Atomic structure 1.1 Flashcards
why do isotopes have similar chemical properties
the same electronic configuration
what are the 4 steps in a mass spectrometer
1) ionisation ( electron impact and electro spray)
2) acceleration
3) flight tube
4) detection
what is the process of electron impact ionisation
- a vaporised sample is injected at low pressure
- an electron gun fired high energy electrons at the sample
- this knocks out an outer electron
- forming positive ions with different charges
what is the process of electro spray ionisation
- sample is dissolved in volatile solvents
- injected through a fine needle giving it a fine mist
- tip of needle has high voltage
- sample molecule gains a proton
- M(g) + H+ => MH+(g)
- The solvent evaporated away the MH+ ions towards a negative plate
when would electron impact be used
for elements with low formula mass .
Can cause larger organic molecules to fragment
when would you use electro spray ionisation
for larger organic molecules .
The softer conditions of this technique means fragmentation does not occur
what is the process of acceleration
- positive ions are accelerated by an electric field
- to a constant kinetic energy
- the velocity of each particle depends on its mass
- lighter particles have a faster velocity
what is the process of the flight tube
- the positive ions with the smaller m/z values will have the same kinetic energy as those with larger m/z and will move faster
- the heavier particles take longer to move through the drift area
- the ions are distinguished by different flight times
what is the process of detection
- the ions reach the detector and generate a small current.
- the current is produced by electrons transferring from the detector to the positive ions
what is the abundance of the species proportionate to
the size of the current
what is the relative atomic mass formula
= sum of ( isotopic mass x %abundance) / 100
what is the relative atomic mass formula if relative abundance is used instead of percentage abundance
sum of ( isotopic mass x relative abundance ) / total relative abundance
define first ionisation energy
the enthalpy change when one mole of gaseous atoms forms one mole of gaseous ions with a single positive charge
H(g) => H+(g) + e-
define second ionisation energy
Enthalpy change when one mole of gaseous ions with a single positive charge forms one mole of gaseous ions with a double positive charge
Ti + (g) => Ti2+ (g) + e-
what are the factors that affect ionisation energy
1) attraction of the nucleus
2) the distance of the electrons from the nucleus
3) shielding of the attraction of the nucleus
why are successive ionisation energies always larger ( ones after first ionisation energy )
Second ionisation is energy of an element is always bigger than the first ionisation energy .
When the first electron is removed on a positive ion is formed.
The ion increases the attraction on the remaining electrons and so the energy required to remove the next electron is larger
EQ : A TOF mass spectrometer can be used to determine the relative molecular mass of molecular substances.
Explain why it is necessary to ionise molecules when measuring their mass in a TOF mass spectrometer (2)
- Ions , not molecules , will interact with and be accelerated by an electric field
- Only ions will create a current when hitting detector
Outline how the TOF mass spectrometer is able to separate these two species to give two peaks (4)
- Positive ions are accelerated by an electric field
- to a constant kinetic energy
- positive ions with m/z of 104 have the same kinetic energy as those with m/z of 118 and move faster
- therefore ions with m/z 104 arrive at detector first
EQ : Why do isotopes have the same chemical properties (1)
- Same number of electrons in the outer most shell
what’s the equation to represent electron impact ionisation
X (g) => X+(g) + e-
EQ : define the term
relative atomic mass (1)
- Average mass of one atom of an element compared to 1/12th the mass of one atom of carbon 12
EQ : In a mass spectrometer , the isotopes of an element are separated . Two measurements for each isotope are recorded on the mass spec . State the two measurements that are recorded for each isotope . (2)
- m/z
- relative abundance
EQ : Predict the element in period 3 that has the highest second ionisation energy . Give a reason for your answer (2)
- Na
- Electron removed from 2p orbital which is nearer the nucleus so more attraction
EQ : explain why the ionisation energy of every element is endothermic (1)
- Energy needed to overcome the attraction between the electron and the nucleus
