2.2.2 Bonding and Structure Flashcards
What is Ionic Bonding?
The electrostatic force of attraction between 2 oppositely charged ions; formed by the transfer of electrons.
What does ionic bonding happen between?
Metals and Non-metals
At which two points is ionic bonding its strongest?
- When the ion is small (small ionic radius)
- When the ion has a larger charge
Example of an ionic compound?
NaCl
What is the arrangement of ionic compounds?
Giant Ionic Lattice
3D structure
In a giant ionic lattice, what do the ions do?
Each ion is surrounded by oppositely charged ions therefore the ions in the lattice attract in all directions.
What are the properties of ionic compounds?
Soluble in Water
High boiling point and melting point
Can conduct Electricity (only molten or aqueous)
High Boiling and melting point (ionic):
Strong electrostatic forces between oppositely charged ions.
A great amount of energy is required to overcome these forces, giving it a high MP and BP
Electrical conductivity (ionic):
When in their solid state, ionic compounds cannot carry charge or electricity because the ions are not free to move and are in a fixed position.
When dissolved or molten, ionic compounds can carry charge and electricity through the structure because the ions become free to move.
Solubility (ionic):
An ionic substances can dissolve in polar solvents (e.g. water)
The polar solvent disrupts the lattice as the slight charges (dipoles) in the polar substance attract the ions in the lattice, creating a solution.
What is covalent bonding?
The electrostatic force of attraction between a shared pair of electrons and the nuclei of the bonded atoms
Single covalent bonds consist of only…
one shared pair of electrons
Multiple covalent bonds consist of …
2 or more shared pairs of electrons
Example of single covalently bonded molecule
Hydrogen H-H
Example of multiple covalently bonded molecule
Oxygen (O=O)
Nitrogen (triple)
Carbon dioxide (O=C=O)
What are lone pairs?
Electron pairs that haven’t been used up in covalent bonding
What can lone pairs do?
Give a concentrated region of negative charge around the atom.
decreases bond angle by 2.5°
What is a dative covalent bond?
When only one atom provides both electrons which will be shared as a pair.
(one atom provides an electron pair –> these are usually its lone pairs)
Examples of Molecules with dative covalent bonds
Ammonium ion (3 covalent and 1 dative)
Oxonium ion (2 covalent and 1 dative)
What is bond enthalpy?
The measure of covalent bond strength
Bond enthalpy shows you …
The average amount of energy required to break a covalent bond.
The larger the enthalpy value…
the stronger the covalent bond
If the covalent bond is strong, the bond enthalpy should have a…
Positive value (endothermic)
Endothermic reactions:
Breaking bonds (+)
Exothermic reactions:
Forming bonds (-)
What are two covalent structures
Simple covalent molecules
Giant covalent molecules
Name the properties of a simple covalent molecule:
Low melting point and boiling point
Low solubility
Poor electrical conductivity
Usually found as a liquid or gas
Low melting point and boiling point (simple covalent molecule):
Weak intermolecular forces of attraction (London forces) between the molecules
A minimal amount of energy is required to overcome these forces in the molecule
Electrical conductivity (simple covalent molecule):
Low because, whether in a molten or aqueous state, no delocalised electrons or ions can carry charge through the structure.
Examples of simple covalent molecules:
Water
Nitrogen
Hydrogen
Ammonia
Carbon dioxide
Oxygen
Iodine
Ice (Solid)
What defines the shape of a molecule or ion?
The number of electron pairs it has
(Either bonded or lone pairs)
Define the electron pair repulsion theory:
All electrons are negatively charged
Electron pairs automatically repel each other
The shape the molecule makes allows the electron pairs to be as far as possible to minimise repulsion
What happens if the central atom is surrounded by bonded pairs of electrons
The bonded pairs repel each other equally
What are the 7 shapes of molecules?
Linear
Trigonal planar
Tetrahedral
Trigonal pyramidal
Trigonal Bipyramidal
Bent (Non-linear)
Octahedral
Linear:
2 bonded pairs (double)
0 lone pairs
180 degrees
Linear example:
Carbon dioxide (O=C=O)
Trigonal Planar:
3 bonded pairs
0 lone pairs
120 degrees
Trigonal planar example:
BF3
Tetrahedral :
4 bonded pairs
0 lone pairs
109.5 degrees
Tetrahedral example:
Methane (CH4)
Trigonal pyramidal:
3 Bonded pairs
1 lone pair
107 degrees
Trigonal pyramidal example:
Ammonia (NH3)
Trigonal Bipyramidal:
3 Bonded pairs
2 Lone pairs
120 and 90 Degrees
Bent (Non-linear):
2 Bonded pairs
2 lone pairs
104.5 degrees
Bent example:
Water (H2O)
Octahedral:
6 bonded pairs
0 lone pairs
90 degrees
Octahedral example:
SF6
Lone pairs…
Reduce bond angles by 2.5 degrees
Lone pairs repel…
More than bonded pairs
What is electronegativity?
The tendency for an atom to attract an electron in a covalent bond to itself
What are the most electronegative elements?
Nitrogen
Oxygen
Fluorine
Chlorine
What happens to electronegativity as you go across a period?
Increases
The nuclear charge increases (proton number increases)
Atomic radius decreases because of the strong electrostatic force between the positive nucleus and negative electrons.
Therefore the electrons get pulled towards the nucleus
Electrons get easier to gain and harder to lose
What happens to electronegativity as you go down a group?
Decreases
Electron shielding increases due to more shells and atomic radius increases
The distance between the nucleus and the outer electron increases so the attraction between them weakens
Electrons become easier to lose and harder to gain
Compounds with slight electronegative differences will be …
Covalent
Compounds with large electronegative differences will be …
Ionic
What side of the periodic table has the most electronegative elements
Top right
What is a permanent dipole-dipole interaction (Polar covalent bond)?
When the elements in a molecule have different electronegativities, it causes an unequal distribution of charge/electrons, producing a separation of charge (A dipole)
This creates a polar molecule.
What element will have the negative dipole?
The element with the larger electronegativity
Symmetrical molecules have…
Bonded pairs and no lone pairs
Example of a symmetrical molecule:
CO2
Symmetrical molecules are…
Non-polar
Why are symmetrical molecules non-polar?
The dipoles cancel out, even if there are individual polar bonds within the molecule since there is an equal distribution of electrons (bonded electron pairs) so the dipoles cancel out.
Properties of permanent dipole-dipole interactions:
Stronger than London forces
Found in asymmetrical molecules
There is always a significant difference in electronegativity.
Induced dipole-dipole interaction only occurs in what?
Simple covalent molecules and noble gases
How are London forces (induced dipole-dipole interactions) created?
electrons are constantly moving in a molecule.
Electron density constantly fluctuates, so parts of the molecule can become more negative than others, inducing temporary dipoles (transient dipoles).
These induced dipoles induced more dipoles in neighbouring molecules
London forces =
Induced dipole-dipole interactions
What can affect London forces?
The more electrons there are, the higher the chance for temporary dipole to be produced
This increases the strength of the interaction, increasing Boiling points.
Hydrogen bonds only occur?
When one of the hydrogen atoms in a molecule is attached to one of the most electronegative atoms of either N,O,F or Cl
Hydrogen bonding requires
A large electronegativity difference and there must be a lone pair
Hydrogen bonding is stronger than both…
Permanent dipole dipole interactions
London forces
Why does water have a high BP?
Because of the surplus amount of strong hydrogen bonds , therefore a high amount of energy is needed to break these hydrogen bonds
