3.1.1 Atomic Structure Flashcards
Dalton and Thomson Models
19th century Dalton - Atoms are spheres, different spheres make up different elements.
Thomson - atoms contains electrons
‘plum pudding model’
Rutherford’s model
Gold foil experiment, fired alpha particles at gold foil.
Most particles passed straight through, very small number reflected backwards.
Nuclear model.
Mostly empty space, positive central nucleus and random electrons in empty space
Problem: cloud of electrons would spiral down into nucleus.
Bohr’s model
Electrons only exist in fixed orbitals with fixed energy levels.
When electrons move between shells electromagnetic radiation is emitted or absorbed.
Level of radiation has a fixed frequency.
Later changed to include sub-shells.
Sub-atomic particles relative charge
Proton +1
Neutron 0
Electron -1
Sub-atomic particles relative mass
Proton +1
Neutron +1
Electron 1/2000
The structure of the atom
All elements are made of atoms.
Atoms contain protons and neutron in a nucleus and orbiting electrons in fixed energy levels (shells)
Mass number
A - number of protons and neutrons
Atomic number
Z - number of protons.
Isotopes
Different atoms of the same element, same number of protons, different number of neutrons.
In general have same chemical reactivity as same electronic configuration.
Purpose of a mass spectrometer
Find relative atomic mass by identifying the mass and abundance of isotopes in a sample.
Finding the relative molecular mass of a substance made of molecules.
Identify elements
Electrospray ionisation
sample is dissolved in a volatile solvent
injected through a fine hypodermic needle to give a fine mist
attached to the positive terminal of a high voltage power supply.
Gains a proton (mass number increases by one)
One plus ion.
Electrospray ionisation Equation
X(g) + H+ —> XH+(g)
Electron impact ionisation
Sample is vaporised
High energy electrons are fired at it from an electron gun
Knocks off one electron to form a one plus ion
Electron impact ionisation Equation
X(g) + e- —–> X+(g) + 2e-
When is electron impact ionisation used?
Elements and substances with low formula mass
When is electrospray ionisation used?
Substances with a high molecular mass.
Biological molecules such as proteins.
Acceleration (TOF)
Ions are attracted towards a negative plate where they are accelerated.
Have the same Kinetic energy.
Uses an electric field
Kinetic energy equation and units
KE = 0.5 x m x v2
KE joules
m Kilograms
v meters per second
Flight tube (TOF)
Positive ions travel through a hole in a negatively charged plate.
Travel through flight tube.
As all have same kinetic energy are separated by mass, lightest ions travel the fastest.
Speed distance time equation and units.
Speed = distance / time
Speed meters per second
Distance meters
Time seconds
Detector (TOF)
Lightest ions travel the fastest so take the less time to reach the detector.
Positive ions hit a negatively charged plate.
Discharged by gaining an electron from the plate.
Movement of charge generates a current.
Size of current is proportional to the abundance.
Mass spectrum tips
Electrospray ionisation
Molecular ion is highest peak to the right
Smaller peaks to the left are due to fragmentation
Surrounding peaks are because of isotopes.
Remove one from the molecular ion due to mass of added proton.
Mass spectrum tips
Electron impact ionisation
Smaller peaks due to different ion eg (2+) efecting mass charge ratio on the x axis.
Height of peaks proportional to abundance of ion.
Relative molecular mass
The average mass of one molecule relative to one twelfth the mass of one molecular of carbon twelve.
