Shapes of molecules
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
- 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:
Example 1 - Beryllium chloride (BeCl₂)
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.
Example 2(a)- Boron Fluoride (BF₃)
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.
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.
Example 3(a)- Methane(CH₄)
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.
Example 3(b)- Ammonia (NH₃)
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
•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.
Example 3(c)- Water (H₂O)
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
That's all for part 1.
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You can read part 2 soon~