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
![broken image](http://custom-images.strikinglycdn.com/res/hrscywv4p/image/upload/c_limit,fl_lossy,h_9000,w_1200,f_auto,q_auto/4936730/779481_780555.jpeg)
- 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:
![broken image](http://custom-images.strikinglycdn.com/res/hrscywv4p/image/upload/c_limit,fl_lossy,h_9000,w_1200,f_auto,q_auto/4936730/371443_402940.jpeg)
Example 1 - Beryllium chloride (BeCl₂)
![broken image](http://custom-images.strikinglycdn.com/res/hrscywv4p/image/upload/c_limit,fl_lossy,h_9000,w_1200,f_auto,q_auto/4936730/901593_338606.jpeg)
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.
![broken image](http://custom-images.strikinglycdn.com/res/hrscywv4p/image/upload/c_limit,fl_lossy,h_9000,w_1200,f_auto,q_auto/4936730/149430_125851.jpeg)
Example 2(a)- Boron Fluoride (BF₃)
![broken image](http://custom-images.strikinglycdn.com/res/hrscywv4p/image/upload/c_limit,fl_lossy,h_9000,w_1200,f_auto,q_auto/4936730/720373_355344.jpeg)
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.
![broken image](http://custom-images.strikinglycdn.com/res/hrscywv4p/image/upload/c_limit,fl_lossy,h_9000,w_1200,f_auto,q_auto/4936730/899728_41881.jpeg)
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.
![broken image](http://custom-images.strikinglycdn.com/res/hrscywv4p/image/upload/c_limit,fl_lossy,h_9000,w_1200,f_auto,q_auto/4936730/29787_452977.jpeg)
Example 3(a)- Methane(CH₄)
![broken image](http://custom-images.strikinglycdn.com/res/hrscywv4p/image/upload/c_limit,fl_lossy,h_9000,w_1200,f_auto,q_auto/4936730/816819_827634.jpeg)
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.
![broken image](http://custom-images.strikinglycdn.com/res/hrscywv4p/image/upload/c_limit,fl_lossy,h_9000,w_1200,f_auto,q_auto/4936730/651799_395041.jpeg)
Example 3(b)- Ammonia (NH₃)
![broken image](http://custom-images.strikinglycdn.com/res/hrscywv4p/image/upload/c_limit,fl_lossy,h_9000,w_1200,f_auto,q_auto/4936730/857480_194183.jpeg)
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
![broken image](http://custom-images.strikinglycdn.com/res/hrscywv4p/image/upload/c_limit,fl_lossy,h_9000,w_1200,f_auto,q_auto/4936730/66345_787399.jpeg)
•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.
![broken image](http://custom-images.strikinglycdn.com/res/hrscywv4p/image/upload/c_limit,fl_lossy,h_9000,w_1200,f_auto,q_auto/4936730/425185_821910.jpeg)
Example 3(c)- Water (H₂O)
![broken image](http://custom-images.strikinglycdn.com/res/hrscywv4p/image/upload/c_limit,fl_lossy,h_9000,w_1200,f_auto,q_auto/4936730/487642_101399.jpeg)
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
![broken image](http://custom-images.strikinglycdn.com/res/hrscywv4p/image/upload/c_limit,fl_lossy,h_9000,w_1200,f_auto,q_auto/4936730/261750_428444.jpeg)
That's all for part 1.
Well done!
You can read part 2 soon~