Inorganic Chemistry: Periodic Table, Chemical Bonding, Coordination Compounds and Metallurgy
Inorganic chemistry is the study of the elements and their compounds, leaving aside the vast family of carbon compounds that belong to organic chemistry. It explains the arrangement of elements in the periodic table, the trends in their properties, the way atoms combine to form molecules and crystals, the structure and behaviour of coordination compounds, and the extraction of metals from their ores. It is the chemistry of minerals, metals, salts, acids, bases and catalysts found all around us.
For students preparing for APSC, ADRE, SSC, Railways, NEET and JEE in Assam, inorganic chemistry is often the most rewarding section because a large part of it is factual and based on clear rules. Once the periodic trends and bonding principles are understood, many questions can be answered by logical reasoning rather than by memorising isolated facts. This article presents a complete deep-study revision of the subject, covering every topic that appears in competitive examinations.
Basic Terms
- Periodic law: the properties of elements are periodic functions of their atomic numbers.
- Group: a vertical column of the periodic table containing elements with similar outer electronic configurations.
- Period: a horizontal row of the periodic table in which the shell number of the outermost electron remains the same.
- Atomic radius: the distance from the nucleus to the outermost shell, which varies across the periodic table in a regular manner.
- Ionisation enthalpy: the energy required to remove the most loosely bound electron from an isolated gaseous atom.
- Electron gain enthalpy: the energy change when an isolated gaseous atom accepts an electron.
- Electronegativity: the tendency of an atom in a molecule to attract the shared pair of electrons towards itself.
- Ligand: an ion or molecule that donates a pair of electrons to a central metal atom or ion in a complex.
- Coordination number: the number of donor atoms directly bonded to the central metal ion in a complex.
- Lanthanoid contraction: the steady decrease in atomic and ionic radii across the lanthanoid series due to poor shielding of the nucleus.
- Metallurgy: the science and technology of extracting metals in a pure form from their ores.
- Gangue: the unwanted rocky and earthy impurities present in an ore.
Core Concepts and Formulas
- Modern periodic law: properties of elements are periodic functions of their atomic numbers, as given by Moseley.
- Effective nuclear charge equals the atomic number minus the screening constant, and it governs the size and the ionisation enthalpy of an atom.
- Periodic trends: atomic size decreases across a period and increases down a group; ionisation enthalpy and electronegativity increase across a period and decrease down a group.
- Bond order from molecular orbital theory = (number of bonding electrons minus number of antibonding electrons) divided by two.
- Magnetic moment in Bohr magnetons = the square root of n times (n plus 2), where n is the number of unpaired electrons.
- Formal charge = valence electrons minus non-bonding electrons minus half the bonding electrons.
- Effective atomic number of a complex = atomic number of the metal minus electrons lost plus electrons donated by the ligands.
- Oxidation state is found by assigning the usual fixed states to common elements and making the sum equal to the charge on the species.
- Equivalent mass of a metal = atomic mass divided by valency.
- In crystal field theory, the crystal field splitting energy decides the colour and the magnetic behaviour of the complex.
Periodic Classification and the Modern Periodic Table
The periodic table is the map of chemistry. Mendeleev arranged the elements in order of increasing atomic mass and left gaps for elements that had not yet been discovered, predicting their properties remarkably well. Later, Moseley showed that atomic number, not atomic mass, is the fundamental property, and the modern periodic table arranges the elements by atomic number into eighteen groups and seven periods. Elements are further classified as s-block, p-block, d-block and f-block based on the orbital receiving the last electron.
The table is divided into metals, non-metals and metalloids. Metals occupy the left and centre, non-metals the upper right, and metalloids lie along the zig-zag boundary. The noble gases form group eighteen and are exceptionally stable because of their complete outer shells. Understanding the position of an element immediately tells us a great deal about its reactivity, its typical valency and the nature of its compounds.
Periodic Properties and Their Trends
Several measurable properties vary smoothly across the periodic table, and these trends are the basis of countless examination questions. Atomic and ionic radii decrease from left to right across a period because the effective nuclear charge increases while the shell remains the same, and they increase down a group because new shells are added. Ionisation enthalpy and electronegativity follow the opposite pattern, increasing across a period and decreasing down a group.
Electron gain enthalpy is generally more negative across a period and less negative down a group, though the halogens show exceptions because of their small size. Metallic character decreases across a period and increases down a group, while non-metallic character shows the reverse trend. These regularities explain why, for example, caesium is the most metallic of the stable elements and fluorine the most electronegative.
Chemical Bonding and Molecular Structure
Atoms combine to achieve a stable electronic configuration, usually that of the nearest noble gas. The ionic bond results from the transfer of electrons from a metal to a non-metal, the covalent bond from the sharing of electrons between two atoms, and the coordinate bond from the donation of a lone pair by one atom to another. VSEPR theory predicts molecular shapes from the repulsion between electron pairs, while valence bond theory introduces the idea of hybridisation, explaining shapes such as linear, trigonal planar, tetrahedral and octahedral.
Molecular orbital theory, on the other hand, describes bonding in terms of molecular orbitals spread over the whole molecule, and it successfully explains the paramagnetism of oxygen and the existence of the hydrogen molecular ion. Bond order, bond length, bond enthalpy and magnetic behaviour are all deduced from the molecular orbital diagram. Other important ideas include resonance, dipole moment, hydrogen bonding, and the metallic bond in solids.
Coordination Compounds
Coordination compounds contain a central metal atom or ion surrounded by ligands. Werner's theory proposed that metals have a primary valency satisfied by ions and a secondary valency satisfied by ligands, and the secondary valency fixes the coordination number and the geometry. The coordination sphere may be neutral or charged, and the charge is balanced by counter ions. Coordination compounds show a rich variety of isomerism, including geometrical, optical, ionisation, hydrate and linkage isomerism.
Crystal field theory explains why many complexes are coloured and why some are paramagnetic. When ligands approach the metal ion, the d orbitals split into two sets separated by the crystal field splitting energy. The nature of the ligand, its position in the spectrochemical series, and the geometry of the complex decide whether the complex is high spin or low spin. Important applications include the use of complexes as catalysts, in the extraction of metals, in photography, in medicine, and in the detection of metal ions.
Metallurgy and Extraction of Metals
Metallurgy is the extraction of metals from their ores. The first step is concentration, or dressing, of the ore, which removes the gangue by methods such as hand picking, gravity separation, froth flotation, magnetic separation or leaching. Froth flotation is used for sulphide ores, while leaching is used when the ore is soluble in a suitable solvent.
The concentrated ore is then converted to the oxide by roasting (heating in the presence of air) or calcination (heating in the absence of air). The oxide is reduced to the metal using carbon, carbon monoxide, hydrogen or a more reactive metal, or by electrolysis in the case of reactive metals. Finally, the crude metal is refined by distillation, liquation, electrolytic refining, zone refining or vapour phase refining. Slag and flux are used to remove acidic and basic impurities respectively, and the choice of reducing agent depends on the position of the metal in the reactivity series.
s-Block, p-Block, d-Block and f-Block Elements
The s-block elements, groups one and two, are highly reactive metals. Alkali metals react vigorously with water, show a strong tendency to lose one electron, and form basic oxides. Alkaline earth metals are somewhat less reactive and form sparingly soluble sulphates and hydroxides. Both groups exhibit characteristic flame colours, which are used in qualitative analysis and in fireworks.
The p-block spans groups thirteen to eighteen and contains metals, metalloids and non-metals. Important families include the boron family, the carbon family, the nitrogen family, the oxygen family and the halogens. Their compounds range from the inert noble gases to the highly reactive halogens and the industrially important nitrogen and sulphur compounds. The d-block, or transition metals, are characterised by variable oxidation states, coloured ions, catalytic activity, paramagnetism and the formation of complexes. The f-block elements, the lanthanoids and actinoids, include the rare earths and the radioactive actinide series.
Qualitative Analysis and Salt Analysis
Qualitative analysis identifies the ions present in a salt. The cation is usually detected through the colour and solubility of its salts, flame test, and reactions with group reagents such as dilute hydrochloric acid, hydrogen sulphide in acidic and basic media, ammonium sulphide and ammonium carbonate. The anion is identified by tests involving dilute and concentrated acids, barium chloride, silver nitrate, and lead acetate, along with the characteristic gases evolved.
A systematic scheme is followed: first the preliminary tests, then the detection of anions, and finally the detection of cations by grouping them according to their precipitation behaviour. Colour, smell, solubility and the nature of the gas released are important clues. This topic rewards careful observation and a clear memory of the standard tests.
How Numerical Problems Are Solved
Most numerical problems in inorganic chemistry are short and rule-based. To calculate the oxidation state, write the species, assign the usual states to familiar atoms such as oxygen and hydrogen, and solve for the unknown. To find the magnetic moment, first determine the number of unpaired electrons from the electronic configuration, then apply the spin-only formula. For coordination compounds, identify the ligand, the coordination number and the oxidation state, and use the spectrochemical series to decide the magnetic behaviour. For metallurgy problems, balance the extraction reaction and use equivalent masses to relate the mass of ore to the mass of metal. Always write units and check whether the answer lies within a reasonable range.
Examination Trends
In state-level recruitment examinations, inorganic chemistry questions are mostly factual: the periodic table and its trends, chemical bonding, common compounds, uses of metals and everyday inorganic chemistry. In NEET and JEE, questions test deeper understanding, including molecular orbital theory, hybridisation, coordination chemistry, the lanthanoid contraction and the mechanism of extraction. Assertion-reason and matching-type questions are common. Numerical work is usually limited to oxidation states, magnetic moments and simple stoichiometry, making accuracy far more important than speed.
Common Mistakes to Avoid
- Confusing atomic number with atomic mass while arranging elements.
- Reversing the trends of atomic size, ionisation enthalpy or electronegativity.
- Forgetting that half-filled and fully filled configurations have extra stability.
- Mixing up the number of unpaired electrons with the total number of d electrons.
- Ignoring the charge when balancing the coordination sphere of a complex.
- Using the wrong method of concentration, such as froth flotation for an oxide ore.
- Confusing roasting with calcination, which differ in the presence or absence of air.
- Writing reaction products without balancing the equation or without the correct state symbols.
How to Prepare This Topic
- Learn the periodic table thoroughly, including the position, symbol and electronic configuration of the common elements.
- Master the periodic trends, because they explain a very large number of questions without rote learning.
- Practise drawing molecular shapes and molecular orbital diagrams, and relate them to bond order and magnetism.
- Build a clear picture of coordination compounds, including nomenclature, isomerism, bonding theories and applications.
- Revise the extraction processes and the standard salt analysis tests, and attempt previous years' papers under timed conditions.
Frequently Asked Questions
State the modern periodic law.
The properties of elements are periodic functions of their atomic numbers. This law was given by Henry Moseley and forms the basis of the modern periodic table, which has seven periods and eighteen groups.
Why does atomic size decrease across a period?
Because the nuclear charge increases while the electrons are added to the same shell, the effective nuclear charge pulling the electrons inward increases, so the atom becomes smaller from left to right across a period.
What is a coordination compound?
A coordination compound contains a central metal atom or ion surrounded by ligands that donate electron pairs. The central atom and its ligands form the coordination sphere, whose charge is balanced by counter ions.
What is the difference between roasting and calcination?
Roasting is the heating of an ore in the presence of air to convert a sulphide ore into its oxide. Calcination is the heating of an ore in the absence of air to remove volatile impurities and to convert a carbonate or hydroxide ore into its oxide.
How is the magnetic moment of a complex calculated?
By first finding the number of unpaired electrons n from the electronic configuration or the crystal field diagram, then applying the spin-only formula, which is the square root of n times (n plus 2) Bohr magnetons.