Module 2: bonding and structure Flashcards
Define Ionic bond
Strong electrostatic forces of attraction between oppositely charged ions.
Name the properties of ionic compounds
- High m.p and b.p
- Giant ionic lattice
- Strong electrostatic attractions which require a lot of energy to overcome
- The higher the charge - the stronger the bond
- Smaller ionic radius - the stronger the bond
- Soluble
Define covalent bond
Shared pair of electrons between atoms
What is a dative covalent bond?
Some compound e.g NH3 have a lone pair of electrons. Some ions have a vacant orbital e.g H+. This causes a bond to occur between the vacant orbital and the shared pair of electrons.
How do you calculate the molecular structure of a molecule?
- Identify group number of central atom
- Add one for each extra atom
- Divide total by 2 - gives number of electron pairs
- Draw out correct shape
Linear
- 2 pairs of electrons
- 2 bonding pairs
- 180 degrees
Trigonal planar
- 3 pairs of electrons
- 3 bonding pairs
- 120 degrees
Tetrahedral
- 4 pairs of electrons
- 4 bonding pairs
- 109.5 degrees
Trigonal pyramid
- 4 pairs of electrons
- 3 bonding pairs
- 1 lone pair
- 107 degrees
Trigonal Bipyramidal
- 5 pairs of electrons
- 5 bonding pairs
- 120 degrees and 90 degrees
Non linear
- 4 pairs of electrons
- 2 bonding pairs
- 2 lone pairs
Octahedral
- 6 pairs of electrons
- 6 bonding pairs
Define electronegativity
The ability of an atom to attract a pair of electrons in a covalent bond
What does electronegativity depend on?
- The nuclear charge (number of protons) of the atom (the higher the greater)
- The atomic radius of the atom (the smaller the greater)
- The number of principle energy levels the atom has (the fewer the greater)
What atom is given the delta negative charge symbol?
- ## The element with a higher electronegativity
What’s a polar bond?
A covalent bond where there is an unequal share of the electrons due to the differing electronegativities of the atoms involved.
How can you work out if a molecule is polar or non polar?
- Add on any partial charges on polar bonds
- Look to see if there is an even or uneven distribution of the electron density in the molecule.
What are common polar molecules?
- H2O
- NH3, PH3
- CH2, F2, CH3BR
- HCl, HF
- Alcohols, carboxylic acids
What are common non-polar molecules?
- Br2, Cl2, O2, N2, H2, I2
- CO2 (linear, 2 dipoles cancel)
- CH4, CF4: tetrahedral
Is CHCl3 or CCl4 non-polar?
CCl4 - symmetrical: dipoles cancel
What do intermolecular forces apply to?
- Simple molecular structures
Name the three types of intermolecular forces from weakest to strongest
- Induced dipole-dipole
2.Permanent dipole-dipole - Hydrogen bonding
What do all molecules have?
Induced dipole-dipole forces
What applies to polar molecules?
Permanent dipole-dipole forces
What does hydrogen bond to?
Nitrogen, oxygen and fluorine
What is an induced dipole-dipole bond?
- A temporary dipole induces a dipole in another molecule
- There is an attraction between the delta positive on one molecule and the delta negative on an adjacent molecule. This is called the induced dipole dipole intermolecular force.
- All simple molecules have induce dipole dipole intermolecular forces but some simple molecules have other intermolecular forces.
What affects the strength of induced dipole-dipole forces?
- Size/Mr of the molecule (bigger molecule, more IDD forces between molecules)
- Surface area contact (more SA contact, more IDD forces between molecules)
How do you explain a permanent dipole-dipole bond?
- There is a polar bond due to the difference in elctronegativities.
-There is an attraction between the delta positive on one molecule and the delta negative on the adjacent one.
Why do hydrogen bonds only apply to oxygen, nitrogen and fluorine?
There’s a large difference in electronegativity as they all have high electronegativities.
What are features of a hydrogen bond diagram?
- Partial charges on all atoms
- All lone pairs clearly shown
- Hydrogen bond shown clearly between the lone pair and the delta positive H on the other molecule.
- Has to be 180 degrees around the hydrogen
Why is ice less dense than liquid water?
- Fewer particles of water per unit volume in the solid state than the liquid state
- In ice the water molecules are further apart than in liquid water.
- This means there are more gaps/ spaces between the molecules
Why does water have a relatively high boiling point?
- It’s due to the presence of hydrogen bonds which are much stronger than the IDD’s in the other hydrides.
What are physical properties of Simple molecular structures?
- Low melting and boiling points due to weak intermolecular forces which require very little energy to overcome. Actual m.p is determined on strength of EMFs
- They will not conduct electricity in the solid state or in the molten state as there are no charged particles free to move. (no mobile charge carriers)
- Non polar molecules do not generally dissolve in water - If the simple molecules can form hydrogen bonds they become more soluble.
What are exceptions of ionic bonds?
BeCl2 and AlCl3
What covalent molecules are happy with less than 8 e-?
BF3 and AlCl3
What covalent molecules are happy with more than 8 e-?
SF6, PCl5
