains an electron to become H^- (hydride anion), which has an charge of -1. Here's another example:

By gaining an electron, the hydride ion obtains the stable electron configuration of a closed n=1 shell, that is, the noble gas configuration of He.

Oxygen

Oxygen is a group 6A element. Elemental Oxygen is found in two forms: oxygen gas (O₂) and and ozone gas (O₃). Different forms of an element in the same state are called Allotropes.

Reactions between Oxygen and Metals

When oxygen reacts with most metals a metal oxide is formed where oxygen has an oxidation state of -2. For example, zinc oxide is formed when zinc metal reacts with oxygen gas:

and aluminum oxide is formed when aluminum metal reacts with oxygen gas:

There are, however, some exceptions, which we consider next.

Group IA Metals - Alkali Metals

Because alkali metals are so active, the product of their reaction with oxygen gas is not what you might expect. While lithium metal reacts with oxygen gas to form lithium oxide, as one might expect:

when sodium metal reacts with oxygen gas under the same conditions it forms sodium peroxide: and the very active alkali metals, potassium, rubidium, and cesium, reacts with oxygen gas to form superoxides:

Group IIA Metals - Alkaline Earth Metals

Oxygen reacts with most alkaline earth metals to form a metal oxide:

For example,

However, oxygen combines with barium metal, the most active of this group, to form a peroxide:

Reactions between Oxygen and Non-metals (except Group 7A and 8A)

When oxygen combines with non-metals in their elemental form, the product is a non-metal oxide. For example, oxygen reacts with solid carbon to form carbon monoxide or carbon dioxide, respectively, as shown below (reaction not balanced):

Similarly, oxygen reacts with solid phosphorus to form tetraphorphorus heptoxide or tetraphorphorus decoxide, respectively, as shown below (reaction not balanced):

When reacted with solid sulfur, oxygen forms sulfur dioxide gas:

Oxide Reactions

Non-Metal Oxide reactions

The oxides of non-metals are acidic. If a non-metal oxide dissolves in water, it will form an acid.

For example,

The non-metal oxides can be neutralized with a base to form a salt and water.

For example,

Metal Oxide reactions

The oxides of metals are basic. If a metal oxide dissolves in water, it will form a metal hydroxide.

For example,

Like any base, these bases can be neutralized by an acid to form a salt and water.

Examples:

Generally, the more metallic character an element has, the more basic its oxide will be. Likewise, the more non-metalic character an element has, the more acidic its oxide will be. The metalic character of an element can be determined by its position on the periodic table:

Finally, we note that a salt can also be formed by the direct reaction of a metal and a non-metal.

For example,