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Metal deficiency defect is a crystal defect that arises due to the absence of anions or the presence of extra cations at the interstitial sites. Metal complexes with a variable valency can have this form of metal defect. Therefore, metal defects can be classified into two types:
- Metal excess defect and
- Metal deficient defect
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Key terms: Metal, Anion, Cation, Ions, FeO, NiO, FeS, Alcohol, Alkali, Valency, Solid, Crystal, Metal defect
Metal Deficiency Defects
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Metal deficiency defect occurs when a vacant lattice site is generated by a missing cation or anions at interstitial sites. Compounds with variable valencies, such as transition metal compounds like NiO, FeS, FeO, and others, are prone to this defect due to their potential to accept an excess positive charge.
| Main aspects of metal deficiency defects are:
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The video below explains this:
Imperfections in Electrical and Magnetic properties of Solids Detailed Video Explanation:
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| Related Articles | ||
|---|---|---|
| Schottky Defect | Vacancy Defect | Interstitial Defect |
| Impurity Defects | Electron Valency | Ferromagnetic |
| Packing Efficiency | Imperfections in Solids | Magnetic Properties |
Types of Metal Deficiency Defects
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Metal Deficiency defects can arise due to two main reasons. They are:-
- Cation Vacancies
- Extra anions occupying interstitial states
Due to Cationic Vacancies
Cationic Vacancies arise when one of the positive ions leaves the lattice site and the additional negative charge balances by receiving two positive charges instead of just one.
| Cation Vacancy defect is usually located in compounds of transition metals that have variable valency or oxidation states. FeO and FeS show this kind of metal deficiency defect. |
FeO is a crystal with a metal shortage. The metal scarcity in FeO occurs due to the fact that the constructive ion present in the crystal is smaller in size than the -ve ions.
- Cationic vacancies occur when the cation is not present in its original Lattice position.
- There are defects where the lattice position of the cation is vacant but the number of cations and anions remains balanced as the missing anions and cations will both be equal or the cation will be occupying an interstitial position.
- The cationic vacancy is mainly due to the absence of the cation in the crystal which leads to an imbalance of charge.
- In order to balance this charge, the neighbouring cation gains two positive charges, one more than the usual.
- This eventually balances the charge on the crystal.
- However, there will be less number of metal ions as compared to the crystals of the vacancies, resulting in a change in the crystal’s composition.
- The defect can only occur when the metal ion has two or more oxidation states.
- Therefore, this defect is possible with the elements of d-block.
Due to Extra Anions Occupying the Interstitial Sites
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Metal Deficiency Defect also occurs due to extra anions occupying the interstitial sites.
- In this case, the additional positive charges might be in the interstitial positions.
- The additional negative charge in the lattice is balanced by the additional charges at the adjoining metal ions.
- This type of metal deficiency defect is not common as negative ions are normally very big and cannot adjust in the interstitial sites.
Solved Example Based on Metal Deficiency DefectsExample 1: The Fe0.94O (nonstoichiometric compound) is formed when the x% of Fe2+ ions are adjusted or replaced by 2/3x% of Fe3+ ions. Determine the value of x. Solution: x% of Fe2+ ions are taken by Fe3+ ions. No. of Fe3+ that are replacing the Fe2+ ions is two-thirds of x, which gives us vacancies of cations. Vacancies of cations = x - 2/3x = 1/3x. Fe0.94O is the formula of the stoichiometric compound. x/3 = 1-0.94 x = 0.06 * 3 x = 0.18 x = 0.18 *100 = 18%. |
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Things to Remember
- Metal deficiency defect is a crustal defect caused due to the absence of anions or presence of excess cations at interstitial sites.
- Metal Deficiency defects can arise due to two main reasons. They are:-
- Cation Vacancies
- Extra anions occupying interstitial states
- Metal Deficiency defect is a kind of disorder that occurs in compounds wherein the metal can show off variable valency. For example- Transition metallic compounds.
- A vacant lattice site formed by a missing cation causes a metal deficiency defect.
- Metal Deficiency defects also occur when additional +ve charges take the interstitial positions.
- The types of non-stoichiometric defects are - Metal Excess defects and Metal Deficiency defects.
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Sample Questions
Ques: Which point defect in a solid's crystal units increases its density? (Delhi 2009, 1 Mark)
Ans: A metal excess defect increases a solid density. The presence of extra cations in the interstitial sites is to blame.
Ques: Why does FeO have a metal deficiency defect? Explain. (2 Marks)
Ans: A -ve charge is lacking from its lattice site. To preserve electric neutrality, one of the nearest metallic ions acquires a further +vecharge. This form of disorder occurs in compounds wherein the metal can show off variable valency like Transition metallic compounds. Fe has variable valency.
Ques: What are the causes of mental deficiency defects? (2 Marks)
Ans: The main reason is the missing negative charges and because of this some metal ions in the nearest vicinity get an additional +ve charge in order to maintain the electrical neutrality. In order for this to happen the metal should exhibit variable vacancy.
Ques: What is a metal deficiency defect? Explain with an example? (2 Marks)
Ans: Transition metals like FeO, NiO, FeS exhibit these kinds of defects. It occurs due to an extra anion in the interstitial site. The additional -ve charge is balanced with the aid of using the additional charges at the adjoining metal ions. This is to maintain Electrical neutrality.
Ques: How many types of non-stoichiometric defects are there? (2 Marks)
Ans: Inorganic solids which are nonstoichiometric have the elements in a ratio that is nonstoichiometric due to these defects present in their crystal structures. The types of nonstoichiometric defects are:
- Metal Excess Defect
- Metal Deficiency Defect
Ques: What is the difference between a Metal deficiency defect and Metal Excess Defect? (2 Marks)
Ans: Metal excess defect and metal deficiency defect are two types of defects that can be found in the crystal lattices of certain substances. The primary distinction between metal excess and metal deficiency defects is that metal excess is caused by anionic vacancies and extra cations in the interstitial sites, whereas metal deficiency is caused by cationic vacancies and extra anions in the interstitial sites.
Ques: Explain how you can determine the atomic mass if you know metal if you know its mass density and the dimensions of a unit cell of its crystal. (All India 2011, 3 Marks)
Ans: Let the edge of the unit cell = a pm
Number of atoms present per unit cell = Z
∴ The volume of unit cell = (a pm)3
= (a × 10-10 pm)3 = a3 × 10-30 pm3
The density of unit cell = Mass of unit cell x Volume of unit cell ……………… (i)
Mass of unit cell = Number of atoms in the unit cell × mass of each atom
= Z × m
Mass of each atom = Atomic mass x Avogadro's no’s= M N0
Substituting these values in equation (i), we get
The density of unit cell = Z × M x a3 × 10-30 pm3 × N0
If a is in cm, d = Z × M x a3×N0 g/cm3
∴ Molar mass can be calculated as
M = d×a3×N0 Z
Ques: Do compounds like NaCl show a metal deficiency defect? (3 Marks)
Ans: No, they exhibit metal excess defects. When NaCl crystal is heated where Sodium vapour is present, The Na atoms get settled on the crystal surface. And the Chlorine ions diffuse and combine with Na from NaCl on the crystal surface. This occurs due to the electron loss by Na to form Na+. The electrons that get diffused occupy the anionic places.
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