week 1 Flashcards
vsepr stand for?
valence shell electron pair repulsion model
how do you determine the central atom of a molecule?
hydrogen is terminal
central atom is the atom in the earliest group
if atoms are in the same group, the larger and least electronegative atom is central
4 steps of vsepr?
- count valence electrons of central atom
- add an electron for every single bond and add 2 for every double bond around the central atom
- add or subtract an electron for negative or positive charges
- half this number to determine the number of electron pairs
what is vsepr used for?
determine 3-D shape of molecules
use vsepr to predict shape of CH4
- 4 valence electrons on C = 4
- add 4 for 4 single bonds around C = 8
- no +ve or -ve charges
- 8/2 = 4 so 4 bonding electron pairs, no lone pairs so tetrahedral
what shape has 3 bonding pairs and 1 lone pair?
trigonal pyramidal
what shape has 2 bonding pairs and 2 lone pairs?
bent
what shape has 5 bonding pairs and 0 lone pairs?
trigonal bipyramidal
what shape has 4 bonding pairs and 1 lone pair?
disphenoidal
what shape has 3 bonding pairs and 2 lone pairs?
t-shaped
what shape has 6 bonding pairs and 0 lone pairs?
octahedral
what shape has 5 bonding pairs and 1 lone pair?
square pyramidal
what shape has 4 bonding pairs and 2 lone pairs?
square planar
advantages and disadvantages of vsepr?
ads:
- simple and mainly correct
- works well for single bonds
- adaptable for double bonds
disads:
- no info on energy, bonding or reactivity
- not 100% correct
- doesn’t work for d-block elements
what does quantisation refer to in terms of subatomic particle energy
refers to the idea that subatomic particles cannot have any energy value and are limited to specific energy values or states
energy is inversely proportional to ?
wavelength
what happens when H2 is exposed to high voltage
what does it tell us about
hydrogen atoms are created and excess energy is released in the form of light
light absorbs in to spectrum and line spectrum shows only specific energies are emitted
the energy of light emitted by hydrogen atoms is restricted to transitions between states identified by integer values
give the equation linking energy and frequency
E = h𝝂
E is energy in J
h is planks constant = 6.262x10^-34 Js
𝝂 is frequency of radiation in s-1 or Hz
energy is proportional to frequency
what is Rydberg’s equation?
what does it tell us?
1/λ = RHZ^2(1/n1^2 -1/n2^2)
RH is Rydberg’s constant which varies for each element
RH for hydrogen = 3.29x10^15 Hz
Z is nuclear charge
n1 and n2 are integers where n1 < n2
equation tells us that the energy of light emitted by hydrogen atoms is restricted to transitions between states, identified by integer values
which value of n is
ground state?
ionisation?
n = 1 is ground state, lowest energy level
n = ∞ is an infinite distance from nucleus = ionisation of electron
radius is proportional to ?
energy is inversely proportional to ?
r is proportional to n^2
E is proportional to -1/n^2
give the equation to find the energy of a certain orbital
E = -RHZ^2/n^2
what kind of system does the rydberg-bohr model work for?
one electron systems He+, Li2+, B3+ etc
why did Sommerfeld modify the Bohr model?
on a line spectrum, more lines are visible than expected
more than one orbit can have the same energy
Sommerfeld’s modification allows orbitals to be non-circular/elliptical so they have angular momentum which must be quantised and orientation to be quantised
by quantising shapes and orbits he introduced additional energy levels corresponding to fine spectral lines
