Shapes, Electronegativity and Polarity Flashcards
When drawing a shape diagram of a molecule what does a solid line mean?
A bond in the plane of the paper
When drawing a shape diagram of a molecule what does a dashed line mean?
A bond pointing behind the plane of the paper ( away from you)
When drawing a shape diagram of a molecule what does a triangular line mean?
A bond pointing in front of the plane of the paper (towards you)
What two factors affect the shape of a covalent molecule or polyatomic ion? What is this theory?
- The number of regions of electron density around the central atom.
- These regions of electron density repel each other as far apart as possible to minimise repulsion.
It is called the electron pair repulsion theory
What is the shape and bond angle for a molecule that has 2 regions of electron density?
Linear - 180°
What is the shape and bond angle for a molecule that has 3 regions of electron density?
Trigonal Planar - 120°
What is the shape and bond angle for a molecule that has 4 regions of electron density?
Tetrahedral - 109.5°
What is the shape and bond angle for a molecule that has 5 regions of electron density?
Trigonal Bipyramidal - 90 and 120°
What is the shape and bond angle for a molecule that has regions of 6 electron density?
Octahedral -90°
How do lone pairs of electrons effect bonding?
Lone pairs repel more strongly than bonded pairs, decreasing the bond angle ( - 2.5° per lone per)
How do double and triple bonds effect the shape of molecules?
They don’t they should be treated the same as a single bond.
Define electronegativity?
A measure of a atom’s ability to attract the bonding electron pairs in a covalent bond.
What is the Pauling electronegative scale?
A scale used to compare the electronegativity of the atoms of different elements
What bond is occurring if there is an electronegativity difference of 0?
Covalent bond
What bond is occurring if there is an electronegativity difference of 0 to 18?
Polar covalent bond
What bond is occurring if there is an electronegativity difference greater than 18?
Ionic bond
What is the bonding and electronegativity in non-polar covalent bonds?
Equal sharing of the bonded electrons, bonded atoms have the same or similar electronegativities.
What is the bonding and electronegativity in polar covalent bonds?
Unequal sharing of the bonding electrons, bonded atoms have significantly different electronegativities.
How are the partial charges distributed in a non-polar molecule?
Non-polar molecules have an equal distribution of partial charges
How are the partial charges distributed in a polar molecule?
Polar molecules have an unequal distribution of partial charges
How is the polarity of water tested?
The water stream bends towards the electrostatically charged rod.
How are polar molecules dissolved in water?
- The water molecules attract the + and - ions.
- The ionic lattice breaks down as it dissolves
- In the resulting solution, water molecules surround the + and - ions.
What are intermolecular forces?
Weak forces that exist between molecules
When do intermolecular forces occur?
Between simple covalent molecules
What are the three main types of intermolecular forces?
- Induced dipole-dipole interactions (London Forces)
- Permanent dipole-dipole interactions
- Hydrogen bonding
Where is a Induced dipole-dipole bond found?
Exists between ALL types of simple covalent molecules (and between the single atoms of noble gases)
When is a Induced dipole-dipole bond found?
They arise from ma temporary dipole (uneven distribution of electrons) in one molecule that induces a dipole in neighbouring molecules. The induced dipoles then attract each other.
How long do induced dipole-dipole bonds last?
As electrons are continuously moving these bonds are only momentarily made.
What does the strength of induced dipole-dipole bonds rely on?
The more electrons, the stronger the induced dipole-dipole interactions and the more energy is then needed to overcome them. (THE MORE ELECTRONS THE STRONGER THE BOND)
Where is a permanent dipole-dipole bond found?
Exists between polar molecules
What do all molecules with permanent dipole-dipole forces also have?
London Forces
How long do permanent dipole-dipole bonds last?
There dipoles are present all of the time and are stronger than London forces
What does the strength of permanent dipole-dipole bonds rely on?
The greater the difference in electronegativity the greater the dipole
What is the state of most simple molecular substances at rtp?
Gas or liquid
Why is the boiling/ melting point of most simple molecular substances low?
Most intermolecular forces are weak
What is the solubility of non-polar molecules and why?
They usually dissolve in non-polar solvents. The forces between the solvent and the molecules will be similar to or stronger than the forces between both.
What is the solubility of polar molecules and why?
Usually dissolve in polar solvents. The forces between solvents and molecules will be similar to or stronger than the forces between both, but is dependant on the strength of the dipole so can be difficult to predict.
What is the electrical conductivity like in simple molecular compounds?
Simple molecular compounds do not conduct electricity as they lack mobile ions/ delocalised electrons to act as charge carriers.
What is a hydrogen bond?
Usually a strong type of permanent dipole-dipole interaction.
When does a hydrogen bond occur?
Occurs when hydrogen is covalently bonded to Oxygen, Nitrogen or fluorine making a significant delta positive charge on the hydrogen. The H+ is attracted to a lone pair on O, N or F in a neighbouring molecule
How strong are hydrogen bonds?
They are the strongest type of intermolecular force
What causes the anomalous properties of water?
Its extensive hydrogen bonding
Why does water have a relatively high boiling point?
- Has 2 lone pairs and 2 H^+ atoms to each oxygen atom.
- Each H2O molecule can form up to 4 hydrogen bonds
- More energy is required to overcome the H-bonds resulting in the high boiling point
Why is solid ice less dense than water?
- In ice, each water molecule is held in an open lattice with large gaps between molecules.
- When ice is heated, some of the hydrogen bonds break and the water molecules move closer together making liquid denser than ice.
- This allows ice to float on water
What are the three effects of hydrogen bonding on a substance?
- Have higher boiling points due to their stronger IMF’s
- Are often soluble in water and other compounds with hydrogen bonds
- Have higher viscosity
