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I/GCSE Chemistry: Shapes of Molecules (Part 1)

Shapes of Molecules

· chemistry,igcse chemistry,Molecules,Molecular Shapes

Shapes of molecules

In I/GCSE Chemistry,

The shape of a DIATOMIC MOLECULE is always “linear”

POLYATOMIC MOLECULES contain 2 categories of atoms:

  • Central atom- act as a hub all other atoms bond onto this atom
  • Ligand atom- atoms that bond onto the central atom
  • The shape of any molecule depends on the number and the nature of electron pairs found in the outer shell of the central atom.

In I/GCSE Chemistry, pairs of electrons in the outer shell of the central atom will repel one another. But the strength of the repulsions are not equal

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  • The basic shape of molecules can be determined by considering the repulsion between bonding pairs only
  • BONDING ANGLE- Angular distance between 2 successive ligand atoms in the molecule.

Representing 3D molecules- “wedge and dash” connection:

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Example 1 - Beryllium chloride (BeCl₂)

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Around the central Be atom, there are 2 bonding pairs, 0 lone pairs.

The 2 bonding pairs repel each other as far apart as they can. Bond angle is 180° such that the shape of molecule formed is LINEAR.

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Example 2(a)- Boron Fluoride (BF₃)

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Around the central B atom: 3 bonding pairs, 0 lone pairs.

The 3 bonding pairs repel each other equally around central atom as far as possible. Bond angle is 120° such that the shape of molecule formed is TRIGONAL PLANAR.

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Example 2(b)- Carbon dichloride (CCl₂)

The carbon uses its ground state as its bonding state

Around the central C atom: 2 bonding pairs, 1 lone pair

Lone pair-bonding pair repulsion is STRONGER than a bonding pair-bonding pair. Each lone pair in outer shell of central atom squeezes bonding pairs together by approx 2.5.

Bond angle is 117.5° such that the shape of molecule formed is V-SHAPED.

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Example 3(a)- Methane(CH₄)

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Around the central Be atom: 4 bonding pairs, 0 lone pairs.

Each bonding pair repel with equal force as far as possible.

Bond angle is 109.5° such that the shape of molecule formed is TETRAHEDRAL.

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Example 3(b)- Ammonia (NH₃)

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Around the central N atom: 3 bonding pairs, 1 lone pair

Bond angle is 107° such that the shape of molecule formed is TRIGONAL PYRAMID

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•Dative covalent bonding changes shape of structure = e.g. NH₃ form NH₄⁺ ion

Behaves like a bond pair: 4 bonding pairs, 0 lone pair

Bond angle is 109.5° such that the shape of molecule is TETRAHEDRAL ION.

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Example 3(c)- Water (H₂O)

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Around the central O atom: 2 bonding pairs, 2 lone pairs

Bond angle is 104.5° (109.5-(2x2.5)such that the shape of molecule formed is TETRAHEDRAL

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That's all for part 1.

Well done!

You can read part 2 soon~

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I/GCSE Chemistry: Electronegativity and Polarity
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I/GCSE Chemistry: Shapes of Molecules (Part 2)
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