Group VIA elements and their physical and chemical properties

Group IVA elements Carbon family

Group IVA comprises carbon (C), silicon (Si), germanium(Ge), tin (Sn) and lead (Pb).

 Physical properties of elements

Melting and boiling points.

Melting and boiling point of group VIA elements decreases down the group. Although decrease in m.p. is not regular order .This decrease in melting and boiling point is due to decrease in inter-atomic forces. 

Question: Why melting point and boiling point of carbon and silicon are higher than other elements?

Answer. The melting and boiling points of carbon and Si is notably higher due to their higher tendency to form giant structure.  

Question: Why a melting point of pain is lower than led?

Reason: The melting point of sn is lower than lead due to formation of distorted 1,2-coordinated structure rather than pure.

Trend from non metal to metal and group 4 elements

The change from non metallic to metallic character is due to increase in atomic size. First two elements, carbon and silicon are non metal although electrical properties of silicon are similar to metalloid (semi metal). The third element geranium is partly metal and partly non metal while last element tin Sn and lead Pb are totally metal. 

i.  Carbon a d silicon------------------non metals    

ii.  Germanium ------------------------ metalloid

iii. Tin and lead ------------------------- metals                                   

Oxidation States

Group VIA elements exist in two oxidation states:  +2  and +4. In carbon and silicon +4 oxidation state is more stable as compared to +2. The + 2 oxidation state is rare and is easily oxidized to + 4. The steady increase in stability of lower oxygen state then higher oxidation state on moving from top to bottom in the group is due to inert pair effect.

Possible oxidation states

Generally valence electronic configuration of group IVA elements is ns2 and np2. Carbon and silicon show + 4 oxygen state while Germanium Tin and lead show + 2 and + 4 oxidation state. We get +2 oxidation state of Ge, Sn and Pb due to loss of np2 electrons. ns2 electrons remain inert and does not take part in bond formation. This ns2  electrons are called inert pair of electron and this effect is called innert pair effect. OR

Non involvement of ns2 electron in bond formation is called innert pair effect.

 The stability order of +2 oxidation state

C²⁺<Si²⁺<Ge²⁺<Sn²⁺<Pb²⁺

The stability order of +4 oxidation state

C⁴⁺>Si⁴⁺> Ge⁴⁺>Sn⁴⁺>Pb⁴⁺

Oxidizing and reducing properties

When group IVA elements lose electrons from ns² and np² and M⁺⁴  cations are formed. On going from top to bottom in the group stability of M⁺⁴ decreases from Ge to Pb.

Ge⁴⁺>Sn⁴⁺>Pb⁴⁺

<Ge²⁺<Sn²⁺<Pb²⁺

Compounds of Ge²⁺ is less stable than Ge⁴⁺  therefore, Ge²⁺ acts as a strong reducing agent. Similarly Sn²⁺ is less stable than Sn⁴⁺ , so Sn²⁺ acts as a strong reducing agent.

e.g., 

Ge²⁺ -------- >   Ge⁴⁺    +  2e-

Sn²⁺ -------- >   Sn⁴⁺    +  2e-

When we compare the stability of Pb²⁺  and Pb⁴⁺is less stable than Pb²⁺ , so

Pb⁴⁺  +   2e-  --------- >  Pb²⁺  

So, Pb⁴⁺ is strong oxidizing agent.

Negative Oxidation State

Since electronegativity of these elements are very low so they don't form negative ions. However, carbon forms -4 and -2 ion in certain compounds. For example , Na2C2, CaC2

Nature of M²⁺ and M⁴⁺ compound

Nature of M²⁺ and M⁴⁺ compound can be predicted by Fajan's rule.

Fajan's rule states at cation with high charge density makes bond more covalent. According to Fajan's rule, molecule containing M⁴⁺ (high charge density) are covalent in nature and molecule containing M²⁺ (low charge density) are ionic in nature.

Question: SnCl₂ is ionic and SnCl₄ is ionic in nature, why?

 Answer. In SnCl₂ the charge on Sn is +2, i.e., Sn²⁺ which has low charge, so Sn²⁺form ionic compound while in SnCl₄ charge on Sn is + 4 i.e.Sn⁴⁺which has high charge density, Sn⁴⁺ forms covalent compound.

Question: Why PbCl₂ is more stable than PbCl₄?

Answer.  PbCl₂ is more stable than PbCl₄ because in PbCl₂ , Pb is present as Pb²⁺ that is more stable and has high ionic character.  On the other hand, in PbCl₄ , Pb is present as Pb⁴⁺ that is less stable and has high covalent character. e.g.,

    Pb⁴⁺    +     2e-         ------- >        Pb²⁺

^{(less stable)                                         (more stable)}  

   PbCl₄        --------  >     PbCl₂   +    Cl₂ 

(less stable)                    (more stable)  

Stability

At the top of group IVA,  carbon and silicon have + 4 oxidation state in PbCl₄ and PbCl₄, respectively, Both have no tendency to be decomposed to dichloride (+2 oxidation state) that is is less stable.  Thermal stability of tetrahalides of  group IVA decrease  down the group.

Hydrolysis of Tetra halides of group 4group IVA elements

Hydrolysis of Tetra halides of group 4group IVA elements takes place in two steps.

Step 1:

In this type oxygen of water donates and electron pair to central atom to form coordinate covalent bond and MX₄.H₂O is formed as intermediate which is unstable.

Step 2:

In second step, 4HX molecules are eliminated from this unstable intermediate and hydroxide of central metal atom is formed.

Question: Why tetrahalide  of carbon are not hydrolyzed while those of Si, Ge, Sn and Pb get readily hydrolyzed?

Answer

Tetra halide of carbon are not hydrolyzed because carbon has no d orbital in its valence shell and hence it is unable to accommodate lone pair of electrons donated by oxygen atom of water molecule to form  intermediate compound. Thus, CCl₄ is not hydrolyzed.

On the other hand, halides of  Si, Ge, Sn and Pb have vacant d orbitals in their valence shell to accommodate (accept) lone pair of electrons and therefore they are readily hydrolyzed.

Trends of hydrolysis

The ease with which tetrahalide are hydrolyzed by water decreases from Si to Sn. As the metallic character central atom increases from Si to Sn. Therefore GeX4 and SnX4 for not easily hydrolyzed than SiX4.  Empty orbitals are always available with any atom and they can you be utilized if sufficient energy is provided for the reaction to occur.

For example

CCl₄  +   H2O   ------------ >  COCl₂   +   2HCl

The hydrolysis of pbcl4 also takes place in the same manner.

CCl₄   +  H2O -------- >   PbO2   + HCl

PbCl₄ is also decomposed to PbCl₂ along with this reaction.

Complex formation

Except carbon, all elements form complex halides and and all form hexahelo complex. 

SiF4    +   2F-   ------------- >[SiF6]-2

Oxides

Group IV elements form three types of oxides. 

1. Monoxides (MO)

2.  Dioxide   (MO2)

3. Other oxides (C3O2, Pb3O4, Pb2O, Pb2O3)

Dioxides               

Carbon dioxide 

Structure 

i. It is a triatomic simple molecule with covalent bond.

ii. CO2 is a gas 

iii. It has two double bonds between carbon and oxygen.

iv. CO2 is linear molecule and net dipole moment is zero.

v. Solid CO2 has face Centre cubic structure 

vi. Bond length between carbon and oxygen is 115 p.m. 

vii. All atems have complete octet, so it is a stable molecule.

SiO2

Structure 

i. It has giant structure with covalent  bond.

ii. Sio2 is a solid.

iii. It has single bond between  silicon and oxygen atom.

iv. Both Si and O are sp3 hybridized.

v.  SiO2 has a  tetrahedral structure and dipole moment zero.

Question: CO2 is a gas and SiO2 is a solid. Why?

OR

 Why physical properties of CO2 and SiO2 are different?

Reason: This is due to the following reasons.

i. Silicon atoms are much larger in size than carbon atoms and  tend to be surrounded by more than oxygen atoms. 

ii. Silicon forms only single bond while carbon farm double bond number.

iii. CO2 form simple molecule which have weak wonder-wall force of attraction while  Si is rounded by 4 oxygen atoms in tetrahedral structure. So Si forms giant structure containing covalent bond throughout the network.

Acid base  behavior of group IV  oxides

 oxides after elements of group 4 elements are acidic and thus acidity decreases down the group and basic of the society increased on the group

Reaction with water reaction with base Germanium 10 and lead oxide monoxide basic nature acidic nature dioxide basics nature is the nature

 ⅚