What is Alkaline Earth Metal?
The elements that constitute group 2 of the modern periodic table are known as alkaline earth metals. Alkaline earth metals are atoms that have their s-subshell packed with two valence electrons. [Noble gas] ns2 is their electronic configuration in general. They are also known as group two metals because they are found in the second column of the periodic table.
Members of Alkaline Earth Metals Include:
Beryllium
Magnesium
Calcium
Strontium
Barium
Radium
The physical and chemical properties of the elements in this group are very similar. This article will study alkaline earth metal physical and chemical behaviour in detail.
Physical and Chemical Behaviour
Atomic and Ionic Radii
Due to charge and the addition of an electron to the same energy level, both ionic and atomic radius decreases down the periodic table column, making them smaller than alkali metals and larger than other atoms of the same age.
Both s-electrons can be lost in alkaline earth elements, making them doubly positive cationic. The radius of a cationic atom is less than that of a neutral atom. Ionic radii continue to increase as you go down the column.
Ionization Energy
Alkaline earth elements can donate all of their valence electrons to form an octet noble gas configuration. They have two ionisation energies as a result:
First Ionisation Energy
The energy required to remove the first electron from a neutral atom is known as the first ionisation energy of alkaline earth metals. Because of smaller radii and the electrons being retained closely by the higher nuclear charge, it is greater than that of an alkali metal atom. Electrons are withdrawn from a completely filled and thus stable subshell.
Second Ionisation Energy
The second ionisation energy needed for the second electron from the cation in alkaline earth metals will be higher than the atom's first ionisation energy, but lower than any alkali metal's second ionisation energy. Despite the strong ionisation energy, both electrons may be removed since the atom:
Adopts a noble gas configuration.
Because of the tight packing of atoms or ions in solids, the smaller size and higher charge help overcome the higher ionisation energy by increasing the lattice energy.
Due to greater solvation, liquids have higher hydration energy.
So alkaline earth elements in group two are all divalent electropositive metals with a set oxidation state of 2. The tiny beryllium atom would need the most ionisation energy to remove the valence electron.
The valence electron is protected by the inner electrons as the atomic size increases, making it easier to remove with less energy. As a result, as the atomic number or size increases, the ionisation energy decreases.
Reactivity of Alkaline Earth Metals
Ionization energy is inversely proportional to reducing ability. From Beryllium to Barium, the reducing property is supposed to increase as the ionisation energy decreases down the column.
From beryllium to barium, the reduction potential decreases, suggesting rising reducing capacities. Due to their higher ionisation energy, alkaline earth metals are weaker reducing agents than alkali metals.
Flame Colouration
The energy required for an electronic transition between available energy levels in Alkaline Earth Metals is in the visible spectrum. As a result, all metals, except beryllium and magnesium, develop a distinct colour in the flame that reflects their emission or absorption spectrum and can be used to identify them when heated.
Melting and Boiling Points
The melting and boiling points of alkaline earth metals are higher than alkali metals due to their smaller size and strong metallic bonding in a close-packed system. Except for magnesium, the melting and boiling points of alkaline earth metals decrease in order from beryllium to barium.
Alkaline Earth Metal Physical and Chemical Behaviour
This subsection discusses the main characteristics of alkaline earth metal compounds as well as their general characteristics.
Hydrides
Beryllium does not react directly with hydrogen. The replacement of beryllium chloride with lithium aluminium hydride yields beryllium hydride.
2BeCl2 + LiAlH4 → 2BeH2 + LiCl + AlCl3
Beryllium and magnesium form covalent hydrides, which have two metal atoms bound to each hydrogen. The "banana Bond" is a type of molecule in which three centres share just two electrons.
Metallic hydrides are formed when calcium, strontium, and barium react with hydrogen. Hydrides ions are generated by metallic hydrides.
M + H2 → 2MH2 → M+ + 2 H-
Hydrogen is formed when hydrides react violently with water. Hydrogen is produced using a calcium hydride called "Hydrolith."
CaH2 + 2H2O → Ca(OH)2 + H2
Reaction of Alkaline Earth Metals with Water
Also at higher temperatures, beryllium does not react with water. Magnesium only forms hydroxides and releases hydrogen as it reacts with hot water. Magnesium receives a protective coat of its oxide, which protects it from further water molecule attack. Other alkaline earth metals produce hydrogen when they react with even cold water.
Carbides
Except for beryllium, alkaline earth metals and their oxides react with carbon to form carbides. Carbides are used as a source of acetylene gas because they react with water to produce it.
Oxides
Beryllium only reacts with oxygen at temperatures above 600°C. Magnesium and strontium form oxides when exposed to oxygen, while barium forms peroxides.
The covalent oxides BeO and MgO are more so than the ionic oxides. Amphoteric beryllium oxide, weakly basic magnesium oxide, and basic calcium oxide, while other oxides are basic.
Hydroxides
Hydroxides are formed when oxides react with water. From beryllium to barium, the basic nature and thermal stability of hydroxides change.
Carbonates
Carbonates are formed when hydroxides react with carbon dioxide.
M(OH)2 + CO2 → MCO3+ H2O
Bicarbonates are water-soluble and only occur in solution. In water, carbonates are stable and insoluble. Carbonates' solubility decreases from Be to Ba. Carbonates degrade into bicarbonates in the presence of carbon dioxide. From Be to Ba, the carbonates' ionic character and thermal stability increase.
Sulphate
Beryllium sulphate, unlike alkali metal sulphates, is water-soluble. The hydration energy of beryllium sulphate increases as its size and a charge density decrease, resulting in increased solubility. Other sulphates' solubility decreases from BeSO4 to BaSO4 as lattice energy increases and hydration energy decreases (due to increasing size).
Nitrates
Nitrates are made by reacting nitric acid with the appropriate oxides, hydroxides, and carbonates. Nitrates are water-soluble. Beryllium nitrate becomes nitrite when heated, while other nitrates become oxide, releasing brown nitrogen dioxide fumes.
2M(NO3)2 → 2MO + 4 NO2 + O2
Halides
Both halogens react with alkaline earth metals from calcium to barium to form strong ionic halides with a definite crystal structure. From fluorine to iodine, reactivity decreases. Because of the strong polarisation of the small covalent ion on the electron cloud of the halogen anion, as suggested by Fajan's law, beryllium halides have more covalent bonding.
Beryllium halides exist as individual molecules in the gas phase, and they form Be-X chains in the solid phase.
Fluorides are water-insoluble. Other halides' solubility decreases as ionic size increases, such as from Mg2+to Ba2+. In their solid-state, halides are hygroscopic and contain the water of crystallisation (CaCl2.6H2O). Dehydrating agents include fused halides.
Reaction of Alkaline Earth Metals with Liquid Ammonia
Alkaline earth metals, including alkali metals, form ammonia solvated cations and electrons. The solution is reductive, electrically conductive, and paramagnetic. The visible region absorbs the solvated electrons, turning the solution blue. Bronze is the colour of the condensed solution. It decomposes into amide, ammonia, and hydrogen after a long period of time.
Complex of Alkaline Earth Metal
Alkaline earth metals are smaller size complexes. Beryllium binds to mono, di, and tetradentate ligands to form a variety of complexes.
Applications of Alkaline Earth Metal
Beryllium is mostly used in military applications, although it has other applications as well. Beryllium is a p-type dopant in some semiconductors used in electronics, and beryllium oxide is a high-strength electrical insulator and heat conductor. Beryllium is also used in mechanics when stiffness, lightweight, and dimensional stability are needed over a broad temperature range due to its lightweight and other properties.
Magnesium has a wide range of applications. It has advantages over other materials such as aluminium, but due to magnesium's flammability, it is no longer widely used. Magnesium is often alloyed with aluminium or zinc to create products with better properties than pure metals. Magnesium is used in a variety of industrial processes, including the manufacture of iron and steel, as well as the production of titanium.
Calcium has a wide range of applications. Its use as a reducing agent in the extraction of other metals from ore, such as uranium, is one of its many applications. It is also used to deoxidize alloys and is used in the manufacture of many metal alloys, including aluminium and copper alloys. Calcium is also used in the production of cheese, mortar, and cement.
Strontium and barium have fewer uses than lighter alkaline earth metals, but they are still useful. Pure strontium is used in the study of neurotransmitter activation in neurons, and strontium carbonate is often used in the manufacture of red fireworks. Radioactive strontium-90 is used in RTGs, which take advantage of its decay heat. Barium is used in vacuum tubes to extract gases, and barium sulphate is used in a variety of industries, including the petroleum industry.
Did You Know?
Due to its small scale, highest ionisation force, high electropositive nature, and best polarising nature, Beryllium has a more covalent nature. Beryllium's properties set it apart from other alkaline earth metals.
Among alkaline earth metals, it is the hardest.
Except at extremely high temperatures, it does not react with water.
Beryllium has the highest melting and boiling points.
It does not form hydride when it comes into contact with hydrogen.
Because of its higher electrode potential, it does not release hydrogen from acid like other alkaline earth metals. Concentrated nitric acid forms an oxide layer that renders it inactive.
Amphoteric beryllium oxide and hydroxide When dissolved in acids, it forms salts, and when dissolved in bases, it forms beryllate.
Beryllium forms a carbide with a different formula when it reacts with water, yielding methane rather than acetylene like other metals.
Beryllium nitride is a flammable material.
It doesn't interfere with nitrogen or oxygen in the air.
FAQs on Physical and Chemical Behaviour of Alkaline Earth Metal
1. What are the alkaline earth metals?
The alkaline earth metals are the elements that make up Group 2 of the modern periodic table. They are characterised by having two valence electrons in their outermost s-subshell, with a general electronic configuration of [Noble gas] ns². The members of this group include Beryllium (Be), Magnesium (Mg), Calcium (Ca), Strontium (Sr), Barium (Ba), and Radium (Ra).
2. Why are these elements called 'alkaline earth metals'?
These elements earned their name for two main reasons. Firstly, their oxides and hydroxides are alkaline in nature. Secondly, their oxides are stable to heat and were found in the Earth's crust, historically referred to as 'earths' by early chemists. Thus, the name combines their chemical nature with their natural occurrence.
3. What are the key physical properties of alkaline earth metals?
The key physical properties of alkaline earth metals show distinct trends down the group:
- Atomic and Ionic Radii: They have smaller atomic and ionic radii compared to the alkali metals of the same period. The radii increase down the group from Beryllium to Barium.
- Ionisation Enthalpy: They have high ionisation enthalpies, but these values decrease down the group, making it easier to remove electrons from larger atoms like Barium compared to Beryllium.
- Density: They are generally harder and denser than alkali metals due to stronger metallic bonding.
- Melting and Boiling Points: These metals have higher melting and boiling points than alkali metals. The trend generally decreases down the group, with Magnesium being an exception.
4. Why are alkaline earth metals harder and denser than alkali metals?
Alkaline earth metals are harder and denser than alkali metals primarily because of stronger metallic bonding. This is due to two factors: they possess two valence electrons (compared to one in alkali metals) that participate in bonding, and they have a smaller atomic size, which allows for a more compact and tightly packed crystal lattice.
5. Despite having high ionisation energies, why do alkaline earth metals readily form M²⁺ ions?
While removing two electrons requires significant energy (high first and second ionisation enthalpies), alkaline earth metals still readily form divalent cations (M²⁺). This is because the high ionisation energy is compensated for by two other factors: the resulting M²⁺ ion achieves a stable noble gas configuration, and the high charge and small size of the ion lead to very high lattice energy in solids and high hydration energy in solutions, which makes the overall process energetically favourable.
6. How does the anomalous behaviour of Beryllium differ from other alkaline earth metals?
Beryllium exhibits several properties that are different from other Group 2 elements due to its exceptionally small size and high ionisation enthalpy. Key differences include:
- It forms predominantly covalent compounds, while others form ionic compounds.
- It is the hardest metal in the group.
- It does not react with water, even at high temperatures.
- Its oxide and hydroxide are amphoteric, meaning they react with both acids and bases, whereas other group member oxides are basic.
- It forms a different type of carbide that yields methane on hydrolysis, unlike others which produce acetylene.
7. How do alkaline earth metals react with key substances like water, oxygen, and hydrogen?
The reactivity of alkaline earth metals increases down the group. Here's a summary of their reactions:
- With Hydrogen: All elements except Beryllium react with hydrogen to form hydrides (MH₂).
- With Water: Beryllium does not react. Magnesium reacts only with hot water. Calcium, Strontium, and Barium react with cold water to form hydroxides and liberate hydrogen gas.
- With Oxygen: Beryllium and Magnesium form oxides (MO). Strontium also forms an oxide, while Barium forms a peroxide (BaO₂).
8. Why does the solubility of alkaline earth metal sulphates decrease down the group?
The solubility of alkaline earth metal sulphates (like BeSO₄, MgSO₄, etc.) in water decreases as we move down the group from Beryllium to Barium. This is because two energy factors are at play: lattice energy and hydration energy. For the sulphates, as the size of the metal ion increases, the hydration energy decreases more rapidly than the lattice energy. The significant drop in hydration energy for larger ions like Ba²⁺ makes its sulphate (BaSO₄) much less soluble than BeSO₄.
9. What are some common uses of alkaline earth metals like Magnesium and Calcium?
Magnesium and Calcium have numerous important applications. Magnesium is used to create strong, lightweight alloys with aluminium and zinc for various industrial products. It is also used in the manufacturing of iron and steel. Calcium is a vital reducing agent in extracting other metals like uranium, deoxidizes alloys, and is a key component in the production of cement, mortar, and cheese.
