Although Zirconium belongs to 4d transition series and Hafnium to 5d transition series even then they show sim

NCERT Exemplar · Q20

Q. Although Zirconium belongs to 4d transition series and Hafnium to 5d transition series even then they show similar physical and chemical properties because ___________.

(i) both belong to d-block.

(ii) both have same number of electrons.

(iii) both have similar atomic radius.

(iv) both belong to the same group of the periodic table.

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Concept understanding — Magnetism and Color

Magnetism and Colour: An Intuitive First Look

You've probably noticed that some materials are magnetic (like iron) and others aren't (like wood). And you've seen that objects have different colours — a rose is red, the sky is blue. At first glance, these two properties seem completely unrelated. But at the deepest level, both magnetism and colour come from the same source: how electrons behave inside atoms.

Let's start with a simple picture.

The Intuition: Electrons as Tiny Magnets and Painters

Imagine an electron orbiting the nucleus of an atom. That moving charge is like a tiny loop of electric current — and any loop of current creates a magnetic field. So every electron is a microscopic magnet. In most materials, these tiny magnets point in random directions and cancel out. But in iron, they align, and the material becomes magnetic.

Now, colour. When light hits an atom, electrons can absorb some of its energy and jump to a higher orbit. The colour we see is the light that wasn't absorbed — the leftover wavelengths. Different atoms absorb different colours because their electrons have different "jump sizes" (energy levels).

So both magnetism and colour are about how electrons move and interact with their environment. One is about the direction of electron spin and orbit (magnetism), the other about the energy of electron jumps (colour).

The Precise Statement

Important

Magnetism and colour are both consequences of the electronic structure of atoms, but they arise from different aspects of electron behaviour:

Magnetism originates from the magnetic moments of electrons — their spin and orbital motion. A material is magnetic when these moments align cooperatively.
Colour originates from the absorption of specific wavelengths of light by electrons, which occurs when the photon energy matches the energy difference between two electron states.

How They Connect (and How They Don't)

The two phenomena are linked because they both depend on the arrangement of electrons in orbitals — the so-called electronic configuration. But they are not the same thing, and one does not cause the other.

Here's a table to make the distinction clear:

Property	Origin	What determines it?	Example
Magnetism	Electron spin and orbital motion	Unpaired electrons, crystal structure	Iron is magnetic because it has 4 unpaired electrons per atom
Colour	Electron transitions between energy levels	Energy gap between orbitals	Copper is reddish because its electrons absorb blue-green light

Note

A material can be magnetic and colourless (like pure iron — it's silvery, not colourful). A material can be brilliantly coloured and non-magnetic (like a ruby). The two properties are independent in most everyday cases.

The Deeper Link: Transition Metals

The most interesting connection appears in transition metals (elements like iron, cobalt, nickel, copper). These atoms have partially filled d orbitals. That partial filling does two things:

It leaves unpaired electrons, which can align to produce magnetism. …

Why this formula?

Magnetism and Color: Why the Key Formulas Hold

This is a fascinating intersection of physics and perception. The core idea is that color is not a property of light itself, but of our brain's interpretation of different wavelengths. Magnetism, in turn, can influence how these wavelengths are produced or absorbed.

Let's break down the key formulas and their why.

1. The Fundamental Link: Energy, Frequency, and Color

The most important formula connecting magnetism and color is the Planck-Einstein relation:

E = h \nu

Where:

$E$ = energy of a photon (light particle)
$h$ = Planck's constant ( $6.626 \times 10^{-34} \text{ J·s}$ )
$\nu$ = frequency of the light

Why does this hold?

Quantum nature of light: Light is not a continuous wave, but comes in discrete packets called photons.
Energy quantization: The energy of a photon is directly proportional to its frequency. Higher frequency means higher energy.
Magnetism's role: When an electron in an atom jumps from a higher energy level to a lower one, it emits a photon. The energy difference ( $\Delta E$ ) between these levels determines the photon's frequency:

\Delta E = h \nu

Color perception: Our eyes detect different frequencies as different colors. For example:
- Red light: $\nu \approx 4.3 \times 10^{14} \text{ Hz}$ (lower energy)
- Blue light: $\nu \approx 6.7 \times 10^{14} \text{ Hz}$ (higher energy)

Key insight: The color you see is determined by the energy gap between electron orbits. Magnetism can alter these energy gaps (via the Zeeman effect, see below).

2. The Zeeman Effect: How Magnetic Fields Split Colors

When a magnetic field is applied to an atom, a single spectral line (one color) splits into multiple lines. This is described by:

\Delta E = \mu_B \cdot B \cdot m_l

Where:

$\Delta E$ = energy shift of the spectral line
$\mu_B$ = Bohr magneton ( $9.274 \times 10^{-24} \text{ J/T}$ )
$B$ = magnetic field strength (in Tesla)
$m_l$ = magnetic quantum number (integer: $-l, ..., +l$ )

Why does this hold?

Electron as a tiny magnet: An electron orbiting a nucleus behaves like a tiny current loop, creating a magnetic dipole moment.
Energy in a magnetic field: This dipole moment interacts with an external magnetic field. The interaction energy depends on the orientation of the electron's orbit relative to the field.
Quantized orientations: The magnetic quantum number $m_l$ tells us which orientation is allowed. Each orientation has a slightly different energy.
Result: A single energy level splits into $2l+1$ sub-levels. Transitions between these sub-levels produce photons with slightly different energies — hence different colors appear.

Example: A sodium lamp emits yellow light. In a strong magnetic field, that yellow line splits into three closely spaced lines (normal Zeeman effect).

3. Faraday Rotation: Magnetic Field Twists Light's Color

When polarized light passes through a material in a magnetic field, its plane of polarization rotates. The rotation angle is:

\theta = V \cdot B \cdot d

Where:

$\theta$ = rotation angle (in radians)
$V$ = Verdet constant (material-specific, depends on wavelength)
$B$ = magnetic field strength
$d$ = path length through the material

Why does this hold?

Circular birefringence: In a magnetic field, the material has different refractive indices for left- and right-circularly polarized light.
Phase difference: These two components travel at different speeds, creating a phase difference.
Recombination: When they recombine, the resulting linear polarization is rotated. …

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The key idea is lanthanoid contraction — the steady decrease in atomic radius across the lanthanide series causes Zr and Hf to have nearly identical atomic radii, leading to their strikingly similar properties. The correct answer is (iii).

This is a classic question from the periodic properties chapter, and it tests a subtle but important consequence of the f-block elements. Let’s unpack why the answer isn’t the obvious one.

Why the obvious answers are wrong

You might think “both belong to d-block” or “both are in the same group” explains their similarity. After all, elements in the same group often behave alike. But that’s too vague — many d-block elements in the same group (like Cr and Mo, or Fe and Ru) show significant differences. So option (i) and (iv) are true but not the specific reason for the exceptional similarity between Zr and Hf.

Option (ii) — “both have same number of electrons” — is simply false. Zirconium (atomic number 40) has 40 electrons; Hafnium (atomic number 72) has 72. So that’s out.

The real reason is far more interesting.

The role of lanthanoid contraction

Here’s the chain of reasoning:

Zirconium is in the 4d series (Period 5, Group 4). Hafnium is in the 5d series (Period 6, Group 4). Normally, moving down a group in the d-block increases atomic size because a new electron shell is added. For example, from Ti (Period 4) to Zr (Period 5), the atomic radius jumps from about 147 pm to 160 pm.
But between Zr and Hf, something unusual happens. Hafnium comes after the lanthanide series (elements 57 to 71). The 14 lanthanide elements fill the 4f orbitals. As you move across the lanthanides, the nuclear charge increases by 14, but the 4f electrons are poor at shielding the outer electrons from this increased pull.
This lanthanoid contraction — the steady decrease in atomic radius across the lanthanide series — means that by the time you reach Hf, its atomic radius has been “pulled in” so much that it is almost identical to Zr’s radius. Zr’s atomic radius is about 160 pm; Hf’s is about 159 pm. They differ by less than 1%.

r(\text{Zr}) \approx 160\ \text{pm}, \quad r(\text{Hf}) \approx 159\ \text{pm}

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Here are the common mistakes students make on this question, along with how to avoid each.

Mistake 1: Choosing "Both belong to d-block" (Option (i))

Why students pick it: It’s a true statement — both are d-block elements. Students often stop at the first correct-sounding fact without checking if it explains the similarity.
Why it’s wrong: Belonging to the d-block is too broad. Many d-block elements (e.g., Fe and Zn) have very different properties. The question asks for the specific reason for the striking similarity between Zr and Hf.
How to avoid: Always ask: Does this fact directly cause the observed similarity? If the fact is true for many dissimilar elements, it’s not the correct explanation.

Mistake 2: Choosing "Both have the same number of electrons" (Option (ii))

Why students pick it: They confuse "same number of electrons" with "same electronic configuration in the outermost shell." Zr (atomic number 40) and Hf (atomic number 72) have different total electrons.
Why it’s wrong: Total electron count is different. Even their valence configurations are not identical (Zr: $4d^2 5s^2$ , Hf: $5d^2 6s^2$ ), though they are analogous.
How to avoid: Memorize atomic numbers of key pairs (Zr = 40, Hf = 72). If the numbers differ, total electrons differ. Never assume "same group" means "same number of electrons."

Mistake 3: Choosing "Both have similar atomic radius" (Option (iii))

Why students pick it: This is actually a true and relevant fact — Zr and Hf have nearly identical atomic radii due to lanthanoid contraction. Students often pick this because it sounds scientific.
Why it’s wrong (in this context): The question asks for the reason behind the similarity. Similar atomic radius is itself a consequence of lanthanoid contraction, not the root cause. The root cause is that they belong to the same group (same valence shell configuration), and lanthanoid contraction makes their radii nearly equal.
How to avoid: Distinguish between a symptom (similar radius) and the cause (same group + lanthanoid contraction). The exam often tests this hierarchy. Option (iv) is the fundamental reason.

Mistake 4: Overlooking "Lanthanoid Contraction" as the hidden link

Why students miss it: They know Zr and Hf are similar, but they don’t connect it to the lanthanoid series (elements 58–71) that sits between them in the periodic table.
Why it matters: The 4f electrons in lanthanoids poorly shield the nuclear charge, causing a steady decrease in atomic radius across the lanthanoid series. This makes Hf’s radius almost equal to Zr’s, despite Hf being one period lower. …

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Q.Although Zirconium belongs to 4d transition series and Hafnium to 5d transition series even then they show similar physical and chemical properties because ___________.

Concept understanding — Magnetism and Color

Magnetism and Colour: An Intuitive First Look

The Intuition: Electrons as Tiny Magnets and Painters

The Precise Statement

How They Connect (and How They Don't)

The Deeper Link: Transition Metals

Why this formula?

Magnetism and Color: Why the Key Formulas Hold

1. The Fundamental Link: Energy, Frequency, and Color

Why does this hold?

2. The Zeeman Effect: How Magnetic Fields Split Colors

Why does this hold?

3. Faraday Rotation: Magnetic Field Twists Light's Color

Why does this hold?

Why the obvious answers are wrong

The role of lanthanoid contraction

Concept: Lanthanoid Contraction and Its Effect on 4d and 5d Elements

Method: Cause-Effect Analysis of Periodic Trends

Mistake 1: Choosing "Both belong to d-block" (Option (i))

Mistake 2: Choosing "Both have the same number of electrons" (Option (ii))

Mistake 3: Choosing "Both have similar atomic radius" (Option (iii))

Mistake 4: Overlooking "Lanthanoid Contraction" as the hidden link

Q.Although Zirconium belongs to 4d transition series and Hafnium to 5d transition series even then they show similar physical and chemical properties because ___________.

Why the obvious answers are wrong

The role of lanthanoid contraction

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