Chapter 6 - Bonding and structure Flashcards
What is ionic bonding ?
Ionic bonding is the electrostatic attraction between oppositely charged ions
What is covalent bonding ?
Covalent bonding is the strong electrostatic attraction between a shared pair of electrons and the nuclei of the bonded atoms
What is the electron pair repulsion theory ?
It explains and predicts the shapes of molecules and polyatomic ions
What is the order of pair repulsion, strongest to weakest?
- Lone-Lone
- Bonded-Lone
- Bonded-Bonded
What is a lone pair of electrons ?
- A lone pair refers to a pair of electrons that are not shared with another atom in a covalent bond
- Lone pairs repel more strongly compared to bonding pairs
- Every lone pair decreases the bond angle by 2.5 degrees
What shape is a molecule with no lone pairs ?
- Tetrahedral, 109.5 degree bond angle
- For example, CH4
What shape is a molecule with one lone pair ?
- Trigonal pyramidal, 107 degree bond angle
- For example, NH3
What shape is a molecule with two lone pairs ?
- Non-linear, 104.5 degree bond angle
- For example, H2O
What shape is a molecule with two bonded pairs ?
- Linear, 180 degree bond angle
- For example, CO2
What shape is a molecule with three bonded pairs ?
- Trigonal planar, 120 degree bond angle
- For example, BF3, boron trifluoride
What shape is a molecule with four bonded pairs ?
- Tetrahedral, 109.5 degree bond angle
- For example, CH4
What shape is a molecule with five bonded pairs ?
- Trigonal Bipyramidal
- There are actually two bond angles, 120 degrees in one plane and 90 degrees in another plane
What shape is a molecule with 6 bonded pairs ?
- Octahedral, 90 degree bond angles
- For example, SF6 Sulfur hexafluoride
Carbonate ion
CO3 (2-), Trigonal Planar, 120 degree bond angles
Nitrate ion
NO3(-), Trigonal planar, 120 degree bond angles
Sulfate ion
SO4(2-), tetrahedral, 109.5 degree bond angles
What is electronegativity ?
The ability of an atom to attract the bonding electrons in a covalent bond
How is electronegativity measured ?
- It is measured on the Pauling scale
- A higher value on the Pauling scale means it has higher electronegativity and thus a greater attraction for an electron pair in a covalent bond
What do Pauling values depend on ?
- Depends on an elements position in the table
- Across the periodic table, the nuclear charge increases and the atomic radius decreases
- As you go up and across the table, the values increase
Covalent or ionic
- Covalent bonds have an electronegativity difference of 0
- Polar covalent bonds have an electronegativity difference between 0 and 1.8
- Ionic bonds have an electronegativity difference of more than 1.8
What makes a bond polar ?
- Two atoms with an electronegativity difference of at least 0.5, the bonding electrons are pulled more towards the more electronegative atom
- A polar bond forms a dipole
How is a dipole formed ?
- A difference in charge between the two atoms
- A dipole in a polar covalent bond is a permanent dipole
What happens if the polar bonds are arranged symmetrically in a molecule ?
The dipoles cancel each other out, so the molecule has no overall dipole and is non polar
What happens if the polar bonds are arranged unsymmetrically in a molecule ?
Uneven distribution of charge and the molecule will have an overall dipole
What are the three types of IMFs ?
- Induced dipole dipole interactions AKA London Forces
- Permanent dipole dipole interactions
- Hydrogen bonding
What are London Forces ?
- Weak IMFs that exist between all molecules, polar or non polar
- They act between induced dipoles in different molecules, only temporary dipoles
How do induced dipoles come around ?
- Movement of electrons produces a changing dipole in a molecule
- An instantaneous dipole will exist, but its position will be changing
- The instantaneous dipole induces a dipole in a neighbouring molecule
- The induced dipole induces further dipoles on neighbouring molecules, which then attract more and more
What effect does the amount of electrons have on boiling point?
- The more electrons
- The larger the instantaneous and induced dipoles
- The greater the induced dipole dipole reactions
- The stronger the London forces
- The higher their boiling point
What are Permanent dipole dipole interactions ?
- Permanent dipole dipole interactions act between the permanent dipoles in different polar molecules
- The delta negative part in one molecule is attracted to the delta positive part in another molecule
How much energy is required to break permanent dipole-dipole interactions ?
- More energy is required as it is stronger than London forces.
- If you have permanent dipoles you are likely to also have London forces
What is the MP and BP like for simple molecular substances ?
They have a low melting and boiling point, this is due to the fact that a little bit of energy is required to break the weak IMFs
What is the solubility of Non-Polar Simple Molecules like in non polar solvent ?
- IMFs form between the two
- Interactions weaken the IMFs in the lattice
- IMF breaks and the compound dissolves
What is the solubility of Non-Polar Simple Molecules like in polar solvents ?
- There are little interactions between molecules in the lattice and in the solvent
- Bonding within the polar solvent is too strong to be broken
What is the solubility of Polar Simple Molecules like ?
Polar covalent substances may dissolve in polar solvents. This is due to the fact that the polar solute molecules and the polar solvent molecules can attract each other
How do intermolecular forces explain simple covalent compounds not conducting electricity?
Covalent molecules are uncharged as the permanent dipoles are not strong enough. This means they can not carry charge.
What is Hydrogen Bonding ?
- Hydrogen bonding is the intermolecular bonding between the lone pair of electrons in molecules containing N, O or F and a H atom
- It acts between a lone pair of electrons and a hydrogen in a different molecule
What are some anomalous properties of water ?
- Ice is less dense than water
- Water has a high MP and BP
- It has a high surface tension
- Water has a high viscosity
Why is ice less dense than water ?
- In ice, water molecules are held together in an open tetrahedral lattice that is full of holes
- When ice melts, hydrogen bonds are broken, so ice has more of these than water. As hydrogen bonds are long, this causes ice to be less dense than water.
Why does water have a high MP and BP ?
Water has both London forces and hydrogen bonds which mean more energy is needed to break these bonds.
