A Survey of the s- and p- Block Elements

In this section we will have a look for a brief survey of the s- and p-block elements. We will have a look a certain properties of the elements across the group and as well their trends for some properties.

Hydrogen

Hydrogen is known as the most abundant element in the universe and the 15th most abundant element on earth. The main properties of hydrogen gas are it is a colourless gas as well highly combustible gas, it forms diatomic molecular H₂, and it has low melting and boiling point (m.p.=14 K and b.p.=20 K). The low melting and boiling point is caused by weak van der Waals interaction due to small size of H₂ molecule. Besides that, the bond dissociation energy for H-H is 436 kJ mol⁻¹. The position of H in periodic table can be debatable because it can be place in group 1 or group 17 with argument H can form H⁺ and H^-. In nature, H exists in 3 different isotopes, H-1 is the most common one (almost 99.9% abundance of H), H-2 or deuterium which the deuterium compound is used for solvents in NMR spectroscopy, and H-3 or tritium which is radioactive with half life of around 12 year and it decay into He-3. Moreover, H is predicted to show metallic behaviour at very high pressure condition, so it is believed that Jupiter have a metallic core of hydrogen.

Hydrogen gas can be manufactured in very different way as shown below (more detail see hydrogen fuel economy).

Steam reforming and coal gasification. Hydrogen gas can be synthesised by reacting methane or coal with water vapour in high temperature to form hydrogen gas and carbon monoxide. Then, to increase the yield of hydrogen gas synthesis, CO is reacted with water vapour to form hydrogen gas and carbon dioxide.

 

Dehydrogenation process is involved the hydrocarbon compound (e.g. ethane) to form double bond compound (e.g. ethene) by releasing hydrogen gas.

Electrolysis of water also produces hydrogen gas at cathode, meanwhile oxygen gas is formed in anode.

Hydrogen gas is very common in industrial process and main uses of hydrogen gas is in ammonia industry, and as well it is the common reducing agent. The uses of hydrogen gas is shown below.

The uses of hydrogen gas

Moreover, in recent technology hydrogen gas can be used to generate energy with helps of fuel cell (see the chemistry of fuel cell for detail).

S-Block Metals

S-block metals consist of group 1 which is known as alkali metals (Li, Na, K, Rb, Cs, and Fr) and group 2 which is known as alkaline earth metals (Be, Mg, Ca, Sr, Ba, and Ra). In this part we will have a look the metallic structure of s-block metals. 

All the group 1 metals have body centre cubic (BCC) structure which is not a close-packed strucuture and it only has 1 valence electron per atom. Therefore, the delocalised electron sea in metallic structure of group 1 is less dense than group 2, which makes the melting point of group 1 is lower than group 2. Because group 2 has relatively smaller than group 1, it can exhibit more efficient packing, so it has close-packed structure (HCP [hexagonal close-packed] and CCP [cubic close-packed]). Moreover, the melting points decreases as down the group because of the metallic bonding becomes weaker as the metallic radius increases. For your information, d-block (transition) metals has more varies mixture of CCP, HCP, BCC, lattice structures.

The reactivity of group 1 metals is dominated by formation of the +1 oxidation state and the reactivity increases down group as the ionisation energy decreases. It means the energy to remove electron become smaller due to bigger radius which means weaker interaction between the valence electron as well the shielding effect of inner-core electrons which decreases the interaction. The reactions of group 1 metals are shown below.

Besides that, group 1 metal for example can have chemical properties as a solution in liquid ammonia (b.p. ammonia -33°C). When, alkali metals in liquid ammonia it forms identical colour for different electropositive metals (for example Na forms deep blue solution) and there is no hydrogen gas evolved. The coloured solution is due to solvated electron in the solution, and higher metal concentration gives bronze solution due to electrons delocalised throughout solution.  

Moreover, solutions of alkali metals in liquid ammonia is electrically conducting and it is paramagnetic (unpaired electrons); Na in liquid ammonia is known as a good reducing agent (decomposition in presence of Fe₂O₃ gives NaNH₂ + H₂). 

In group 2, it is dominated by formation of the +2 oxidation state (e.g. halides MX₂, oxide MO, and peroxide MO₂,). In the other sides, Be has special case due to very small ionic radius (59 pm), so its ionic compounds often contain hydrated cation, e.g. [Be(H₂O)₄]Cl₂. Moreover, anhydrous BeCl₂ has a polymeric chain structure with ionic/covalent character and the structure is similar to BeH₂ but is not electron deficient as Cl has lone pair.

P-block elements

The structures of p-block elements is varied from metallic structures into only monoatomic molecules (group 18 elements). Besides that, there are also elements can exhibit covalent polyatomic molecule or network which are mainly metalloid elements; and as well the diatomic molecules (N, O, and group 17).

From the structure as we see on figure above, the structural variation leads to irregular trends in group 13-16. In the other sides, the melting point of group 17 and 18 increase as down the group due to stronger intermolecular forces (van der Waals interaction). In p-block elements there are some structural features of p-block elements which are:

• Allotropy. Elements which can exist in more than one (structural) form in the same state exhibit allotropy and the different forms are called allotropes. The chemical bonding between atoms is different, with different discrete structural or molecular units. For example: C, Sn, P, O, and S.

• Polymorphism. Element or compounds which can exist in different crystal form but contain identical repeating units exhibit polymorphism and the different forms are called polymorphs. For example: rhombic and monoclinic sulphur, both contain S₈ molecules.

• Catenation. Elements or compounds which form structures containing sequences of element-element bonds exhibit catenation. For example C as element or in alkanes.

Group 13 elements

B₁₂ icosahedron (left) and part of one layer of the infinite lattice of
α-rhombohedral boron.

B is approximated to have close packed arrangement of B₁₂ icosahedra (not separate B₁₂ molecules). Besides that, due its metalloid structure it still have some electrons mobility within the structure and other more complex structures is existed as allotropes of boron.

The construction of B₈₄ unit, the main building block of β-rhombohedral B. (a) In the centre of the unit is B₁₂ icosahedron, and to each these, another B atom is covalently bonded. (c) A B₆₀-cage is the outer 'skin' of B_84-unit. (d) The final B_84-unit can be described in terms of covalently bonded subunits (B₁₂)(B₁₂)(B60)

Besides that, Al and Tl have CCP structures, In hase distorted close-packed, and Ga has irregular structure due to less metallic in character (higher electronegativity).

Group 14

Diamond crystal structure

Graphite

In nature C exists as 2 allotropes, diamond and graphite. Diamond has FCC structures with C atoms fill in half of the tetrahedral holes. Diamond exhibits 3D covalent network with 4-coordinate tetrahedral C (sp³ hybridisation). Diamond is a non-metal but it has semiconducting property at high temperature, and it is a good heat conductor due to its rigid covalent bond. Moreover, diamond has high melting point (3800 K) and it is the hardest naturally occurring substance. Meanwhile, graphite is 2D covalent network (planar sheets) with 3-coordinate trigonal C (sp² hybridisation). It has metalloid properties as it conducts electric current within sheets but not between them (delocalised π electrons). Moreover, a single layer of graphite was isolated for the first time in 2004 which is called graphene.

graphene

The discovery of graphene gave Andre Geim and Konstantin Novoselov the 2010 Physics Nobel Prize for the groundbreaking experiments regarding the two-dimensional material graphene. Initially, graphene was isolated using Scotch tape and it is almost transparent. Besides that Graphene is 100 times stronger than stell, has higher conductivity than Cu, and it is a light material (density of 0.77 mg m⁻²). Moreover, there are some potential applications of graphene such as touch-screens and solar cells.  

buckminsterefullerene

Another allotrope of C is C₆₀ which is known as buckminsterfullerene (fullerenes or buckyball). It was discovered in 1985 by Harry Kroto and from that discovery, Harry Kroto, Robert F. Curl, and Richard Smalley won the 1996 Chemistry Nobel Prize. Fullerene is formed from the vapourisation of graphite. It is a molecular compound with coordination number of C is 3 (approximation sp² hybridisation) and all C atoms are equivalent. The structure comprises of 12 pentagonal faces and 20 hexagonal faces that forms 1.1 nm in diameter. The appearance of fullerene is black solids and form coloured solutions in organic solvent. Moreover, fullerenes with higher number of C atom is possible

Fullerenes solutions in toluene

For heavier elements in group 14 exhibit varied structures. Si and Ge both adopt the diamond structure with metalloids and semiconductors characteristic due to less electronegative than C. Sn exhibits allotropy of grey tin (α-Sn) with diamond structure (coordination number of 4 and stable below 286 K) and white tin (β-Sn) which is more dense and has metallic character (coordination number of 6 and stable above 286 K). Moreover, Sn is close to metal/metalloid boundary. Lastly, Pb has CCP metal structures.

Group 15 (The Pnictogens)

Nitrogen forms dinitrogen (N₂) which has triple bond with bond energy of 946 kJ mol⁻¹. Nitrogen gas is a colourless gas and the most abundant gas in the atmosphere (78%) with melting point of 63 K and boiling point of 77 K.

White phosphorus

Black phosphorus

Meanwhile, phosphorus has 2 main allotropes structure, white phosphorus (P₄) and black phoshorus (P_n-right) which is a covalent network. P₄ is a molecular compound with tetrahedral structures with P atom in the corners. It is very reactive with melting point of 317 K and boiling point of 550 K. In the other sides, black phosphorus is more stable compound due to less strained bonding if P-P. Moreover, there are also red phosphorus (used in safety matches) and violet allotropes. 

The argument behind this different structural form of P and N can be seen from energetical sides or from the structural itself. From the structure, N has smaller atomic radius, so it has better overlap of p-orbital to form π-bonds than P. Besides that, it also can be seen from the thermochemical sides by using Hess's law. Hess's law states that the standard enthalpy of an overall reaction is the sum of the standard enthalpies of the individual reactions into which a reaction may be devided. This is the law upon which thermochemical (e.g. Born-Haber) cycles are based. Therefore for N and P are shown below.

Born-Haber cycle

From the cycles above, we see that N₂ is favourable than N₄(because the enthalpy is positive), but P₄ is favourable than P₂ (becaue the enthalpy is negative).

For heavier elements in group 15, As, Sb, Bi all have room temperature solid state structures resembling black phosphorus. The separation between layers decreases down the group and its behaviour becomes more metallic as As and Sb are metalloids and Bi is metallic. High pressure induces increased metallic character, As₄ and Sb₄ molecules exist in the vapour phase.

Group 16 (The Chalcogens)

Oxygen allotropes

Oxygen can form 2 allotropes, O₂ and O₃ and it is the main constituent of air (21%), and also abundant in water and minerals. Dioxygen is a paramagnetic substance and it forms blue liquid dioxygen with melting point of 54 K and boiling point of 90 K. The small atomic radius of O provides a strong π-bonding due to good overlaps of p orbitals.

S_n

Then, sulphur exists in nature as sulphide minerals and volcanic deposits and it also has 2 main allotropes forms, S₈ ring structure (S₆ - S₁₃ rings also known) and polymeric chains of S_n. Besides that, sulphur can have polymorphs structure as yellow solid of monoclinic S₈ and yellow solid rhombic S₈. The scheme below is shown the allotropes and polymorphs of S.

Allotropy and polymorphism of sulphur

Heavier elements in group 16 has varies structures, Se has Se₈ and Se_n allotropes like sulphur, and Te has Te_n chains as Te is metalloid. Lastly, Po is a metallic solid with a simple cubic lattice.

Group 17 (the Halogens) and Group 18 (the Noble Gases)

All elements in group 17 exist as diatomic molecules, X₂. F-F bonds is anomalously weak because it has short bond and lone pairs electrons repel each other and it makes F₂ highly reactive. Besides that, I₂ crystals have metallic lustre and it can conduct electric current at high region (iodine is close to metalloid region) and the solid of I₂ is sublimes. The melting point and boiling point increases as down the group due to stronger van der Waals interaction, and the the bond enthalpy decreases as down the group due to increasing in atomic radius creates less efficient overlap which means weaker covalent bonding.

Meanwhile, group 18 elements are all monoatomic gas and it solidifies at low temperature with close-packed crystal lattice. In general, group 18 elements are inert but heavier elements (Xe and Kr) form some compounds. The main use of group 18 elements is as a light sources such as neon lamps. Moreover, liquid He is used as a cooling agent of magnet in LHC (Large Hadron Collider) and some instrument such as NMR spectoscopy.