Chemistry is basically the study of various elements and these elements are classified into categories. The two most important categories of elements are called the representative elements. The s-block and p-block elements are a part of the representative elements of the periodic table. Today, in this article, we’re throwing light on s-block elements which are also one of the most important chapters of your school chemistry. This article lays down an overview of the s-block elements for our readers.
Introduction to s-block elements
The elements in which the last electron enters the outermost s-orbital are called s-block elements. The s-block elements have two groups (1 and 2).
- The Group 1 elements are called alkali metals. These elements are called alkali metals as they form hydroxides by reacting with water that is strongly alkaline in nature.
- The Group 2 elements are called alkaline earth metals as their oxides and hydroxides are alkaline in nature and exist in the earth’s crust.
The following table outlines the metals contained in each s- Block group:
|Alkali Metals- Group 1||Alkaline Metals- Group 2|
|Lithium, Sodium, Potassium, Rubidium, Caesium, and Francium.||Beryllium, Magnesium, Calcium, Strontium, Barium, and Radium.|
Before deep-diving into the s-block elements, let’s understand the periodic table and the modern periodic law.
Modern Periodic Law
The systematic arrangement of elements into groups and periods is called periodic table. In 1913, when Henry Moseley proved that the atomic number of an element is a more fundamental property than its atomic mass, Mendeleev’s Periodic Law was modified and the atomic number was adopted as the basis of the Modern Periodic Table.
The modern periodic table classifies elements on the basis of the modern periodic law i.e. properties of elements are a periodic function of their atomic numbers. In this tabular arrangement where elements are arranged in rows and columns, the elements in horizontal rows have their valence electrons occupying the same shell or energy level whereas the vertical columns have the same number of valence electrons in their outermost shell. In this article, we discuss the s-block elements of the periodic table.
s- Block Elements
As knowledge of orbitals and sub-shells came into light, there occurred a need to study the elements as a part of more general classification according to the orbital occupied by the valence electrons which form the basis of the blocks within the periodic table. There are four such blocks – s-block, p-block, d-block and f-block – depending on whether the valence electrons occupy the s-orbital, p- orbital, d-orbital or f-orbital. Such a classification leads to an easier way of generalization and analysis of the properties of elements. S-block elements are perhaps the simplest yet interesting block in the table.
As explained earlier, the s blocks comprise of the group-I and group-II elements – also known as the alkali metals and alkaline earth metals respectively. They have the general outer electronic configuration ns1 for alkali metals and ns2 for alkali earth metals. Since they have a ns1 or ns2 configuration, s-block elements readily form M+ or M2+ cations respectively by losing electrons. Thus they are very reactive and are never found in their free state in nature.
The Main Features of the Modern Periodic Table
- Elements are arranged on the basis of increasing order of atomic numbers.
- The horizontal rows in the periodic table are called Periods and the vertical columns are known as the Groups.
- The elements in the Modern Periodic Table are arranged in 7 Periods and 18 Groups.
- The elements in the Modern Periodic Table are classified into four categories viz. Representative Elements, Transition Elements, Inner Transition Elements, and Noble Gases.
General Characteristics of Compounds of Alkali Metals
- Alkali metals are like silver in appearance and are not very dense.
- They can easily be cut and one of the elements, cesium can even melt in the palm of your hand. It is a soft, silvery-gold alkali metal with a melting point of 28.5 °C, which makes it one of only five elemental metals that are liquid at or near room temperature.
- They are usually stored in specially made solutions and containers to not allow unintended reactions to take place. This is because their melting points are quite low and hence they are extremely reactive. Part of what makes alkali metals so reactive is that they have one electron in their outermost electron layer. Like so many other metals, the alkali metals want nothing more than to have electronic structures like their famously stable and unreactive cousins, the noble gases.
- It takes very little energy to remove that outermost electron from an alkali metal. Thus, alkali metals easily lose their outermost electron to become a +1 ion. This happens so often that it is rare to find a sample of an alkali metal with all of its electrons; most alkali metals occur in their ionic +1 form.
- The energy needed to remove an electron from an element is called the first ionization energy. The alkali metals have the lowest first ionization energies of all of the elements. In fact, as you go down the 1A column, the first ionization energies get lower and lower, making cesium the most easily ionized element on the periodic table.
Physical properties of S Block elements
S-block elements have their sub-shells filled with electrons. Alkali metals have one electron in their outermost s-sub shell. Let’s take a look at the physical properties of Alkali metals.
- Atomic radii: As we know atomic size decreases as we move from left to right in the periodic table, s-block metals – alkali metals to be precise- have the largest atomic radii in a particular group. Also, the atomic and ionic sizes increase as we move down the group.
- Physical State: All the elements exist as silvery white, soft, light metals and are considered metals due to very low ionization energies. These elements can be compressed into sheets or drawn into wires. When freshly cut, they’re lustrous but the lustre tarnishes easily with exposure to air.
- Density: The density increases as we move from Li to Fr but there’s an anomaly in case of sodium and potassium where K is lighter than Na. Li is the lightest among all the metals. Li, Na, and K are lighter when compared to water.
- Ionization Enthalpy: s-block metals have low values of ionization enthalpies as it is easier to remove their valence electrons to attain stability. Ionization enthalpy decreases as we move down a group as the outer electrons feel a lesser pull from the nucleus. This is predominant in the last members of each group, where the shielding of nuclear charge by inner electrons means the outer electrons are very loosely attached to the atom (the effect is called as the Screening effect), further decreasing its ionization enthalpy
- Melting & Boiling Points: The presence of large atomic radius makes these elements bind very weakly at the time of forming crystals. These elements have very low melting and boiling points because of a weak inter-atomic bonding.
- Colour: When we heat most of the s-block elements or their salts, the outermost electrons get excited and jump to higher energy levels. When they come back to their ground state, colours characteristic to each metal is imparted to their flames. This is rather used in the laboratories to identify the action in salts using a test called the `flame test’. This can’t be applied for Magnesium and Beryllium as their outer electrons are too bound to get thermally excited.
- Hydration Enthalpy: Alkaline earth metals and Lithium generally form hydrated salts. The extent of hydration decreases with size and hence decreases down the group.
After understanding the physical properties of alkali metals, let’s take a look at their chemical properties i.e. how they react with oxygen, air, etc.
- Reaction with Air: s-block metals react with oxygen in the air to form alkaline oxides. Lithium forms monoxides, sodium form peroxides whereas other alkali metals form superoxides. As for alkaline earth metals, they form monoxides, but Beryllium and Magnesium are kinetically inert towards air and water unless powdered, due to the formation of oxide film on their surface.
- Reaction with Water: On reaction with water, s-block elements from hydroxides. These hydroxides are extensively used in laboratories and industry as bases and for other purposes.
- Reaction with Hydrogen: Metal hydrides can be prepared by reaction of all s-block elements, except Beryllium. BeH2is prepared by reaction of BeCl2 with LiAlH4.
- Reaction with Halogens: s-block elements react readily with halogens to form ionic metal halides. However, Li and Be Halides are generally covalent in nature.
- Reducing Nature: s-block elements are strong reducing agents. Alkali metals are stronger reducing agents than alkaline earth metals.
- Solutions in Liquid Ammonia: Alkali metals dissolve in liquid ammonia to give deep blue, paramagnetic, conducting solutions. Alkaline earth metals give a deeper blue-black solution on dissolution in liquid ammonia. The blue colour is due to ammoniated electrons.
- Ability to form Alloys: Alloys are mixtures of metals. Alkali metals can form alloys with other elements in the same period or with metals in other groups. Alkali metals form amalgams very easily by dissolving in mercury.
- Ability to form Complexes: Alkali metals are unable to form complexes because of large size, low nuclear charge, and poor ability to attract electrons. Their inherent properties are opposite to the requisites for the formation of complexes. Lithium, the smallest element in the group has the ability to form certain complexes. The complex forming ability decreases as we move down the group.
Anomalous properties of Lithium and Beryllium & Diagonal relationship
As mentioned in various cases, the first member of both the groups – Li and Be differ somewhat in properties from the rest of the members. This is mainly because of the small sizes of these elements compared to the other members and high polarizing power, which can be rooted back to its high charge to radius ratio.
In fact, they resemble the elements diagonally right-down to them in the periodic table. Lithium resembles Magnesium and Beryllium resembles Aluminium is many aspects. This is due to the fact that sizes and charge/size ratio of elements shows similarity across the diagonal in the table. This is commonly referred to as the diagonal relationship in the periodic table.
The compounds of s-block elements find extensive use in various industries. The oxides, hydroxides, halides, hydrides, carbonates, and bicarbonates formed by s-block elements are of utmost importance in many organic and inorganic chemical applications. Moreover, ions of s-block elements, especially sodium, potassium, calcium, and magnesium are very crucial in biological systems as well, stressing the importance of extensive learning and understanding of the s-block elements.
The s-block elements also play key roles in biological systems. Covalent Hydrides form the basis of organic compounds, other compounds, and ions containing s-block elements are found in tissues and cellular fluids.
Electronic configuration of Alkali metals
Electronic configuration of alkaline earth metals:
Applications of S-Block elements
“Lime” refers to both limestone (CaCO3) and its derivatives burnt lime (CaO) and
hydrated lime (Ca(OH)2). These substances are of enormous importance to New Zealand
industry: Ground limestone is one of the most important fertilizers. Burnt and hydrated
lime are used in many industries to neutralise acid waste, and are used as causticisers in
the pulp and paper industry and as flux in the steel industry. Lime is also used in road
stabilisation and gold recovery and is exported around the South Pacific region.
Limestone occurs widely in New Zealand, but it needs to be quarried and processed before
being ready for use. Burnt lime is produced from limestone by heating to 1100C and
allowing the following reaction to take place:
CaCO3 + heat → CaO + CO2 ↑
Hydrated lime is produced by adding water to calcium oxide in a continuous hydrator:
CaO + H2O → Ca(OH)2 + heat ↑
As stated earlier, the term ‘lime’ refers to three related substances: limestone, burnt lime and
hydrated lime. These are each used for different purposes. Limestone is used as a fertiliser
and in the manufacture of glass and cement. Burnt lime and hydrated lime are used in:
• the steel industry (as a flux)
• the pulp and paper industry (as a causticiser)
• gold mining
• road stabilisation
• water treatment
• wastewater treatment
• fellmongery (to treat hides)
That’s all on the s-block elements for now. Hope this was an informative article.