- Include one nonmetal, Boron (B), and Four elements that are metallic in their properties: Aluminium (Al), Gallium(Ga), Indium (In) and Thallium (Tℓ).

- The elements in Grp 13 have the general configurations ns2np1 i.e., one electron in the outermost p-orbitals and two electrons in the s-orbital.

Group 13 (IIIA)

Occurrence

- Aluminium is most abundant metal. It is the third most abundant element (after oxygen and silicon) by weight in the earth’s crust. It is a major constituent of the common aluminosilicate rocks such as feldspars and micas. The most abundant ore of aluminium is bauxite Al2O3. H2O.

- Boron is quite rare element but is well known because it occurs as concentrated deposits of borax Na2B4O7.10H2O and kernite Na2B4O6(OH)2 .3H2O. - Gallium, indium and thallium are very much less abundant and occurs in traces in sulphide minerals rather than oxides. Small amounts of gallium are also found in the ores of elements adjacent to it in the periodic table (Al, Zn and Ge). Traces of In and Tℓ are found in ZnS and PbS ores.

Physical properties of these elements are shown in the table below:

Boron's chemistry is so different from that of the other elements in this group.

Chemically, boron is:

- A nonmetal (tendency to form covalent bond);

- More similar with carbon and silicon than with aluminum and the other members of his group;

- Like carbon, boron forms many hydrides (Boranes): BH3, BxHy;

- Like silicon, it forms oxygen-containing compounds (borates);

- Extremely difficult to purify.

The pure element has many allotropes (different forms of the pure element), many of which are based on B 12 unit.

Boron (B)

In the boron hydrides, hydrogen forms a bridge between boron atoms. For example:

- Diborane has 12 valence electrons, eight of these electrons are involved in bonding to the terminal hydrogens. Thus, four electrons remain for bonding in the bridges.

This type of bonding (bridge), involving three atoms and two bonding electrons per bridge, is described as three-center, two-electron bonding.

- Similar bridging hydrogen atoms occur in many other boranes, as well as in carboranes (contain both boron and carbon atoms) . - In addition, bridging hydrogens and alkyl groups are found in aluminum chemistry. - A few examples of these compounds are shown in the following figure:

Similarities between Boron-nitrogen chemistry and carbon chemistry:

1/ Because of the similarity in the structures (see figures below) there are some similarities between the organic molecule benzene C6H6 and the borazine, B3N3H6 specially in their physical properties.

Their chemistry is quite different.

2/ Another similarity is between Boron nitride, BN and Carbon :

Like Carbon, Boron nitride exists in:

a/ A diamond-like form:

Cubic and each nitrogen coordinated by four borons and each boron by four nitrogens.

BN comparable to diamond in hardness.

b/ Graphite –like form

Boron has two stable isotopes: 11B (80.4%) and 10B (19.6%). 10B is a good absorber of neutrons. This property has been use in the treatment of cancer in a process called boron neutron capture therapy (BNCT).

Boron Isotopes:

Other Chemistry of the group 13 (IIIA) Elements:

Metallic Characters and Oxidation States:

- Due to high ionisation energy the elements of Grp 13 are less electropositive as compared to elements of Grp 2. On moving down the group, the electropositive (metallic character) increases because ionisation energy decreases. For example, Boron is a non-metal, while the other elements are typical metals.

- Boron shows only +3 oxidation state in its compounds. Except Boron, other elements also

show +1 oxidation state. The +1 oxidation state becomes more stable as we move down the group from Boron to Thallium. In case of last element , Thallium , +1 oxidation state has been found to be more stable than +3 oxidation state.

This is attributed to “inert pair effect”. In the case of last element, after the removal of one electron from p-orbital, the remaining 6S2 electrons behave like stable noble gas and do not take part in compound formation. The reluctance of the s-electron pair to take part in chemical combination is called “inert pair effect”. This helps to explain the stability of lower oxidation states for the heavier elements of a group.

Reactivity of Group 13 (IIIA)

- The group 13 elements form the hydrides of the type MH3, whose thermal stability decreases as we moves down the group.

- The group 13 elements react with halogens to

give halides of the type MX3. - All elements of this group 13 form oxides of

general formula M2O3 and hydroxides M(OH)3.

GROUP 14 (IVA)

- The group 14 of the periodic table contains five elements Carbon(C), Silicon(Si ), Germanium (Ge), Tin (Sn) and Lead (Pb). This group is known as carbon family.

- The atoms of these elements have four

electrons in the outermost shell , 2 in S and 2 in P-subshell.

Occurrence

- All the elements are well known except Germanium. Carbon occurs both as free element (graphite or diamond) and in the combined form mainly as carbonates of Ca, Mg and other electropositive elements. It also occurs as CO2, an important constituent of atmosphere. Carbon is seventeenth in the order of abundance in earth’s crust.

- Germanium occurs rarely. Germanium minerals are extremely rare but the element is distributed in trace amounts in coal and zinc carbonates. - Tin occurs as cassiterite, SnO2 and lead is found as galena, PbS.

- Silicon is the second most abundant element in the earth’s crust after oxygen as silica (sand or quartz) and silicates.

Physical properties of these elements are shown in the table below:

Metallic characters increases down the group:

- Carbon is a nonmetal

- Silicon and germanium are semi-metals

- Tin and Lead are metals.

- Group 14 elements can exhibit two oxidation state, which +2 and +4.

- Although +4 is the group characteristic oxidation state, the +2 state becomes more stable down the group.

The increasing stability of the +2 state is another example of the so-called "inert-pair effect“.

- Stable Carbon and Silicon compounds have +4 state.

- At Ge and Sn, both +2 and +4 states are stable.

Mettalic Characters and Oxidation States:

Carbon occurs as the isotope: 1/ 12C ( main isotope) 2/ 13C and 3/ 14C 13C, can be used as the basis of NMR observations for the characterization of carbon-containing compounds. . 14C is formed in the atmosphere in very small amounts from nitrogen .

- At Pb, the bond strength is too low to compensate for the slightly higher ionization energy requirement of the Pb(IV) state in many cases. Hence, the +2 state is favored.

Isotopes of Carbon:

14c entered into plant and animal tissues by biological processes. When a plant or animal dies, the exchange of its carbon with the environment by respiration and other biological processes stops, and the 14C in its system is trapped. Therefore, by measuring the remaining amount of 14c, one can determine to what extent this isotope has decayed and, in turn, the time passed since death. called "radiocarbon dating," used to estimate the ages of many samples.

- Carbon's two naturally occurring allotropes are graphite and diamond, with graphite the more stable form.

- Graphite is the stable form because it has shorter and stronger bonds, owing to π-delocalization in the hexagonal rings that comprise the sheets. - Diamond being a more ordered structure.

Fullerenes:

- A fullerene is a molecule of carbon with the formula C2n (2n = 20, 60, 70, 76, 78, 82, 84, 96) in the form of a hollow sphere, ellipsoid, tube, and many other shapes.

C70 C60

- Carbon nanotubes are prepared by arc-evaporating graphite

- Nanotubes are metallic, semiconducting, or insulating, depending on their preparation.

It was wrongly believed that carbon can form only 4 b0nds, many examples are now known in which carbon can coordinate to 5, 6, or more atoms.

Five-coordinate carbon is common, with methyl and other groups usually forming bridges between two metal atoms, as in A12(CH3)6 and in organometallic compounds. Examples of carbon atoms having coordination numbers of 5,6,7, and 8 are shown in the figure below.

They are colorless, odorless gases.

Carbon monoxide CO:

- Stable compound;

- Carbon formally only three bonds;

- Extremely toxic, forming a bright red complex with the iron in hemoglobin, which has a greater affinity for CO than for O2.

Carbon dioxide CO2:

- Fifth in abundance, after nitrogen, oxygen, argon, and water;

- Formed as the product of respiration, combustion, and other natural and industrial processes;

- It was the first gaseous component to be isolated from air and it has many industrial uses.

Oxides of carbon:

Carbon forms several anions, especially with the most electropositive metals called the carbides, with covalent as well as ionic bond.

Classifying carbides is not simple, but some useful categories are:

1/ saline (salt-like) (Methynide, C4-)

Example: Be2C.

- Prepared by heating the two elements at high temperature;

- React with water (hydrolysis) to gives CH4.

2/ Those containing the [C2]2- ion (Acetylide)

Example: Na2C2 , MgC2 and CaC2

- Hydrolsis gives acetylene C2H2.

Carbides:

4/ Interstitial carbides: - Formed by heating C with d-block metals Example: WC. - Very hard (WC used as cutting tool); - Do not react with water.

3/ Those containing (C3)4- ion: - They are rare. Example: Mg2C3. - Hydrolysis gives propyne

- These carbides, as indicated, liberate organic molecules on reaction with water. For example:

Calcium carbide, CaC2, used to prepare acetylene for lighting and welding.

- Silicon, with its semimetallic properties, is very important in the semiconductor industry, with wide applications in computers and solar energy. - Silicon dioxide (Silica), SiO2, occurs in sandstone and granite and as the major component of glass. - A number of hydrides, called Silanes, are known and are alkane analogues.

Silicon

Silanes (SinH2n+2) only up to eight silicon atoms in length prepared.

- SiH4 is stable and, like methane, tetrahedral.

- The stability decreases with chain length; Si2H6 (disilane) undergoes only very slow decomposition, while Si8H18 decomposes rapidly.

- Few compounds containing Si=Si bonds have been synthesized.

- There is no chemistry of multiply bonded Si species comparable with the unsaturated organic compounds.

Silanes:

Stannanes: Have SnH4 and Sn2H6

Plumbane: Possibly, PbH4.

Germanes : Formulas GeH4 to Ge5H12 have

been made.

Silanes are believed to decompose by elimination of :SiH2 by way of a transition state having a bridging hydrogen (see below). This reaction, is used to prepare pure silicon.

- The elements germanium, tin, and lead show increasing importance of the 2+ oxidation state (inert pair effect). - For example, all three show two sets of halides, of formula MX4 and MX2. - For germanium, the most stable halides have the formula GeX4. - For lead, it is PbX2, the dihalides is the most stable.

Why are the Silanes less stable (more reactive than the corresponding hydrocarbons) ?

1/ The Si-Si bond (222 kj/mol) is weaker than the C-C bond (347 kj /mol)

2/ Si-H bonds (393) are weaker than C-H bonds (413).

3/ Silicon is less electronegative (1.92) than hydrogen (2.30) and is, therefore, more susceptible to nucleophilic attack, whereas carbon, is more electronegative (2.54) than hydrogen.

4/ Silicon atoms are also larger than carbon atoms and therefore provide greater surface area for attack by nucleophiles.

5/ Slicon atoms have d-orbitals that can accept electron pairs from nucleophiles (no d-orbitals in carbon).

- Similarly, Germanes, Stannanes, and Plumbanes are more reactive than hydrocarbons.