Lesson Explainer: Alkali Metals Chemistry • Second Year of Secondary School

In this explainer, we will learn how to describe the compounds and reactivities of alkali metals and trends in their physical and chemical properties.

The alkali metals are some of the most unusual and interesting elements because some of them have seemingly contradictory properties. Some alkali metals are solid metals that are so soft and light that they can be cut with scissors and even float on water. Other alkali metals have such incredibly low melting points that they can melt into a liquid on a hot summer day.

The alkali metals are sometimes called the group one metal elements because they make up the leftmost column of the periodic table. The alkali metals include the elements lithium, sodium, potassium, rubidium, cesium, and francium. Hydrogen is also found in the leftmost column of the periodic table, but it is not classed as an alkali metal or group one metal element because it forms a nonmetal gas under standard conditions.

Lithium is the lightest alkali metal element and it has such a low density that it can float on water. The density of lithium is just 0.53 g/cm³ and the density of tap water is approximately 1.00 g/cm³ at room temperature and atmospheric pressure. Lithium atoms are incredibly small and only contain three electrons. Francium is one of the most reactive pure metal elements that is known to exist and it also has the largest atoms of any element in the periodic table. Francium is the most unstable naturally occurring element and it is thought that there is less than thirty grams of francium on Earth at any given moment. The alkali metals have many interesting physical properties and most of these properties can be understood if we recall and compare their different electronic configurations.

The alkali metals all have one valence electron, but they have different inner shell electron configurations. Lithium has just one inner shell of electrons; sodium has two, while cesium and francium have as many as five and six inner shells of electrons. It is more challenging to remove an electron from an alkali metal atom if it is small and has fewer inner shells of electrons. The number of inner shells, and hence electrons, influences the reactivity of the metal; for example, cesium metal atoms are relatively large and react explosively with water. Lithium and sodium atoms are much smaller than cesium and react much less intensely. The following figures show the electron configuration of a lithium atom and cesium atom.

Density values generally increase as we move down the column of group one metals in the periodic table because the mass number systematically increases from lithium all the way through to francium. Density is essentially determined as the ratio of the mass number to the average atomic diameter. Density values are high when the ratio of the mass number to atomic diameter is high. Density values are low when the ratio of the mass number to atomic diameter is low. The mass number systematically increases as we move down the first column of the periodic table, but the atomic diameters (atomic radii) increase in a less predictable way. This means that the density values generally increase as we move down the first column of the periodic table, but there are exceptions and potassium is in fact less dense than sodium.

The density values of the alkali metals seem to be very low when they are compared with the density values for the transition metals and the post-transition metals. Lithium has a density of 0.53 g/cm³ and sodium and potassium have density values of 0.97 g/cm³ and 0.89 g/cm³ respectively. Transition metals like gold and silver have density values of 19.32 g/cm³ and 10.49 g/cm³, respectively, and post-transition metals such as tin and bismuth have density values of 7.29 g/cm³ and 9.79 g/cm³ respectively.

Metallic bonding can be described as the electrostatic attraction between positively charged cations and delocalized electrons. The electrostatic interaction forces are stronger when the positively charged cations and negatively charged electrons are packed into a small volume and there is less average distance between them. The oppositely charged particles can be packed closer together when the metal atoms and ions are small and do not take up too much space. It takes more energy to break apart a metallic lattice that is made up of smaller particles because the particles can pack closer together. Melting points and hardness values generally increase as we move up the column of alkali metals in the periodic table because the group one metal atoms become smaller and their metallic bonding becomes stronger.

Some of the alkali metal elements are abundant, and others are not. Sodium and potassium are the sixth and seventh most common elements in Earth’s crust, and they both form common salt compounds and minerals. Rock salt is primarily composed of the sodium chloride compound that has the chemical formula . The carnallite mineral contains the potassium element and has the formula . The other alkali metal elements are relatively rare. Francium is particularly rare, because it is highly radioactive and has a short half-life of minutes. Scientists first discovered francium in 1939 as a disintegration product of actinium. The following equation describes the radioactive process that led to the discovery of francium:

The alkali metal elements are reducing agents, because they have a single valence electron. They readily reduce many nonmetal substances as they lose their single outer shell electron during a chemical reaction. Alkali metals usually have a oxidation number when they form compounds with nonmetals. The alkali metal atoms gain this oxidation number as they lose their valence electron and reduce a nonmetal atom. Chemists describe alkali metals as the most electropositive chemical elements because they readily lose an electron and form positive ions in chemical reactions.

Alkali metals tend to have a larger atomic diameter and volume than other elements in the same period of the periodic table. Their relatively large diameters mean that they have low first ionization energies and low electronegativity numbers. They do, however, have a second ionization energy that is much higher than their first.

Manufacturers use the potassium and cesium elements to make photoelectric cells, because potassium and cesium atoms have a large volume and low first ionization energy. The two chemical elements readily emit electrons when bombarded with relatively low-energy light through the photoelectric effect phenomenon.

The alkali metals are capable of making relatively stable ionic salt crystals when they react with one of the halogen elements. The following balanced chemical equation shows how sodium metal can be reacted with chlorine gas to make sodium chloride salt crystals.

The balanced equation shows how potassium metal can be reacted with bromine gas to make potassium bromide salt crystals.

The reactions between halogen gases and alkali metals can be summed up with the equation that uses the symbol to represent alkali metals and the symbol to represent halogen atoms.

There are other chemical reactions that can be used to make stable ionic salt crystals from alkali metals. One example is the reaction of alkali metals with aqueous solutions of acids. The chemical equation shows how hydrochloric acid can be reacted with sodium metal to make the sodium chloride compound.

The balanced chemical equation shows how hydrochloric acid can be reacted with lithium metal to make the ionic lithium chloride compound.

Many of the reactions between alkali metals and acids can be summed up with a single generic equation that uses the symbol to represent alkali metals and the symbol to represent hydrogen ions:

The alkali metals can also react with diatomic oxygen molecules to make an entirely different set of ionic compounds. The balanced chemical equation shows how solid lithium metal can be reacted with gaseous oxygen molecules to make a solid lithium oxide compound.

Sodium can also be reacted with oxygen gas to produce sodium oxide ():

Sodium metal can also be reacted with oxygen molecules to form a different sodium compound, sodium peroxide ():

Potassium metal can be reacted with oxygen gas in a similar way to make the potassium peroxide () material:

Potassium and the other heavier alkali metals can also be reacted with oxygen molecules to make unusual superoxide molecules. The balanced equation shows how potassium can be reacted with oxygen gas to make the potassium superoxide () solid.

Potassium superoxide reacts with carbon dioxide according to the following reaction equation:

We can see that the reaction rate is higher in the presence of a copper(II) chloride catalyst. Manufacturers use the potassium superoxide compound to replenish oxygen in closed spaces, such as in airplanes or submarines. The passengers exhale carbon dioxide, and this exhaled carbon dioxide gas reacts with the potassium superoxide substance and makes more oxygen molecules. The reaction ordinarily happens in onboard filters that contain the potassium superoxide compound and some copper(II) chloride catalyst material.

All of the alkali metals can react with water but some react much more vigorously than the others. The reactions almost always produce hydrogen gas and a soluble metal hydroxide compound. The reactions can be summed up with the equation that uses the variable to represent the alkali metals.

The equation shows how solid sodium metal can be reacted with liquid water to make hydrogen gas and aqueous sodium hydroxide molecules.

It was already stated that the alkali metals generally become more reactive as you move down the periodic table and this general rule is true for the reaction of alkali metals with both oxygen gas and liquid water. Lithium, at the top of the group, reacts quite vigorously with water. Sodium and potassium are beneath lithium in the periodic table and they react even more vigorously when they are mixed with liquid water. Rubidium is beneath potassium in the periodic table and it is generally not allowed to be reacted with water in classroom settings because the reaction is so violent that it might cause injury.

The outermost electrons of the larger alkali metal atoms have weaker electrostatic interactions with positively charged protons, and it takes less energy to displace them and induce a chemical reaction. We can use this line of reasoning to predict that cesium metals must react even more violently with water than rubidium. The balanced chemical equation shows how cesium metal can be reacted with water to produce cesium hydroxide and hydrogen gas.

Chemists usually store alkali metals in an airtight container under mineral oil to prevent them from reacting with airborne moisture or oxygen molecules. Alkali metals tend to quickly tarnish if they are not in an airtight container under mineral oil or of another relatively unreactive hydrocarbon.

The alkali metals can also be heated with hydrogen gas to make alkali metal hydrides such as the lithium hydride () or sodium hydride () compounds. The balanced equation shows how sodium metal can be reacted with hydrogen gas to produce the sodium hydride compound.

The reactions between the alkali metals and hydrogen gas molecules can be summed up with the general equation that uses the symbol to represent alkali metals and the symbol to represent metal hydride products.

Some alkali metals react with sulfur at high temperatures and make a metal sulfide compound. The following balanced equation shows how sodium makes the sodium sulfide compound during a reaction with sulfur at a high temperature:

The following equation describes how chemists can similarly combine lithium with sulfur at a high temperature to make the lithium sulfide compound:

Some alkali metals react with phosphorus at high temperatures and make a metal phosphide compound. The following equation describes the reaction of sodium and phosphorus at a high temperature:

The sodium and phosphorus combine during the reaction process and make a sodium phosphide compound. The following equation shows how the high-temperature reaction of potassium and phosphorus can make the potassium phosphide compound:

We have seen how all of the alkali metals react with substances like water and hydrogen. It is important to realize however that some of the alkali metals undergo chemical reactions that set them apart from all the other group one metals. Lithium, for example, can be reacted with nitrogen gas to make lithium nitride () and this lithium nitride product can then be reacted with liquid water to make ammonia. The balanced equation shows how lithium metal can be reacted with nitrogen gas to produce lithium nitride.

The balanced equation shows how the same lithium nitride product can then be reacted with liquid water to produce a valuable ammonia product.

Lithium carbonate is a rather unusual alkali metal compound because it decomposes at about and produces lithium oxide and carbon dioxide gas products. The following chemical equation describes this straightforward thermal decomposition process:

The other group one carbonate compounds do not decompose through such simple decomposition processes. They might, for example, melt into a liquid before they can thermally decompose into some combination of solid and gaseous product molecules.

Alkali metal nitrates partially decompose and produce metal nitrite and oxygen products. The following equation describes the thermal decomposition of sodium nitrate:

The thermal decomposition of potassium nitrate is unusual because the reaction is explosive.

Let’s finish by recapping some of the important points from this explainer.