The Group One Alkali Metals belong to the s-block elements of the periodic table, occupying the leftmost side. Alkali metals are highly reactive due to their tendency to readily lose electrons. In this article, we will discuss the electronic configurations, ionization enthalpy, hydration enthalpy, atomic and ionic radii, and other physical and chemical properties of these elements.

Alkali Metals Guide

Overview of Alkali Metals

Electronic Configuration of Alkali Metals

Physical Properties of Alkali Metals

Chemical Properties of Alkali Metals

Reaction of Alkali Metal with Water

Anomalous Behaviour of Lithium

Uses of Alkali Metals

Answer: Alkali metals are a group of chemical elements in the periodic table consisting of lithium (Li), sodium (Na), potassium (K), rubidium (Rb), caesium (Cs), and francium (Fr).

In general, ‘alkali’ refers to the basic or alkaline nature of metal hydroxides. These compounds are known as alkali metals, as when they interact with water they typically form alkalies, which are strong bases that can effectively neutralize acids.

Alkali metals have a corresponding Noble gas ns1 electronic configuration [1]. They occupy the first column of the periodic table and are represented by the elements Lithium(Li), Sodium(Na), Potassium (K), Rubidium (Ru), Cesium (Cs) and Francium (Fr) which occupy successive periods from first to seven. Francium is a radioactive element with a very low half-life.

However, the main reason why hydrogen (H) is not classified as an alkali metal is because it is usually found in a gaseous state under normal temperature and pressure conditions. Hydrogen can exhibit properties of an alkali metal or even transform into an alkali metal when exposed to extremely high pressure.

Also Read:

Alkaline Earth Metals

S-Block Elements

Alkali Metals are highly reactive and exist only in the form of compounds. They are electropositive metals with a single valence electron.

Overview of Alkali Metals

| Metals | Lithium | Sodium | Potassium | Rubidium | Cesium |

| Atomic Number | 3 | 11 | 19 | 37 | 55 |

| Configuration | He$2s^1$ | Ne$3s^1$ | Ar$4s^1$ | Kr$5s^1$ | Xe$6s^1$ |

| Abundance (ppm) | 65 | 28300 | 25900 | 310 | 7 |

| Atomic Size (pm) | 152 | 186 | 227 | 248 | 265 |

| Density (g/cm³) | 0.53 | 0.97 | 0.86 | 1.53 | 1.9 |

| Ionization Energy (kJ/mol) | 520 | 496 | 419 | 403 | 376 |

| Hydration Enthalpy (kJ/mol) | -506 | -406 | -330 | -310 | -276 |

| Reduction Potential (V) | -3.04 | -2.714 | -2.925 | -2.930 | -2.927 |

| Flame Color | Crimson Red | Yellow | Violet | Red Violet | Blue |

Electronic Configuration of Alkali Metals

Alkali metals have only one electron in their valence shell.

The electronic configuration of Lithium is given by 1s¹ 2s¹.

They tend to lose the outer shell electron, resulting in the formation of cations with a charge of +1 (monovalent ions).

These elements have the highest electron affinity, making them the least likely to form compounds. Thus, they are not found in a pure state.

#Trends in Physical Properties of Alkali Metals

Down the column, the nuclear charge increases and a new orbital gets added to each alkali atom. Here, we have discussed some important trends in physical properties of alkali metals, such as electronegativity, ionization energy, atomic radius, and melting and boiling points, as we go down the column.

Atomic and Ionic Radii of Elements

Every alkali metal has the largest radii than any other element in the corresponding period, and atomic and ionic radii of elements increase, regularly down the column.

The relative ionic radii of Alkali metals increases down the column as they readily lose an electron and become cationic, resulting in a smaller cationic radius than the neutral atom.

Decreasing order of Atomic and Ionic Radius: Cs ˃ Rb ˃ K ˃ Na ˃ Li and Cs+ ˃ Rb+ ˃ K+ ˃ Na+ ˃ Li+

Density of Alkali Metals

Alkali elements have the lowest density due to having the largest radius and volume. This property makes them very soft and able to be cut with a knife. Additionally, lithium, sodium, and potassium are all lighter than water, with potassium having the lowest density among alkali metals.

Electropositive Metallic Character and Ionization Energy

Alkali metals have a single valence electron, which they donate in order to achieve a noble gas configuration. This makes them all univalent electropositive metals. The ionization energy required to remove the valence electron is highest for the smallest atom, lithium.

With increasing atomic size, the valence electron is shielded more and more by the inner electrons, making it easier to remove with less energy required. Therefore, the ionization energy decreases as the atomic number increases.

Li < Na < K < Rb < Cs

Hydration and Solubility of Alkali Metal Ions

Lithium-ion is the most soluble of the alkali metal ions, but as the ion size increases, solubility decreases, with Cesium ion being the least water-soluble. This is due to the fact that the smaller ions have a higher charge density and can be solvated by more water molecules, resulting in a higher enthalpy of hydration and making the hydrated ions more stable.

Solubility of Cs+ > Solubility of Rb+ > Solubility of K+ > Solubility of Na+ > Solubility of Li+

Reduction Potential

The reducing agents are those substances which can donate electrons. Their reducing ability is related to the ease of electron donation or lower ionization energy. Learn more about reducing agents here.

The decreasing ionization energy down the column is indicative of an increase in reducing property from Lithium to Cesium. Lithium is the strongest reducing agent with the highest reduction potential of -3.04V.

The combined energy difference of three processes determines the reduction potential and reducing ability.

Sublimation of the Atom

Ionization of the metal ion

Hydration of the ion with water.

Lithium, being the smallest ion, has a hydration enthalpy much higher than other ions, compensating for its higher ionization enthalpy: ENa ˂ EK ˂ ERb ˂ ECs ˂ RLi.

Flame Colouration

In s-block elements, the energy required for an electronic transition between the available energy levels lies within the visible spectrum region.

Elements can be identified by the characteristic colour of the flame they produce when heated, which is reflective of their emission or absorption spectrum.

How Do Alkali Metals Have Low Melting and Boiling Points?

Being very soft, alkali metals have low melting and boiling points compared to the other period elements. The melting and boiling points decrease from Lithium to Cesium.

Chemical Properties of Alkali Metals

Alkali Metal Compounds and Their Characteristics

Alkali metals are highly electropositive, forming compounds that are ionic in nature. In this article, we will discuss the various compounds of alkali metals and their general characteristics.

Hydrides:

Hydrides are chemical compounds in which one or more hydrogen atoms are covalently bonded to another element.

At higher temperatures, Alkali metals react with hydrogen to form metallic hydrides, which release hydride ions.

2M + H₂ → 2MH₂ → M⁺ + H^-

Nitrides and Phosphides:

Nitrides and phosphides are compounds composed of nitrogen and phosphorus, respectively. They are both important materials in a variety of applications, such as semiconductors, catalysts, and coatings.

Alkali metals can react with even atmospheric nitrogen to form nitrides.

2M<sup>3</sup>N = 6M + N<sup>2</sup>

Phosphorus forms phosphides similarly, and water hydrolyses phosphides to produce phosphine.

3M + P → M₃P → (H₂O) 3MOH + PH₃

Oxides of Carbon:

Alkali metals react with atmospheric oxygen and lose their shining nature. They combine with oxygen to form oxides, but the nature of the oxides formed is different.

Oxygen has a different oxidation state in them. Whereas, smaller Lithium forms a normal oxide, sodium forms peroxides and the larger atoms form superoxides.

2Li + O2 → 2Li2O (Oxide, Oxidation Number of Oxygen= -2)

2Na + O2 → Na2O (Peroxide; Oxidation Number of Oxygen= -1)

(K/Rb/Ce)O2 → K/Rb/Ce + O2 (Superoxide; Oxidation Number of Oxygen= -1/2)

Since the alkali metals react with nitrogen, oxygen and water in the air, they are always stored under kerosene.

Properties of Hydroxides of Alkali Metals

Reaction of Alkali Metals with Water

The reaction of the alkali metals with water is exothermic, and the enthalpy increases from lithium to cesium. During the reaction, the alkali metal floats on the surface of the water, producing basic hydroxides and liberating hydrogen.

The density of Sodium and Potassium is lower than that of water. As the alkali metal gets heavier, its reaction enthalpy increases, causing it to melt and rise to the surface. This makes the reaction with water highly exothermic and explosive, leading to fire from Lithium to Cesium.

2M + 2H₂O → 2M⁺ + 2OH^- + H₂ + ∆H

Lithium is expected to be highly reactive and exothermic due to its large electrode potential and high hydration energy. However, its reaction with water is slow and not explosive. This is due to its higher ionization energy and more covalent nature than other alkali metal ions, limiting its solubility and the amount of reaction. On the other hand, cesium is an ionic and soluble in water.

Moreover, the enthalpy of reaction is higher than the latent heat of fusion. This causes the cesium to melt into liquid which increases the amount of reactants available for the reaction, creating a cycle. Alkali metals can replace hydrogen from any proton donor molecules such as alkynes, ammonia, alcohol, etc.

Oxides and Water:

Oxides are chemical compounds that contain oxygen and one or more other elements. When oxides come into contact with water, a chemical reaction occurs that can produce an acid or a base.

Metal and their oxides react with water to ultimately yield hydroxides.

2LiO + H2O → 2LiOH

2NaO2 + 2H2O → 2NaOH + H2O2

2KOH + H2O2 + O2 → 2KOH + 2H2O

Carbonates and Bicarbonates:

Carbonates and bicarbonates are chemical compounds that contain carbon and oxygen atoms. They are often found in rocks and minerals, as well as in many natural waters and in the atmosphere. Carbonates and bicarbonates are important components of many biological processes and are essential for the proper functioning of organisms.

The hydroxides are alkaline, and they react with carbon dioxide to form carbonates.

2NaOH + CO2 → Na2CO3 + H2O

Except lithium carbonate, alkali metal carbonates are ionic, thermally stable, and water-soluble. These properties become more pronounced as the alkali metal moves down the group. Bicarbonates, except lithium bicarbonate, are solid, water-soluble, and release carbon dioxide upon heating.

Sulphates:

Sulphates, except lithium, are soluble in water. Carbon can be used to reduce sulphates to sulphide, and they form double salts with trivalent metal sulphates (e.g. alum).

Nitrates:

Lithium nitrate is the exception as it decomposes to nitrites when heated, whereas other nitrates are soluble in water and remain stable on heating.

2MNO3 → 2MNO2 + O2

Halides:

Alkali metals react vigorously with all the halogens to form solid ionic halides with a definite crystal structure. The reactivity of the alkali metals decreases as the atomic number of the halogen increases. Lithium halides are an exception with more covalent bonding because of the high polarization of the small covalent ion on the electron cloud of the halogen anion as indicated by the Fajan’s rule.

Lithium Halides are insoluble in water. Halides of bigger metals form Poly Halides by combining with more halogens.

KI + I2 → K3

The Interaction of Alkali Metals with Liquid Ammonia

The cations and electrons produced by the ionization of alkali metals in liquid ammonia become solvated by the ammonia molecules, resulting in an electrically conductive, reductive, and paramagnetic solution.

The solvated electrons absorb in the visible region and the solution turns blue in colour. On standing, the colour changes to a bronze hue and the solution becomes diamagnetic. In dilute solutions, the cation, electron and ammonia react to form sodamide and release hydrogen gas.

M + (x + y)[M(NH3)x]+ + [M(NH3)y]– → MNH2 + ½H2

When hot metal reacts with dry ammonia gas, an amide is formed. This amide can be hydrolyzed to produce ammonia.

2M + 2NH3 → 2MNH2 + H2

NaNH2 + H2O → NaOH + NH3

Extraction of Alkali Metals

Being the highest electropositive metals, the usual method of extraction is not applicable to the extraction of alkali metals. Displacement by other metals and electrolysis are not applicable due to their high electrode potential, which restricts reducing agents like carbon to reduce them.

In electrolysis of aqueous solution, hydrogen ions get preferentially reduced to gaseous hydrogen than sodium ion. Hence, Sodium and potassium are obtained only by the electrolysis of the fused salts of sodium hydroxide and sodium chloride. Alkali metals form alloys with themselves, other metals, and amalgams with mercury.

Anomalous Behaviour of Lithium

Lithium differs from other alkali metals due to its smallest size, highest ionization energy, strongest electropositive and polarizing nature. This gives it a more covalent nature. Additionally, its smaller size, larger solubility, and highest electrode potential make it the strongest reducing element.

Anomalous Behavior

It is harder than other alkali metals.

Reacts slowly with oxygen to form a normal oxide that does not get tarnished quickly.

Lithium reacts very slowly with water.

Only lithium hydroxide decomposes back into oxide and water, making it less basic than other hydroxides. Here is more information on the decomposition of lithium hydroxide.

Lithium carbonate is less stable because of its covalent nature, which causes it to decompose into oxide and carbon dioxide.

It reacts with atmospheric nitrogen to form nitride.

Lithium nitrate decomposes into nitrogen dioxide, oxygen, and an oxide, while the other nitrates of alkali metals produce nitrites and oxygen.

Lithium forms an imide when reacted with liquid ammonia, whereas other alkalis form an amide.

Lithium salts are less soluble than other alkali metal salts.

The Diagonal Relationship between Lithium and Magnesium

Lithium of the alkali metal group resembles more with magnesium of the alkaline earth metal group.

Lithium and Magnesium are relatively harder metals with higher melting points.

Both react slowly with water to release hydrogen.

Water hydrolyzes both nitrides, releasing ammonia.

Both form normal oxides which are less soluble in water.

Both form carbide, which upon hydrolysis yields acetylene.

Lithium and magnesium bicarbonates are only stable when dissolved in a solution, and not in their solid form.

Uses of Alkali Metals

Peroxides of Sodium and Potassium:

Oxidation of sodium and potassium, two alkali metals, with moisture-free oxygen gas at approximately 300°C produces peroxides.

2M + O2 → M2O2

Peroxides react with cold water and oxygen to form hydrogen peroxide at higher temperatures.

2M + 2H2O → 2MOH + H2O2

2M2O2 + 2H2O → 4MOH + O2

Alkali metal peroxides are utilized to generate other peroxides, bleach, prepare perborate, and purify air in confined areas.

Potassium Superoxide

Superoxides of alkali metals are a powerful oxidizing agent due to the release of hydrogen peroxide and oxygen in aqueous solution when Potassium Hydroxide (an orange and paramagnetic solid) is prepared by heating potassium with excess oxygen or passing ozone through it.

2KOH + 2 H₂O → H₂O₂ + O₂ + 2 KOH

Sodium Carbonate (Na2CO3)

Ammonia and carbon dioxide react to form ammonium bicarbonate, which is an essential step in the Solvay process. Limestone is calcined to obtain carbon dioxide, which is then combined with ammonia to form the ammonium bicarbonate. This is then used to precipitate the less soluble sodium bicarbonate from the aqueous solution using brine.

On heating, bicarbonate produces sodium carbonate. Calcium oxide when treated with water gives calcium hydroxide which, when treated with the byproduct, releases ammonia for reuse.

CaCO3 + 2NaCl → Na2CO3 + CaCl2

Sodium Bicarbonate

Sodium carbonate, when placed in a concentrated aqueous solution, will precipitate out sodium bicarbonate when exposed to carbon dioxide.

Na2CO3 + H2O + CO2 → 2NaHCO3

Na$_2$CO$_3$ + H$_2$O + CO$_2$ → 2NaHCO$_3$

The aqueous solution is alkaline. The bicarbonate ion is amphiprotic, i.e. it can act as both a proton donor and acceptor.

Baking Soda

Baking soda is a combination of sodium bicarbonate and weak organic acids such as tartaric acid, along with a diluent like cornstarch. When the acid and carbonate react, carbon dioxide is produced, resulting in a porous structure in baked goods.

Hydroxides

Hydroxides of alkali metals are strong bases which can dissolve in an excess of alkali. They are produced by the electrolysis of an aqueous solution of brine and Hydrogen and Chlorine are obtained as the by-products. Additionally, these hydroxides are deliquescent and react with carbon dioxide to form carbonates. Furthermore, some metal salts of Zn and Al, precipitate metallic hydroxides.

2 NaOH + ZnCl2 → 2NaCl + Zn(OH)2

2 NaOH + Zn(OH)2 → Na2ZnO2 + 2 H2O