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Allotropes: one element, more than one structure

Your guide: Miss SanaCounts atoms the way you count rupees: carefully.

The problem

A diamond in a ring and the graphite in a pencil are made of exactly the same atoms: carbon and nothing else. So why do they look and behave so differently? The answer is in how the atoms are arranged.

Some elements exist in more than one structural form, and these forms can have quite different physical and chemical properties. The forms are called allotropic forms, and the phenomenon is called allotropy.

Oxygen has two allotropic forms: ordinary oxygen (O2) and ozone (O3). Sulphur has two crystalline forms, rhombic and monoclinic; rhombic is the more stable of the two.

Three main forms of carbon (p. 8)

Diamond: a giant macromolecular structure.

Graphite: a layered structure of hexagonal rings of carbon.

Buckminster fullerene (C60): a cage-like sphere of carbon atoms arranged in pentagons and hexagons.

Fig 1.5 also shows graphene alongside these three.

What your book says about C60 (p. 8)

Stable at high temperatures and high pressures.

Covalent, so it dissolves in organic solvents.

Not charged, no boundaries, no unpaired electrons.

Low melting point, soft, does not conduct electricity.

Allotropes are the same element with a different arrangement of atoms, and a different arrangement gives different properties.

Step 1 / 7

Notes, short questions and MCQs

Read the full notes: key terms, model answers and MCQs with answers

The problem

A diamond in a ring and the graphite in a pencil are made of exactly the same atoms: carbon and nothing else. So why do they look and behave so differently? The answer is in how the atoms are arranged.

Some elements exist in more than one structural form, and these forms can have quite different physical and chemical properties. The forms are called allotropic forms, and the phenomenon is called allotropy.

Oxygen has two allotropic forms: ordinary oxygen (O2) and ozone (O3). Sulphur has two crystalline forms, rhombic and monoclinic; rhombic is the more stable of the two.

Three main forms of carbon (p. 8)

Diamond: a giant macromolecular structure.

Graphite: a layered structure of hexagonal rings of carbon.

Buckminster fullerene (C60): a cage-like sphere of carbon atoms arranged in pentagons and hexagons.

Fig 1.5 also shows graphene alongside these three.

What your book says about C60 (p. 8)

Stable at high temperatures and high pressures.

Covalent, so it dissolves in organic solvents.

Not charged, no boundaries, no unpaired electrons.

Low melting point, soft, does not conduct electricity.

Allotropes are the same element with a different arrangement of atoms, and a different arrangement gives different properties.

Key terms

Allotropy
Existence of an element in more than one structural form with different properties.
Diamond
Carbon atoms joined into one giant structure in every direction.
Graphite
Carbon atoms in flat hexagonal rings stacked in layers.
Buckminster fullerene
Sixty carbon atoms in pentagons and hexagons forming a sphere.

Short questions with model answers

  1. Q1. Show that O2 and O3 are allotropes, not two different substances made of different elements.

    • Both contain only oxygen atoms.
    • O2 has two atoms per molecule; O3 has three.

    Same element, different structures: they are allotropic forms of oxygen.

  2. Q2. Using your book, explain why Buckminster fullerene dissolves in organic solvents and state two of its other properties.

    • C60 is covalent in nature.
    • It is soft with a low melting point, and it does not conduct electricity.

    It is covalent; it is also soft, has a low melting point and does not conduct electricity.

  3. Q3. Name the two crystalline allotropes of sulphur and say which is more stable.

    • Sulphur forms rhombic and monoclinic crystals.
    • Rhombic is the more stable form.

    Rhombic and monoclinic; rhombic is more stable.

Common mistakes

  • ✗ Calling ozone (O3) a compound of oxygen.

    ✓ A compound needs different elements. O3 is only oxygen, so it is an allotrope of the element.

  • ✗ Assuming allotropes share properties because they are the same element.

    ✓ Your book says they can have quite different physical and chemical properties; structure decides.

  • ✗ Writing that allotropy is a property of carbon only.

    ✓ Oxygen (O2, O3) and sulphur (rhombic, monoclinic) show it too.

MCQs

  1. 1. The crystals of which substance have a rhombic shape?

    1. (a) Brass
    2. (b) Sulphur
    3. (c) Graphite
    4. (d) Bronze
    Show answer

    (b) Rhombic is one of sulphur's two crystalline allotropes (p. 8). Graphite is carbon in layers; brass and bronze are alloys.

  2. 2. Ozone (O3) is:

    1. (a) An isotope of oxygen
    2. (b) A compound of oxygen
    3. (c) A mixture of oxygen gases
    4. (d) An allotrope of oxygen
    Show answer

    (d) O3 is oxygen atoms only, arranged three to a molecule: a different structural form of the same element.

  3. 3. Which allotrope of carbon has a layered structure of hexagonal rings?

    1. (a) Graphite
    2. (b) Diamond
    3. (c) Buckminster fullerene
    4. (d) Rhombic sulphur
    Show answer

    (a) Graphite is the layered one. Diamond is a giant macromolecule, C60 is a cage, and rhombic sulphur is not carbon.

  4. 4. Buckminster fullerene dissolves in organic solvents because it is:

    1. (a) Ionic
    2. (b) Metallic
    3. (c) Covalent
    4. (d) Charged
    Show answer

    (c) Your book: being covalent in nature, fullerenes are soluble in organic solvents. It also says C60 is not charged.

  5. 5. Which statement about allotropes is correct?

    1. (a) They are different elements
    2. (b) They are forms of one element with different structures
    3. (c) They always have identical properties
    4. (d) They exist only for carbon
    Show answer

    (b) Allotropy is one element in more than one structural form; oxygen and sulphur show it as well as carbon.

Quick revision

  • Allotropy: one element in more than one structural form, with quite different properties.
  • Carbon: diamond (giant macromolecule), graphite (layers of hexagons), C60 (a cage of pentagons and hexagons).
  • Oxygen: O2 and ozone O3. Sulphur: rhombic (more stable) and monoclinic.

Before you start

What is Class 9 Chemistry Allotropic Forms of Substances?
Allotropic Forms of Substances is the open Class 9 Chemistry lesson for Matric Part 1, Punjab board. It is a step-by-step lesson, not a PDF of the book.
Which boards is this Class 9 Chemistry lesson for?
The same Punjab textbook: Lahore, Rawalpindi, Gujranwala, Faisalabad, Multan, Sargodha, Bahawalpur, Dera Ghazi Khan, Sahiwal.
Can I study this in Urdu?
Yes. English, Urdu or Roman Urdu. Science terms stay as the exam writes them.
Is this a notes PDF?
No. The book stays with you. This page is the lesson and the check.

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