Atomic Structure Flashcards
1
Q
Explain how ions are accelerated, detected and have their abundance determined in a TOFMS.
A
- Ions accelerated by electric field
- Detected by gaining electrons, producing current
- Abundance is proportional to size of current
1
Q
Describe and explain the operation of a mass spectrometer.
A
- VAPORISING the sample in a vacuum, usually by heating
-
IONISATION, either by
* electron impact (high energy electrons are fired at the sample from an electron gun to knock off an electron, forming 1+ gaseous ion)
* electrospray ionisation (voltage applied to a dissolved sample, and each particle gains a H+ ion, making the sample a gas made of cations) - ACCELERATION through a slit in a negatively charged plate (ions get the same KE regardless of its mass)
- DRIFT through a time of flight chamber
- DETECTION: the 1+ ions each gain an electron from the detector. This movement of electrons is electrical current; output recorded on computer
2
Q
Why do we ionise the atoms in a TOFMS?
A
- only ions can accelerate in an electric field
- to deflect the beam
3
Q
How do we keep the ions in a TOFMS moving?
A
air is sucked out of the tube, forming a vacuum
4
Q
State how, and explain why, the values of the first ionisation energies of the elements aluminium & sulfur deviate from the general trend.
A
-
For aluminium: (third) electron in (3) p sub-shell.
* Sub-shell further away from nucleus OR of higher energy OR extra shielding. -
For sulphur: Pair of electrons in (3) p orbital.
* Repulsion between electrons.
Note:
Group 3 elements: valence electron further from nucleus
Group 6 elements: pair of electrons = repulsion
