1.1 - 1.6 Atomic structure Flashcards
Exam question from card 29
Relative atomic mass =
(isotope mass x abundance) + (isotope mass x abundance) / Total abundance
Potential energy (J) =
charge(Q) x voltage(v)
Kinetic energy (J) =
1/2 x mass (kg) x [velocity(m/s)]^2
Velocity (m/s) =
distance(m) / time(s)
What are the relative mass and charges of the 3 sub-atomic particles?
Particle / Relative mass / Relative charge
Proton / 1 / +1
Neutron / 1 / 0
Electron / Negligible / -1
What is the name of the force that attracts protons and neutrons together in the nucleus?
Nuclear force
What is the name of the force that attracts protons and electrons?
Electrostatic attraction
Describe the development of the atomic model
1803 - John Dalton, recognised atoms of particular elements differ from other elements
1904 - J.J Thomson, discovered electrons and established plum-pudding model
1911 - Ernest Rutherford, gold and alpha particles experiment discovers nucleus
1913 - Niels Bohr, proposed stable electron orbits
1926 - Erwin Schrodinger, discovered orbitals
1932 - James Chadwick, discovered neutrons
What is the purpose of mass spectometery?
To find out the relative atomic mass, relative molecular mass and relative abundances of isotopes
What are the four stages of mass spectometery?
- Ionisation
- Acceleration
- Ion drift
- Detection
What are the 2 types of ionisation?
- Electron impact ionisation
- Electrospray ionisation
Describe electron impact ionisation
- Sample vaporised
- Electron gun fires high energy electrons at it
- Knocks 1 electron off each atom (the outermost electron)
- Forms positive ion
This method often causes fragmentation
Describe electrospray ionisation
- Sample dissolved in polar solvent
- Sample gains protons
M + H+ –> MH+ - Pumped through capillary needle
- Voltage applied
- Causes aerosol to be formed
- Solvent evaporates and leaves MH+ ions
This method does not cause fragmentation
Describe the process of acceleration in mass spectometery
- An electrical field is applied to a series of plates
- This gives the same kinetic energy to all the ions
- Ions with a lower m/z ratio (lighter ions) will be accelerated more than those with higher m/z
(Attracted to negative plate)
Describe the process of ion drift in mass spectometery
- All the ions leave the electrical field with equal kinetic energy
- In the drift region, lighter ions travel faster than heavier ions
- Deflected by electromagnet
Describe the process of detection in mass spectometery
- Ions hit detector and gain electrons, generating a current
- The computer records the length of time it took the ion to pass through the machine and its abundance and uses this to create a mass spectrum
What does an m/z peak on a mass spectrum mean if electron impact ionisation has been used?
m/z peak = Mr of molecule
What does an m/z peak on a mass spectrum mean if electrospray ionisation has been used?
m/z peak = Mr of molecule + 1
What is the maximum number of electrons each energy level can hold?
Energy level Max no. of electrons
1 2
2 8
3 18
4 32
What is the Aufbau Principle?
An electron will always fill the available orbital of the lowest energy
What is the Pauli Exclusion Principle?
Each orbital can hold a maximum of 2 electrons
What is Hund’s rule?
Atomic orbitals of the same energy fill singly with electrons of the same spin before pairing of electrons occurs
Define ionisation energy
The energy required to remove 1 mole of electrons from 1 mole of an element in its gaseous state
What is the trend in ionisation energy going down the group?
Decreases because:
- More protons/larger nuclear charge
HOWEVER
- Bigger atomic radius
- More shielding
This means weaker attraction between the nucleus and electrons so less energy is required to remove an electron
What is the trend in ionisation energy going across a period?
Increases because:
- Smaller atomic radius
- More protons/stronger nuclear charge
- Same shielding
This means stronger attraction between the nucleus and electrons so more energy is required to remove an electron
Explain the dip in ionisation energies between the 2nd and 3rd group
The outermost electron comes from the P orbital which is slightly further away from the nucleus so the electron can be removed more easily. P-orbital is higher energy and more shielded.
Explain the dip in ionisation energies between the 5th and 6th group
The 1st and 4th electrons in the 3p orbital are paired. As they are both negatively charged they slightly repel each other, meaning it requires less energy to remove one.
Describe the equation for calculating Ar from a mass spec
(m/z x abundance) + (m/z x abundance) / total abundance
State what is adjusted in a mass spectrometer in order to direct ions with different values onto the detector
Electromagnet. This is maintain the correct link between deflection and m/z.
What would cause an ion to form a mass spec peak at half the expected m/z value?
The formation of a 2+ ion.
For example, 72Ge2+ peaks at 36.
Which device in a mass spec ionises particles?
Electron gun
What factors determine how much a particle is deflected in a magnetic field of given strength?
- m/z
- Speed
Give three reasons why ionisation is necessary
To allow ions to be:
- accelerated (by an electric field)
- deflected (by a magnet/magnetic field)
- detected / description of current formed at the detector/sensor
Explain why it would be difficult to distinguish between 48Ti2+ and 24Mg+ ions using a mass spectrometer
- Both ions have the same m / z value (of 24)
- Deflected equally (so detected together)
