L7 - Chemical Bonding (1) Flashcards
What is a chemical bond?
An attractive force that holds two or more atoms together in a molecule.
What are valence electrons?
Outer shell electrons.
What is valency?
How many electrons an atom can give / take.
Why do atoms combine?
To gain stability.
How do electrons in atoms organise themselves?
Electrons in atoms distribute themselves in orbitals in the most stable / lowest energy configuration.
What does rearranging electrons between atoms allow?
Rearranging electrons between atoms allows the combined molecule to achieve even lower energy.
State the Octet Rule.
“Atoms tend to adjust their valence shells to 8e- by losing, gaining or sharing electrons in an attempt to gain a Noble Gas configuration.”
What is significant about shells above n=3?
Atoms have the option of making more bonds than typically predicted.
Describe Expanding the Octet.
When some atoms hold more than 8 valence electrons in an attempt to obtain stable electronic configuration.
The Octet Rule only applies to main group elements. Explain some exceptions.
Transition metals using d-orbitals obey the 18-electron rule instead.
Describe forces between atoms.
- Attractive / repulsive forces operate between 2 atoms when they come close to each other
- Like charges repel: electron-electron and nucleus-nucleus
- Opposite charges attract: electron-nucleus
Describe the strength of attraction between atoms.
- Attraction is strong over a short distance, then strength rapidly decreases
- Negative energy value in order to stabilise interaction (until minimum = optimum bond length)
Describe what is meant by removing an electron to infinity.
When an electron is moved so far from the atom that it is no longer attracted to the nucleus.
State Coulomb’s Law.
“Like charges repel and opposite charges attract, with a force proportional to the product of the charges, and inversely proportional to the square of the distance between them.”
Define Electronegativity.
The ability of an atom to attract electrons towards itself in a chemical bond.
What does electronegativity depend on?
- Number of protons in the nucleus
- Distance from the nucleus
- Size of the atom
- Amount of shielding by inner electrons
What does shielding refer to?
The number of full shells between the valence shell and the nucleus.
What is the Valence Shell?
The outer shell.
What is used to measure the electronegativity of elements?
The Pauling Scale
What is the most electronegative element, and what are its properties?
- Fluorine
- Small atomic radius
- High nuclear charge
Name some highly electronegative elements.
- Nitrogen
- Oxygen
- Fluorine
- Chlorine
- Krypton
Name some intermediate electronegative elements.
- Hydrogen
- Carbon
- Sulfur
- Selenium
What can electronegativity values and the difference between them be used to predict?
Electronegativity values and the difference between them can be used to predict the type of bond which is formed in a particular compound.
Describe the different bonds formed by different elements, with reference to electronegativity.
- Left hand side = low values of electronegativity = bonds with same / similar elements, metallic bonds
- Right hand side = high values of electronegativity = non-metallic compounds when bonding with similar elements, covalent bonds
What does a large difference in electronegativity lead to?
Complete transfer of electrons = Ionic bonding
What does electronegativity predict in different types of bonding?
- Covalent bonding: Electronegativity predicts polarity of bonds and the direction of polarity
- Ionic bonding: Electronegativity predicts direction of initial transfer of electrons
When are ionic bonds formed?
When one or more electrons are fully transferred between atoms.
What does ionic bonding take place between?
Mets and non-metals.
Describe the transfer of electrons with reference to the type of atom.
- Metal atoms try to attain noble gas structure by losing electrons to form positive ions (cations)
- Non-metal atoms try to attain noble gas structure by gaining electrons to form negative ions (anions)
What is electrostatic attraction?
When positive and negative ions attract each other to give an ionic bond.
What does formation of an ionic bond involve?
A decrease in energy compared to the two isolated atoms.
What is the charge of an ion equal to?
The valency of an ion.
What is an ionic bond?
An electrostatic attraction between positively and negatively charged ions.
Describe the properties of ionic bonds.
- Non-directional (the strength of interaction between two ions depends on the distance, but not on the direction)
- Ionic bonds break when ionic compounds dissolve and dissociate in polar solvents
- Ionic bonds also break when the substance is melted
Describe the features of ionic compounds.
- Formed between metals and non-metals
- Dissolve / dissociate easily in water and other polar solvents (separated ions are solvated)
- Conduct electricity when molten and in solution
- Form crystalline solids with high melting temperatures
Describe an Ionic Lattice.
- Every ion is attracted to all other ions with the opposite charge
- Results in a repeating 3D pattern of anions and cations in the solid state
- Giant structure
Define Lattice Energy.
The enthalpy of formation (energy change) of forming one mole of the ionic solid from its gaseous ions.
What does lattice energy measure?
Bond strength
What favours easy cation formation?
- Low ionisation energy of the metal
What favours easy anion formation?
- High electron affinity / electronegativity of the non-metal
- Small size of the non-metal
- Low charge on anion
Describe the factors favouring ionic bonding.
- Large difference in electronegativity between the combining atoms
- High lattice energy of the ionic compound
Describe changes in the atom once ions are formed.
- Positive Ions: Loss of electrons = atomic radius becomes smaller as + charge increases
- Negative Ions: Gain of electrons = atomic radius becomes larger as - charge decreases
Define Polarisation.
Distortion of the electron cloud of an atom.
Describe polarisation with reference to the size of ions.
- The smaller an ion, the higher its charge, the more polarising power it has (ability to distort the electron cloud of another atom).
- The larger an atom and the larger its number of electrons, the higher its polarisability.
Describe polarisation with reference to anions and cations.
Cations are typically smaller than anions and thus, cations tend to have more polarising power while anions tend to be more polarisable.
Describe polarisation with reference to the size of charges.
- Small charge / positive charge = generally not highly polarised = strong attraction for electron, so high charge density = ionic bonds (Cations tend to be polarising)
- Large charge / negative charge = easily polarised = some covalent character in bonds (Anions get polarised)
Describe the process of polarisation.
- Negative ion is distorted = has some covalent character
- Electron density is starting to be pulled from the negative ion to the positive ion so bonding is starting to become directional
What is an ideal ionic bond / compound?
- Ionic bond with no polarisation
- Ionic compound which is composed of completely separate, spherical ions
What does polarisation result in?
- Non-spherical ions
- Covalent character in the bond
When is an ionic compound likely to have some covalent character?
- Cation is small and/or has a high charge = highly polarising
- Anion is large and/or has a high charge = highly polarisable
State Fajan’s Rules.
Pure ionic compound:
- Low positive charge
- Large cation
- Small anion
Ionic compound with covalent character:
- High positive charge
- Small cation
- Large anion
State the formulae of the following polyatomic ions:
- Ammonium
- Cyanide
- Hydroxide
- Peroxide
- Acetate
- Oxalate
- Nitrate
- Sulfate
- Carbonate
- Phosphate
- Permanganate
- Dichromate
- Ammonium: NH4+
- Cyanide: CN-
- Hydroxide: OH-
- Peroxide: O2 2-
- Acetate: CH3 CO2 -
- Oxalate: C2 O4 2-
- Nitrate: NO3 -
- Sulfate: SO4 2-
- Carbonate: CO3 2-
- Phosphate: PO4 3-
- Permanganate: MnO4 -
- Dichromate: Cr2O7 2-
State the important anions and cations in the body.
Anions:
- Cl-
- HCO3 -
- HPO4 2-
- SO4 2-
- PO4 3-
Cations:
- Na+
- K+
- Ca 2+
- H+
- Mg 2+
What type of bonding is present between polyatomic ions?
Covalent
