Periodicity Flashcards
Zeff across a period and down a group
Across a period increased attractions to nucleus overcome increased repulsions
Other electrons in the same shell don’t fully shield
Zeff rises across period
Zeff is roughly constant down a group
Zeff =
Z - S
Z = no. of neutrons in the nucleus
S = shielding (calculated using Slater’s rules)
Slater’s rules to calculate shielding
Write out electron configuration
Any electrons to the right of the electron of interest have no effect
if in s or p orbital:
S = 0.35 x (no of electrons in same group) + 0.85 x (no of electrons in one group down) + 1.00 x (no of electrons in 2 groups down or lower)
if in d or f orbital:
S = 1.00 x (no of electrons to left of electron of interest)
Periodicity: E =
- (Z(eff)^2 R(H)) / n^2
How to calculate exchange energy for B - Ne
Draw out the p orbitals and electrons
Exchange energy = -K x (no of pairs of electrons with parallel spin)
Definition of elecronegativity
The power of an atom in a molecule to attract electrons to itself
Electronegativity increases…
Across a period and up a group as elements get smaller with a higher nuclear charge, as outer electrons are closer to the nucleus
Any element that comes immediately after the d-block have higher electronegativity because d orbitals are filled –> higher Zeff
Electronegativity of group 13
Although order down group is B, Al, Ga, In
Ga actually has higher electronegativity than Al, because Ga comes after 3d, so there are extra protons in the nucleus, so it is not fully shielded, and Zeff is higher than expected
What are the 4 factors relating to Zeff?
Ionisation energy
Electron attachment enthalpy
Electronegativity
Size of atoms
Definition of bond strength and what effects it
The energy required per mole to break a chemical bond
Bond strength is effected by atom size and electronegativity differerence
Increase in electronegativity difference –> stronger bond
Larger atoms –> worse overlap between orbital –> weaker bonds
Bonds such as F-F and O-F are weaker due to lone pair- lone pair repulsion
Also consider if multiple bonding is possible
Definition of boiling point and what effects it
The temperature at which the vapour pressure of a liquid equals the pressure of the atmosphere
Determined by intermolecular forces
VDW attractions are instantaneous dipole-dipole interactions and increase as atoms get larger and more polarisable
Also consider permanent dipole interactions and if hydrogen bonding is possible
Definition of bond length and what effects it
The X-X distance in a homonuclear bond
As atoms get larger, X-X distance increases
What are permanent dipole forces?
Caused by a change in electronegativity between atoms
Stronger than VDW
Dipole moment definition and what effects it
A measure of the asymmetry of charge distribution in a chemical bond
Larger electronegativity difference –> highest dipole moment
Melting point definition and what effects it
Temperature at which element melts at atmospheric pressure
Depends on form of element and so intermolecular forces
Diatomic molecules have very low MP, S exists as S8, relatively low MP, ionic / metallic structures much higher MPs
