Alkali Metal in Periodic Table

Alkali metals are part of the periodic table that contains six chemical elements like lithium (Li), sodium (Na), potassium (K), rubidium (Rb), caesium (Cs), and francium (Fr). Together with hydrogen, alkali metals constitute group 1 of the periodic table. Alkali metals like sodium and potassium are extensively involved in our life processes, and many of their compounds have been used since the early days of human civilization. The alkali metal lithium, sodium, or potassium have wide industrial applications in the present-day world.

Consistent with their ns¹ outer electronic configuration, the alkali metals occupy group 1 in the periodic table. The physical and chemical properties of alkali metals can be readily understood by their outer electronic configuration.

The alkali metals Na, K, and Rb are distributed very widely in the Earth’s crust. With the increasing atomic number, the alkali metals become rarer. Lithium occurs in small amounts because it disappears through the natural process of nuclear reaction.

Francium is very rare due to its high radioactivity. It occurs in very small amounts in nature. The half-life of the longest-lived isotope is 22.8 minutes.

Properties of Alkali Metal

An alkali metal is a soft, shiny, and highly reactive element at standard temperature and pressure. They normally adopt a body-centered cubic crystal structure. At low temperatures, lithium forms a hexagonal close-packed crystal lattice.

Among alkali metals, lithium and sodium are used widely in fireworks. The alkali metal lithium imparts a red color to fireworks while the other alkali metal sodium imparts a gold or yellow color to fireworks.

Alkali metals readily lose their outermost electron to form uni-positive cation (M⁺).

M → M⁺ + e⁻

Periodic Trends of Alkali Metals

The six elements lithium, sodium, potassium, rubidium, caesium, and francium constitute the family of alkali metals. They are placed extremely left in the periodic table. The periodic trends of alkali metals are given below the topics,

Atomic and Ionic Radius of Alkali Metals

The atomic and ionic radius of alkali metals increases systematically from lithium to caesium.

When we proceed downward on the alkali group, the electron is added to higher energy levels. It decreases the electrostatic attraction between the nucleus and valence shell electrons. This decreases electrostatic attraction and increases atomic and ionic radii.

The shielding effect also decreases the electrostatic attraction between the nucleus and valence shell electrons. With increasing shielding electrons the effective nuclear charge of the atom decreases. Therefore, the nucleus and outermost electron separation increase.

Melting and Boiling Points

Alkali metals are soft and low-melting because only the outer ns electron participates in metallic bonding.

With the increasing atomic number, the metallic character of alkali metals increases along the group. Therefore, francium is the most metallic element among alkalis.

However, francium is rare in nature due to its high radioactivity. Hence, the natural element that shows the highest metallic character is caesium.

The melting point, boiling point, and hardness decrease with increasing atomic number. With the increasing atomic number, the size of the atom increases. It weakens the strength of metallic bonding between two alkali atoms.

Density of Alkali Metals

All the alkali metals contain a body-centered cubic crystal lattice. Therefore, the densities of these metals depend on the volume of the atom or atomic radii and atomic weight. The atomic radii and atomic weights of metals increase going down the group.

The observed densities of alkali metals increase down the group with the exception of potassium. Lithium, sodium, and potassium are the only three metals that are less dense than water.

Ionization Energy

All the elements in the alkali metals family have one electron in the s-orbital over a noble gas core. It is easily lost to give very low ionization energy. With the increasing atomic number, the distance between the nucleus and valence s-electron increases. It leads to a steady decrease in their ionization energy.

Chemical Bonding

The alkali metal contains the largest size among the elements of each period. Large size and a small charge of cation favor the formation of ionic bonding. The tendency to form ionic compounds increases down the group.

When the alkalis combine with other elements, there results in a wide difference in electronegativity. This fact leads to the formation of ionic compounds.

Oxidation Number

All the alkali metals show a +1 oxidation state or number. There is no scope for alkalies to exhibit an oxidation number of +2. To show a +2 oxidation state, alkali metals need to utilize electrons from the very stable noble gas configuration.

Standard Electrode Potential

The standard electrode potentials of alkali metals are the most negative among all periodic table elements. The electrode potential values become more negative from sodium to cesium.

The alkali metal lithium may not follow the general trend of electrode potential. Lithium shows the most negative reduction potential. Lithium is the strongest reducing agent among alkalis due to its highest electrode potential value.

The hydration energy of lithium is the highest among the alkali metals due to its small size and it disrupts the structure of water molecules during electrode reactions. These two factors are quite significant in describing the reduction potential of lithium.

Flame Coloration

All the alkali metals give characteristic flame coloration due to the easy excitation of outermost electrons.

• Lithium produces a crimson color
• Sodium produces a yellow color
• Potassium produces a violet color
• Rubidium gives a red violet color
• Caesium gives a blue color

It led to the development of analytical chemistry for the precise estimation of these metals by flame photometry and atomic absorption spectroscopy.

Chemical Reactivity of Alkali Metals

The reactivity of the alkali metals increases along the group with the increasing atomic number. With the increasing atomic number, the distance between the nucleus and the valence shell electron increases. Therefore, francium is the most reactive alkali metal present on the periodic table.

The large difference between the first and second ionization energies of these elements suggests that the preferred oxidation state of alkali metals will be +1. Therefore, the major chemistry of alkali metals is in the form of M⁺ ions.

The chemical reactivity of the alkali metals increases from lithium to caesium. The poor reactivity of lithium is explained by the fact that Li cannot replace the weakly acidic hydrogen from phenylacetylene.

Reaction with Oxygen

The metals react with oxygen to form respective oxides, peroxides, superoxides, and suboxides.

• Lithium reacts with oxygen to form Li₂O and Li₂O₂.
• Sodium gives principally Na₂O₂.
• Other metals give superoxides like MO₂.

Reaction with Water

An alkali metal reacts with water to form hydroxyl ions and hydrogen gas. At 25 °C, the metals react with water to create the following condition,

• Lithium reacts slowly
• Sodium reacts vigorously
• Potassium inflames
• Rubidium and caesium react with an explosion.

Reaction with Carbon

• Lithium and sodium react with carbon on heating to form the acetylides Li₂C₂ and Na₂C₂.
• The other alkalis give non-stoichiometric interstitial compounds.

Reaction with Nitrogen

Lithium reacts uniquely with nitrogen. At room temperature, lithium reacts slowly with nitrogen. At 400 °C, it reacts rapidly with nitrogen to give the ruby red crystalline solid Li₃N.

Reaction with Liquid Ammonia

<p>Alkali metals dissolve in liquid ammonia to give blue solutions which contain free electrons. These solutions are used as a powerful reducing agent in various organic syntheses.