6.2.2 Physical Characteristics

In the previous section, we looked at the general physical properties of transition metals. Now, we will examine the specific trends in these physical characteristics across the first row of the d-block elements. Understanding these trends—particularly in density, atomic radii, melting and boiling points, and magnetic properties—provides deeper insight into the nature of metallic bonding in these elements.

Main Idea

The main idea is that the physical characteristics of transition metals show distinct trends across the period (from scandium to zinc). These trends are not always smooth, as they depend on the complex interplay of increasing nuclear charge, the filling of the 3d subshell, and the resulting changes in metallic bond strength.

Core Concept

The core concept is that the physical properties of transition metals are primarily determined by their electronic configuration and the strength of their metallic bonds. As you move across the period, the number of d electrons increases, which affects the number of unpaired electrons available for bonding and the shielding of the nucleus. This leads to trends in properties like density, atomic radius, and melting point. However, anomalies occur at certain elements (like manganese and zinc) due to the special stability of half-filled and fully-filled d subshells.

Detailed Explanation

The textbook provides a detailed look at several physical characteristics of the first row of transition metals.

1. Density

• Definition: Density is the mass per unit volume expressed in the unit gram per cubic centimetre (g/cm^3). It is an important physical property of metals which shows how closely the metal atoms are packed in a metallic lattice.
• General Trend: Moving through the first period of transition metals, from scandium to copper, the density of transition metals generally increases.
• Reason for the Trend: This is because of two factors:
• Exceptions: There are some exceptions to the general trend. The density of manganese and copper is different from what is generally expected. The atomic radius of zinc is larger than that of copper because its full 3d sub-shell (3d¹⁰) leads to less tight packing of zinc atoms in its metallic lattice, resulting in reduced density.

2. Atomic Radii

• General Trend: In the first-row transition elements, the atomic radii decrease across the period because the nuclear charge increases, pulling electrons closer to the nucleus.
• Key Difference from Main Group Elements: Unlike main group elements, this decrease is less steep because electrons being added in the 3d sub-shell do not shield one another sufficiently from the nuclear charge.
• Detailed Trend:
• In the beginning, the atomic radii decrease because of increasing nuclear charge which overcomes the shielding effect.
• The decrease in atomic radii is not that pronounced while going through the middle of the period because of the two factors, i.e., increasing nuclear charge and shielding effect, counterbalance each other. In this way, the atomic radii remain almost constant at the centre of the period.
• Moving towards the end, atomic radii increase from nickel to copper and zinc. The reason is that the 3d sub-shell becomes full which causes shielding effect, pushing 4s sub-shell away from the nucleus. Similarly, when moving from copper to zinc, we see an unexpected increase in atomic radius. The atomic radius of zinc is larger than that of copper because the additional electron in the 4s sub-shell in zinc increases the shielding effect, which outweighs the effect of the higher nuclear charge.

3. Melting and Boiling Points

• General Trend: The melting and boiling points of the transition elements are generally much higher than those of the s- and p-block elements. They have strong metallic bonding because of the involvement of electrons of both 3d and 4s sub-shells.
• Irregular Behaviour: When we plot melting and boiling points against atomic number, both graphs have some irregular behaviour.
• In the beginning, melting and boiling points increase as atomic number increases.
• Then we get two minima at manganese which reflect that the full 4s and half 3d sub-shells are much stable and not readily available for metallic bonding.
• Zinc Anomaly: Zinc has the lowest melting and boiling points because of full 3d and 4s sub-shells which cannot participate in metallic bonding. This is why mercury is liquid at room temperature as it belongs to the zinc family (with full 3d and 4s sub-shells).

4. Magnetic Properties

• Cause: An electron spinning around its own axis generates a magnetic field just like an electric current flowing through a wire does. It means that a single unpaired electron acts like a small magnet. Based on unpaired and paired electrons in outermost shells, transition metals and their compounds show different magnetic properties.
• Paramagnetic Substances: These are weakly attracted in a magnetic field. For example, aluminium and sodium. The reason is that they have unpaired electron(s) in valence shells. The unpaired electrons on atoms or ions are not influenced by the electrons on surrounding atoms so they are directed randomly. When a paramagnetic solid is placed in a magnetic field, the magnetic moments are aligned parallel to one another and a net weak attraction is observed. However, when the magnet is removed, the solid loses its paramagnetic properties.
• Ferromagnetic Substances: These are strongly attracted in a magnetic field, for example, iron, cobalt, and nickel. They have also unpaired electrons in the valence shells of their atoms and ions in solid states. Their unpaired electrons are influenced by the electrons of surrounding atoms and ions and align in the same direction. When a ferromagnetic substance is placed under a magnet, a very strong attraction is observed—nearly a million times more than a paramagnetic substance. Ferromagnetic substances maintain their magnetic moment even after removal of the magnetic field, so they are permanent magnets.
• Diamagnetic Substances: These are weakly repelled by a magnet because they have paired electrons in outermost shells. For example, copper and iron. The paired electrons in an orbital cancel out their individual magnetic moments making net magnetic moment zero. (Note: The textbook mistakenly lists iron as a diamagnetic substance. Iron is actually ferromagnetic. This is a factual error in the source material.)

5. Alloys

• Definition: Alloys are mixtures of metals which carry useful properties.
• Formation: Transition metals can form alloys because of their similar atomic size. They can substitute one another in their metallic lattices.
• Examples:
• Steel: An alloy of iron, chromium, nickel, and manganese possessing more useful properties, like hardness, corrosion resistance, and shiny.
• Brass: An alloy of copper and zinc.
• Bronze: An alloy of copper and tin.

Intermediate Level

The physical characteristics of the first-row transition metals show clear trends that are linked to their electronic structure. The density increases across the period because the atomic mass increases and the atomic radius decreases, leading to more closely packed atoms. The atomic radius decreases initially due to increasing nuclear charge, but the shielding effect of the 3d electrons causes the radius to level off in the middle of the period and then increase slightly at the end (Ni to Zn) as the 3d subshell becomes full. The melting points are high due to strong metallic bonding involving both 3d and 4s electrons. The maximum melting point occurs around vanadium and chromium, where the number of unpaired electrons available for bonding is highest. There is a dip at manganese because of the stable half-filled 3d⁵ configuration. Zinc has the lowest melting point because its full 3d and 4s subshells are not available for metallic bonding. The magnetic properties depend on the number of unpaired electrons. Iron, cobalt, and nickel are ferromagnetic and can be permanently magnetized, while others are paramagnetic or diamagnetic.

Important Points

• - Density: Generally increases across the period (exceptions: Mn, Cu, Zn).
• - Atomic Radii: Decrease initially, remain nearly constant in the middle, then increase at the end (Ni to Zn).
• - Melting/Boiling Points: High, with a maximum around V/Cr and a minimum at Mn. Zn has the lowest.
• - Magnetic Properties: Paramagnetic (weakly attracted, unpaired electrons), Ferromagnetic (strongly attracted, permanent magnets, Fe/Co/Ni), Diamagnetic (weakly repelled, paired electrons).
• - Alloys: Formed due to similar atomic sizes (steel, brass, bronze).
• - Zinc Anomaly: Lowest melting point due to full 3d and 4s subshells.
• - Manganese Anomaly: Dip in melting point due to stable half-filled 3d⁵ configuration.

Remember

• - Density: Increases across the period.
• - Atomic radii: Decrease, then flat, then increase.
• - Melting point: High, dip at Mn, lowest at Zn.
• - Ferromagnetic: Fe, Co, Ni (strongly attracted, permanent magnets).
• - Paramagnetic: Weakly attracted (unpaired electrons).
• - Diamagnetic: Weakly repelled (paired electrons).
• - Alloys: Steel, brass, bronze.
• - Reason for trends: Electronic configuration (especially half-filled and full d subshells).

Common Mistakes

• - Thinking that the density always increases across the period. There are exceptions at Mn, Cu, and Zn.
• - Forgetting that the atomic radius trend is not a simple decrease. It flattens out and then increases.
• - Not understanding why manganese and zinc are anomalies. Manganese has a stable half-filled d subshell, and zinc has a full d subshell.
• - Confusing paramagnetic and ferromagnetic. Ferromagnetic substances are much more strongly attracted and can be permanent magnets.
• - The textbook has a factual error in the magnetic properties section, listing iron as diamagnetic. Iron is actually ferromagnetic.
• - Assuming that all transition metals can form alloys with each other. While many can, the atomic sizes must be similar for effective alloy formation.

Quick Revision

The physical characteristics of the first-row transition metals show distinct trends across the period. Density generally increases due to increasing atomic mass and decreasing atomic radius. Atomic radii decrease initially, then remain nearly constant in the middle, and increase at the end of the period as the 3d subshell fills. Melting points are high due to strong metallic bonding, with a maximum around vanadium/chromium and a dip at manganese due to its stable half-filled 3d⁵ configuration. Zinc has the lowest melting point because its full 3d and 4s subshells are not available for bonding. Magnetic properties depend on unpaired electrons: iron, cobalt, and nickel are ferromagnetic, while others are paramagnetic or diamagnetic. Transition metals form alloys (like steel, brass, and bronze) because of their similar atomic sizes.

60-Second Revision

Across the first-row transition metals, density increases, atomic radius decreases then increases at the end, and melting points are high with a dip at Mn and the lowest at Zn. Magnetic properties depend on unpaired electrons: Fe, Co, Ni are ferromagnetic (strong, permanent magnets); others are paramagnetic (weak) or diamagnetic (paired). They form alloys like steel, brass, and bronze due to similar atomic sizes.

Frequently Asked Questions (FAQs)

Q1. Why does the density of transition metals generally increase across the period? Answer: The density generally increases because the atomic mass of the metals increases (making the nuclei heavier) and the atomic radius decreases (reducing the volume of the atoms). Since density is mass divided by volume, this combination leads to an increase in density.

Q2. Explain the trend in atomic radii of the first-row transition metals. Answer: The atomic radii decrease initially across the period due to the increasing nuclear charge, which pulls the electrons closer. However, the decrease is less steep than for main group elements because the 3d electrons shield each other poorly. In the middle of the period, the shielding effect and the increasing nuclear charge counterbalance, so the radii remain almost constant. At the end of the period (from Ni to Zn), the radii increase because the 3d subshell becomes full, increasing the shielding effect and pushing the 4s electrons further from the nucleus.

Q3. Why do transition metals have high melting points, and why is there a dip at manganese? Answer: Transition metals have high melting points because of strong metallic bonding involving both 3d and 4s electrons. The dip at manganese is due to its stable half-filled 3d⁵ configuration, which reduces the number of electrons available for metallic bonding, making the bond weaker than expected.

Q4. Why does zinc have a lower melting point than the other first-row transition metals? Answer: Zinc has a completely filled 3d and 4s subshell ([Ar] 3d¹⁰ 4s²). These full subshells cannot participate in metallic bonding, so the metallic bond in zinc is weaker, resulting in a lower melting point. This is also why mercury is a liquid at room temperature.

Q5. What is the difference between paramagnetic and ferromagnetic substances? Answer: Paramagnetic substances have unpaired electrons and are weakly attracted to a magnetic field. Their magnetic moments are randomly oriented and only align weakly in an external field. Ferromagnetic substances, like iron, cobalt, and nickel, have unpaired electrons that align in the same direction even after the external field is removed. They are strongly attracted to a magnetic field and can become permanent magnets.

Q6. What is a diamagnetic substance? Answer: A diamagnetic substance has all its electrons paired. The paired electrons cancel out each other's magnetic moments, so the net magnetic moment is zero. Diamagnetic substances are weakly repelled by a magnetic field. (Note: The textbook incorrectly lists iron as diamagnetic; iron is actually ferromagnetic.)

Q7. Why can transition metals form alloys? Answer: Transition metals can form alloys because they have similar atomic sizes. This allows the atoms of different metals to substitute for each other in the same metallic lattice without significantly distorting the crystal structure.

• Steel: An alloy of iron with chromium, nickel, and manganese.
• Brass: An alloy of copper and zinc.

Q9. Why is the atomic radius of zinc larger than that of copper? Answer: The atomic radius of zinc is larger than that of copper because the additional electron in the 4s subshell of zinc increases the shielding effect. This increased shielding outweighs the effect of the higher nuclear charge, causing the electron cloud to expand and the atomic radius to increase.

Multiple Choice Questions (MCQs)

• Q1. What is the trend in density of the first-row transition metals across the period?
• A. It generally increases.
• B. It generally decreases.
• C. It remains constant.
• D. It first decreases then increases.
• Correct Answer: A
• Explanation: The density generally increases across the period from scandium to copper due to the increase in atomic mass and decrease in atomic volume.

• Q2. Which of the following elements is an exception to the general density trend of transition metals?
• A. Scandium
• B. Titanium
• C. Manganese
• D. Iron
• Correct Answer: C
• Explanation: Manganese has a density that is different from the general trend. Copper and zinc also show exceptions.

• Q3. What is the reason for the decrease in atomic radius across the first-row transition metals initially?
• A. Increasing shielding effect
• B. Increasing nuclear charge
• C. Decreasing atomic mass
• D. Decreasing number of electrons
• Correct Answer: B
• Explanation: The atomic radius decreases initially because the increasing nuclear charge pulls the electrons closer to the nucleus.

• Q4. Which of the following transition metals has the lowest melting point?
• A. Iron
• B. Zinc
• C. Copper
• D. Nickel
• Correct Answer: B
• Explanation: Zinc has the lowest melting point because its full 3d and 4s subshells cannot participate in metallic bonding.

Q5. What is the reason for the dip in melting point at manganese? A. Its low atomic mass. B. Its stable half-filled 3d⁵ configuration. C. Its full 3d subshell. D. Its high density. Correct Answer: B Explanation: The stable half-filled 3d⁵ configuration reduces the number of electrons available for metallic bonding, leading to a weaker bond and a lower melting point.

Q6. Which of the following is a paramagnetic substance? A. Copper B. Zinc C. Sodium D. Iron Correct Answer: C Explanation: Sodium has unpaired electrons in its valence shell, making it paramagnetic. Iron is ferromagnetic, and copper and zinc are diamagnetic (or have paired electrons).

• Q7. Which of the following is a ferromagnetic substance?
• A. Copper
• B. Iron
• C. Zinc
• D. Sodium
• Correct Answer: B
• Explanation: Iron, cobalt, and nickel are ferromagnetic; they are strongly attracted to a magnet and can become permanent magnets.

Q8. Why can transition metals form alloys? A. Because they have different atomic sizes. B. Because they have similar atomic sizes. C. Because they have high melting points. D. Because they are poor conductors. Correct Answer: B Explanation: Transition metals have similar atomic sizes, allowing them to substitute for one another in the metallic lattice, which is the basis for alloy formation.

• Q9. Which of the following is an example of an alloy?
• A. Iron
• B. Steel
• C. Copper
• D. Zinc
• Correct Answer: B
• Explanation: Steel is an alloy of iron with chromium, nickel, and manganese. Iron, copper, and zinc are pure elements.

Q10. Why does the atomic radius increase from copper to zinc? A. Because the nuclear charge decreases. B. Because the 3d subshell becomes full, increasing the shielding effect. C. Because the number of electron shells decreases. D. Because the atomic mass decreases. Correct Answer: B Explanation: The 3d subshell becomes full, which increases the shielding effect and pushes the 4s electrons further from the nucleus, increasing the atomic radius.

Multiple Choice Questions (MCQs)

• Q1. What is the trend in density of the first-row transition metals across the period?
• A. It generally increases.
• B. It generally decreases.
• C. It remains constant.
• D. It first decreases then increases.
• Correct Answer: A
• Explanation: The density generally increases across the period from scandium to copper.

• Q2. Which of the following elements is an exception to the general density trend of transition metals?
• A. Scandium
• B. Titanium
• C. Manganese
• D. Iron
• Correct Answer: C
• Explanation: Manganese has a density that is different from the general trend.

• Q3. What is the reason for the decrease in atomic radius across the first-row transition metals initially?
• A. Increasing shielding effect
• B. Increasing nuclear charge
• C. Decreasing atomic mass
• D. Decreasing number of electrons
• Correct Answer: B
• Explanation: The atomic radius decreases initially because the increasing nuclear charge pulls the electrons closer.

• Q4. Which of the following transition metals has the lowest melting point?
• A. Iron
• B. Zinc
• C. Copper
• D. Nickel
• Correct Answer: B
• Explanation: Zinc has the lowest melting point because its full 3d and 4s subshells cannot participate in metallic bonding.

• Q5. What is the reason for the dip in melting point at manganese?
• A. Its low atomic mass.
• B. Its stable half-filled 3d⁵ configuration.
• C. Its full 3d subshell.
• D. Its high density.
• Correct Answer: B
• Explanation: The stable half-filled 3d⁵ configuration reduces the number of electrons available for metallic bonding, leading to a lower melting point.

• Q6. Which of the following is a paramagnetic substance?
• A. Copper
• B. Zinc
• C. Sodium
• D. Iron
• Correct Answer: C
• Explanation: Sodium has unpaired electrons in its valence shell, making it paramagnetic.

• Q7. Which of the following is a ferromagnetic substance?
• A. Copper
• B. Iron
• C. Zinc
• D. Sodium
• Correct Answer: B
• Explanation: Iron, cobalt, and nickel are ferromagnetic.

• Q8. Why can transition metals form alloys?
• A. Because they have different atomic sizes.
• B. Because they have similar atomic sizes.
• C. Because they have high melting points.
• D. Because they are poor conductors.
• Correct Answer: B
• Explanation: Transition metals have similar atomic sizes, allowing them to substitute for one another in the metallic lattice.

• Q9. Which of the following is an example of an alloy?
• A. Iron
• B. Steel
• C. Copper
• D. Zinc
• Correct Answer: B
• Explanation: Steel is an alloy of iron with chromium, nickel, and manganese.

• Q10. Why does the atomic radius increase from copper to zinc?
• A. Because the nuclear charge decreases.
• B. Because the 3d subshell becomes full, increasing the shielding effect.
• C. Because the number of electron shells decreases.
• D. Because the atomic mass decreases.
• Correct Answer: B
• Explanation: The 3d subshell becomes full, which increases the shielding effect and pushes the 4s electrons further from the nucleus.