chemical bonding Flashcards
(22 cards)
What is electronegativity?
Electronegativity of an atom is a measure of its ability to attract the electrons in a covalent bond to itself.
What affects electronegativity and how?
- Across the Period
Nuclear charge increases while shielding effect remains relatively constant.
Effective nuclear charge increases, hence electronegativity increases across the period. - Down the Group
Elements in the same group experience roughly the same effective nuclear charge as both nuclear charge and shielding effect increase down the group.
However, as the number of quantum shells increases, atomic radius increases. Hence, electronegativity
decreases down the group.
What affects strength of metallic bonding and how?
number of valence electrons contributed per metal atom
The larger the number of valence electrons contributed per atom, the greater the number of delocalised
electrons, the stronger the metallic bonding.
* charge and radius of the metal cation
The higher the charge and the smaller the radius of the metal cation, the higher its charge density*, and
the stronger the metallic bonding.
Why are metals good electrical and heat conductors?
Metals are good electrical conductors because the delocalised electrons act as mobile charge
carriers.
The delocalised electrons also explain why metals are good conductors of heat. Thermal energy causes the
electrons to move more quickly. The energy is then transferred to the other parts of the metal by the mobile
electrons.
Describe how metals change when a large stress is applied.
Metals are usually malleable and ductile. the layers of ions will
slide over one another into new positions. The overall shape of the metal changes (the metal deforms) but it does not break apart because the ‘sea’ of delocalised electrons prevents repulsion between the cations as they move past one another, and metallic bonding remains intact.
How can we compare strength of ionic bonding?
Lattice energy — the heat evolved when 1 mole of a pure ionic solid is formed from its constituent gaseous ions. This is dependent on the charge of the ions and their radii.
lattice energy
q+ × q−
r+ + r−
The higher the charge and the smaller the radii of the ions, the larger the magnitude of lattice energy, the stronger the ionic bonding.
Describe the electrical conductivity of ionic compounds.
In the solid state, ionic compounds do not conduct electricity as the ions are in fixed positions and are not free to move.
In the molten or aqueous state, ionic compounds are good electrical conductors because the ions can act as mobile charge carriers.
Describe the physical strength of ionic compounds.
In an ionic lattice, oppositely charged ions are held in fixed positions throughout the crystal lattice by strong ionic bonding. Moving the ions out of position requires large amounts of energy to overcome these bonds.
Ionic lattices are therefore quite hard and rigid. If a strong enough force is applied, it will force ions of like charges to move next to each other. Repulsion between ions of like charges will cause the lattice to shatter. Ionic lattices are
therefore brittle.
What is bond energy/strength of covalent bonds dependent on?
Bond strength depends on the degree of orbital overlap between the two bonded atoms and also how strongly the electrons in the overlap region are attracted to the nuclei of the atoms. This depends on bond length (distance between nuclei of two bonded atoms) and bond order (number of covalent bond between a pair of atoms)
What elements have more than 8 electrons in their valence shell? Why is this so?
They are period 3 elements and onwards. They can use their energetically accessible d orbitals to expand their octet.
Period 2 elements (e.g. nitrogen) have only four orbitals in their valence shells (2s, 2px, 2py and 2pz). Their 3d orbitals are not energetically accessible, because they are in an outer quantum shell which is at a much higher energy level. Thus, elements in Period 2 cannot accommodate more than four pairs of electrons in
their valence shells.
What are the principles of Valence Shell Electron Pair Repulsion Theory?
The electron pairs around the central atom of a molecule arrange
themselves as far apart as possible so as to minimise their mutual repulsion.
The repulsion between lone pair and lone pair > lone pair and bond pair > bond pair and bond pair.
Explain how the hybridisation theory came about.
Carbon has 2s^2 and 2p^2 orbitals — with only 2 unpaired electrons, it is unable to form 4 bonds. If carbon forms bonds in the ‘excited’ state where it has 1 electron each in the 2s, 2px, 2py, 2pz orbital, it would form 1 strong bond and 3 weaker bonds, which would result in a different shape from the tetrahedral shape on CH4. Thus, the hybridisation theory was proposed, where s and p orbitals mix to form sp orbitals, that are all of the same energy level to form 4 sigma bonds.
Why are there only 2 sp orbitals in ethene?
1 p orbital is being used to form the pi bond, as sp orbitals cannot be used to form pi bonds in the C=C bond.
What affects the strength of bonds in hybridised orbitals?
The percentage of orbitals that are S in character. The bond formed by a sp3 orbital is longer and weaker in character than a sp orbital.
What affects the degree of covalent character in ionic bonding and how?
The polarising power of the cation: Cations that are small and highly charge have high charge densities and high polarising
power. These cations have a high tendency to distort the anion’s electron cloud, resulting in greater covalent character in the ionic bonding.
Polarisability of the anion: Anions that are relatively large have high polarisability. Their valence electrons are further from and less
strongly attracted by the nucleus so the electron cloud is easily distorted by a cation, resulting in greater covalent character in the ionic bonding.
How are dispersion forces formed?
The electrons in a molecule are constantly moving. Fluctuations (or distortions) in electron distribution can cause an instantaneous dipole in one molecule, which induces a dipole in a nearby molecule. An attraction arises between the two molecules. This ‘short-lived’ and generally weak attraction is called instantaneous dipole-induced dipole (id-id) attraction or dispersion forces.
Factors affecting strength of dispersion forces
Number of electrons in molecule/size of electron cloud (related to Mr) linked to polarisability of electron cloud
Surface area of contact between adjacent molecules (depends on shape and branching of molecule)
Factors affecting strength of hydrogen bonding
- The electronegativity of the atom that H atom is bonded to
- The extent of hydrogen bonding depends on the number of H atoms bonded to F, O or N and the number of lone pairs available.
The hydrogen bonding in H2O is more extensive than that in HF or NH3. H2O has two lone pairs of
electrons and two electron deficient hydrogen atoms. H2O can form on average two hydrogen bonds
per molecule. On the other hand, NH3 has only one lone pair (despite having three hydrogen atoms); HF
has only one electron deficient hydrogen atom (despite having 3 lone pairs). NH3 and HF can only form an average of one hydrogen bond per molecule.
Applications of hydrogen bonding
Intramolecular hydrogen bonding occurs when the electron deficient H atom in a molecule is in close
proximity to the lone pairs on atoms (F, O, N) in the same molecule. Intramolecular hydrogen bonding — less intermolecular hydrogen bonding, lower boiling point
Dimerisation of carboxylic acids in non-polar solvents
Ice is less dense tan water
Describe the structure and physical properties of graphite
In graphite, each (sp2 hybridised) C atom is covalently bonded to three other C atoms, forming a two-dimensional layer of hexagonal carbon rings. Each C atom in graphite has a 2p orbital containing one electron (not used in forming the C–C sigma bonds). Within each layer, the 2p orbitals side-on overlap with each other to form a
delocalised pi bonding system. Between the layers of carbon atoms, there is (significant) dispersion forces due to the large surface area. Graphite is a good conductor of electricity and heat as the delocalised electrons are free to move along the layers. Graphite is soft and slippery as the layers can slide over each other due to the dispersion forces between them.
Graphite is insoluble in water. Attractions between the solvent molecules and carbon atoms are not strong enough to overcome the strong covalent bonds between the carbon atoms.
How to determine if a solute is soluble in a solvent
In terms of energy changes, a solute is soluble in a solvent if
the energy released from solute-solvent interactions is greater than or comparable to the energy needed to overcome solute-solute interactions and solvent-solvent interactions.
Why are some ionic compounds insoluble in water?
The energy released from ion-dipole interactions depends on the charge densities of the ions. Ions with lower charge densities (e.g. Ag+, Cl−) form weaker ion-dipole interactions so less energy is released. This explains why some ionic compounds e.g. silver chloride are insoluble in water.
