Atomic structure Flashcards
Definition of isotopes
Isotopes are atoms with the same number of protons, but different number of neutrons
Describe Electron impact
- A vaporised sample is injected at low pressure
- An electron gun fires high energy electrons at the sample
- This knocks out an outer electron
- Forming positive ions with different charges
e.g Ti(g) —> Ti+(g) +e-
Describe electrospray ionisation
- The sample is disabled in a volatile, polar solvent
- injected through a fine needle giving a fine mist or aerosol
- the tip of the needle has high voltage
- at the tip of the need the sample molecule, M, gains a proton from the solvent forming MH+
M(g) + H+ —> MH+(g) - The solvent evaporates away while the MH+ ions move towards a negative plate
Describe stage 2 of TOF
Acceleration:
- positive ions are accelerated by an electric field
- to a constant kinetic energy
- given that all the particles have the same kinetic energy, the velocity of each particle depends on its mass. Lighter particles have a faster velocity, and heavier particles have a slower velocity.
Describe stage 3 of TOF
Flight tube:
*The positive ions with 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
Describe stage 4 of TOF
Detection:
The ions reach the detector and generate a small current, which is fed to a computer for analysis. The current is
produced by electrons transferring from the detector to the positive ions. The size of the current is proportional to the
abundance of the species
What type of ionisation leads to fragmentation
Electron impact ionisation
Define the first ionisation energy
The first ionisation energy is 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 the second ionisation energy
The second ionisation energy is the 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 affecting ionisation energy
1.The attraction of the nucleus
(The more protons in the nucleus the greater the attraction)
2. The distance of the electrons from the nucleus
(The bigger the atom the further the outer electrons are from the nucleus and the
weaker the attraction to the nucleus)
3. Shielding of the attraction of the nucleus
(An electron in an outer shell is repelled by electrons in complete inner shells,
weakening the attraction of the nucleus)
Why do first ionisation energies decrease down a group?
As one goes down a group, the outer electrons are found in shells
further from the nucleus and are more shielded so the attraction of the nucleus becomes smaller
Why is there a general increase in first ionisation energy across a period?
As one goes across a period the electrons are being added to the same shell which has the same distance from the nucleus and same shielding effect. The number of protons increases, however, making the effective attraction of the nucleus greater
Why is there a small drop from Mg to Al?
Al is starting to fill a 3p sub shell, whereas Mg has its outer electrons in the 3s sub shell. The electrons in the 3p subshell are slightly easier to remove because the 3p electrons are higher in energy and are also slightly shielded by the 3s electrons
Why is there a small drop from P to S?
With sulfur there are 4 electrons in the 3p sub shell and the 4th is starting to doubly fill the first 3p orbital. When the second electron is added to a 3p orbital there is a slight repulsion between the two negatively charged electrons which makes the second electron easier to remove
why is the TOF apparatus set in a vacuum
To prevent the ions that are produced colliding from the molecules from the air.
