Chapter 6 - Shapes Of Molecules And Intermolecular Forces Flashcards
What is the electron-pair repulsion theory?
- electrons repel one another
- electron pairs surrounding a central atom determines the shape of the molecule/ion
- electron pair repel to be as fair apart as possible
- arrangement of electron pair minimises repulsion holding the bond in a specific shape
- different number of electron = different shape
What are three-dimensional shape s used to represent molecules 3D shape?
- a straight line = bond IN plane of the page
- a triangle filled = bond is coming OUT the page
- a triangle made out of multiple lines = bong going INTO the page
The shape of molecules depend on …………………………………….
Number of pairs of electrons
What must be included for a molecule to have a LINEAR shape?
- 2 electron pairs
* 180° between each bond
What must be included for a molecule to have a TRIGONAL PLANAR shape?
- 3 electron pairs
* 120° angles between each bond
What must be included for a molecule to have a TETRAHEDRAL shape?
- 4 electron pairs
- 109.5° between each bond
- 0 lone-pairs
What must be included for a molecule to have a OCTOHEDRAL shape?
- 6 electron pairs
* 90° between each bond
How do lone pairs effect the shape of a molecule?
- it is closer to the central atom and occupies more space
- higher repulsion decreases the bond angle
- bond angle reduce by 2.5° for each lone pair bench changing the shape of the molecule
What must be included for a molecule to have a pyramidal shape?
- 3 bond pairs
- 1 lone-pair
- 107 ° between bonds
What must be included for a molecule to have a non-linear shape?
- 2 bond pairs
- 2 lone-pairs
- 104.5° between each bond
What’s electronegativity?
The attraction of a bonded atom for the pair of electrons in a covalent bond
How is electronegativity measured?
Depends on the position of the periodic table
Increases across and increase up the periodic table
Complete the graph
Bond type Electronegativity difference
Covalent a)
Polar covalent b)
Ionic C)
a) 0
b) 0 to 1.8
c) greater than 1.8
Complete the sentence:
•if electronegativity are the same the ……………………….
•if the electronegativity are vastly different …………………… occurs
•if the electronegativity are slightly different ………………………. exists
•different electronegativity causes a ………………
- molecules is non-polar
- ionic bonding
- a polar molecule
- permeant dipole
What is a non-polar bond?
The bonded electron pair is shared equally between the bonded atoms
What molecules of elements have a pure covalent bond?
Hydrogen, oxygen and chlorine etc
What’s a polar covalent bond?
The bonded electron pair is shared unequally and have different electronegativity values
What’s a dipole?
Separation of opposite charges
Why are water molecules polar?
Each O-H bond has induced dipole which act in different directions and do not oppose directly
Why is carbon non-polar?
The double bond between C and O has a permanent dipole which act in different directly and exactly oppose. Hence, cancels out the dipole making the dipole 0
Complete the sentence:
Ionic solutes dissolve in ……
• polar solvents
What properties do covalent bonds have?
- intermolecular forces
* very strong
What are the three types do intermolecular forces? And in order of strength from lowest to highest
- induced dipole-dipole interaction ( known as London Forces)
- permeant dipole-dipole interactions
- hydrogen bonds
Outline the intermolecular force known as LONDON FORCES
- weakest intermolecular forces
- exists in all molecules
- at the beginning, there is an even distribution, the movement of electrons forming a dipole known as instantaneous dipole
- it is constantly changing
- the instantaneous dipole induces a dipole in another
- only temporary
What happens to the London Forces when there are more electrons?
- larger the instantaneous/induced poles
- greater the London force
- stronger attractive force between the molecules
- more energy needed to overcome the intermolecular force, increasing the boiling point
Outline the intermolecular force known as PERMEANT DIPOLE-DIPOLE INTERACTION
- acts between the permanent dipoles in different polar molecules
- high amount of energy need to break the intermolecular forces
What is a simple molecular substance?
Made of simple molecules
What is a simple molecular lattice?
Simple molecules form a regular structure.
•molecules held by weak intermolecular forces
•molecules bonded together strongly by covalent bonds
List the properties if simple molecular substances
- low melting/boiling point - weak intermolecular forces can be broken easily.
- during melting point - weak intermolecular break but covalent bonds are strong so they don’t break
- solubility - non-polar substance aren’t soluble in polar solvents. This is because it would involve breaking stronger attractions between the molecules of the polar substance. But in a non-polar solvents, they are soluble as they can form weak attraction between the solvent and solute
Outline the intermolecular force known as HYDROGEN BONDING
Occurs between molecules which contain an electronegative atom with a lone pair of electrons and a hydrogen atom attached to an electronegative atom.
How is an hydrogen shown by?
Dashed line between lone pair and hydrogen atom
Are hydrogen bonds stronger than London Forces?
Yes
What are the anomalous properties of water?
- the solid (ice) is less dense than the liquid (water) - hydrogen bonds hold water molecules apart in open lattice structure but in ice they are held even further
- high surface tension and viscosity
- high melting/boiling point
