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.
Definition: Electronic Configuration
The electronic configuration of an atom describes how many electrons it has and how all of these electrons are arranged in different electron shells and subshells.
Example 1: Identifying Which Element Is an Alkali Metal
Which of the following is an alkali metal?
The alkali metals are sometimes called group one metals because they make up the leftmost column of the periodic table. Cesium can be found close to the bottom of the leftmost periodic table column and this demonstrates that cesium is an alkali metal. Radium and calcium are both group two elements and definitely not alkali metals. Lanthanum and cerium are both lanthanide elements and they are also definitely not alkali metals. We can use these statements to determine that option B is the correct answer for this question.
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.
Example 2: Determining Which Alkali Metal Is the Most Reactive
Which of the following alkali metals is the most reactive?
The alkali metals react by losing their single outer shell electron. It is easier to take an outer shell electron from an alkali metal if there are weaker electrostatic forces between the positively charged protons in the atomic nucleus and the negatively charged electrons in the outer valence shell. The electrostatic interactions are weakest when the alkali metal atoms have lots of inner valence shell electrons and there is more distance between the valence electrons and the positively charged protons in the nucleus.
Cesium is further down the periodic table than any of the other listed options. Cesium must be the most reactive metal listed because it has the most inner shells of electrons and its valence electrons experience the weakest electrostatic interaction forces from positively charged protons in the atomic nucleus. It will be relatively easy to take a valence electron from cesium and more challenging to take a valence electron from any of the other listed alkali metals. This line of reasoning can be used to determine that option A is the correct answer for this question.
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.
Density is a measure of mass per unit of volume.
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.
Example 3: Identifying Which Alkali Metal Has the Greatest Density
Which of the following alkali metals has the greatest density?
The alkali metals generally have higher densities when they have higher mass numbers. Mass numbers increase as we move from the top to the bottom of the alkali metal column in the periodic table. Cesium has the highest mass number of the listed alkali metals and it also happens to have the highest density. We can use these statements to determine that option A is the correct answer for this question.
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.
Definition: Metallic Bonding
Metallic bonding is the strong electrostatic attraction that exists between positively charged metal cations and delocalized electrons.
Example 4: Identifying Which Alkali Metal Has the Lowest Melting Point
Which of the following alkali metals has the lowest melting point?
The melting point of the group one metals depends on the strength of the metallic bonding between their positively charged metal cations and their negatively charged delocalized electrons. The electrostatic interactions tend to be stronger when the metal cations are small because the oppositely charged particles can pack very close together. There is less distance between the oppositely charged particles when the metal cations are small, and it takes more energy to break them apart because the electrostatic attraction forces are larger. Cesium metals have larger atoms than all of the other listed options, and we can use this information to infer that cesium metals must have the weakest metallic bonding and the lowest melting point. We can use this line of reasoning to determine that option A is the correct answer for this question.
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.
Example 5: Identifying the Chemical Equation That Describes the Reaction between Potassium and Chlorine
Which of the following equations properly describes the reaction between potassium and chlorine?
The formation of potassium chloride () from potassium metal and chlorine gas can be shown in terms of a simple chemical equation. The chlorine gas term would have to be assigned the appropriate gaseous state symbol (g) and the solid potassium metal and solid potassium chloride would have to be assigned the appropriate solid state symbol (s). The product term has to have a stoichiometric coefficient of two because chlorine gas is made up of diatomic molecules and the potassium reactant term must also have a stoichiometric coefficient of two to balance the overall chemical equation. These statements can be compared with the listed options to determine that A is the correct answer for this question.
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.
Example 6: Balancing Chemical Equations for the Reaction between Sodium Metal and Oxygen
Which of the following is the correct balanced equation for the reaction of sodium with oxygen?
Sodium metal can produce sodium oxide () and sodium peroxide () molecules when it reacts with oxygen gas. Sodium peroxide is shown as a chemical product in answer A, but it is shown with the incorrect stoichiometric coefficient. We can discount option A and focus on the other chemical equations that have sodium and oxygen gas reacting together to make sodium oxide products. We will also need to ensure that the chemical equations have the correct state symbols and the correct stoichiometric coefficients.
The oxygen molecules should be assigned the gaseous state symbol (g) and the sodium oxide and pure sodium metal should both be assigned the solid state symbol (s). The sodium oxide product term has to have a stoichiometric coefficient of two because oxygen gas is made up of diatomic molecules, and the sodium metal has to have a stoichiometric coefficient of four to balance the overall chemical equation. These statements can be compared with the list of possible answers to determine that the correct answer is D.
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.
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.
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.
- The alkali metals are sometimes called the group one metals because they make up the leftmost column of the periodic table of elements.
- The six alkali metals are lithium, sodium, potassium, rubidium, cesium, and francium.
- The alkali metals that have higher mass numbers are generally softer and denser and they also have lower melting points.
- The reactivity of the alkali metals increases with mass number.
- The alkali metals () can react with halogen elements () according to the equation
- The alkali metals () can react with hydrogen gas according to the equation
- The alkali metals () can react with water molecules according to the equation
- Many of the reactions between alkali metals () and acid solutions can be summarized using the generic equation
- The heavier alkali metals can react with oxygen according to the equation