Atomic Structure Flashcards
Define atomic number, mass number, isotopes and give relative mass of an electron
Atomic number - number of protons in nucleus of an atom
Mass number - number of protons and neutrons in nucleus of an atom
Isotopes - atoms of same element with the same number of protons but a diff number of neutrons
Relative mass of an electron = 1/1840
T.O.F Mass Spectrometer : Ionisation (Electron Impact)
-Sample is injected into spectrometer and vaporised into a gas
-High energy e- from an e- gun are fired at the sample
-High energy e- knock off an electron from each particle to form positive ions
Na(g) —> Na+(g) + e-
-Used for elements and smaller compounds
T.O.F Mass Spectrometer : Ionisation
(Electrospray)
-Sample dissolved in polar solvent (provides H+)
-Injected through a fine hypodermic needle to give a fine mist
-Tip of the needle is attached to the positive terminal of a high voltage supply
-Particles gain a proton (H+ ion) from solvent to form positive ions
Volatile solvent is used to ensure the solvent evaporates and to leave only positive ions.
-Used for larger compounds
Distinguish between electron impact and electrospray ionisation
In electron impact you lose an electron.
In electrospray ionisation you gain a H+ ion.
T.O.F Mass Spectrometer : Acceleration
An electric field is applied to accelerate the positive ions.
This gives all the ions with the same charge the same kinetic energy.
T.O.F Mass Spectrometer : Separation
The positive ions travel through the mass spectrometer to the detector and the time of flight of each particle depends on its velocity.
T.O.F = d/v
Lighter ions travel faster and reach detector first, heavier ions travel slower.
T.O.F Mass Spectrometer : Detection
A detector records the different ions as they arrive.
-At the detector each positive ion gains an electron which generates an electric current.
-The size of the current is proportional to the abundance of each isotope.
T.O.F Mass Spectrometer : Key principles
-The sample must be ionised so that it can be accelerated and detected.
-The while spectrometer is kept under vacuum to prevent the ions from colliding with particles in the air.
How to calculate the mass of an ion (kg)
(Mass number X 10^-3) / 6.022x10^23
Calculating the T.O.F of an ion
All ions w the same charge are given the same kinetic energy.
M1/(T1)^2 = M2/(T2)^2
In this calculation the mass number can be used instead of actual mass.
Analysis of Mass Spectra
In a mass spectrum, each peak represents an isotope of the element.
The highest peak is the most abundant so it generates the largest current at the detector.
The atomic mass of elements in the periodic table is a weighted average of their isotopes.
Define relative isotopic and relative atomic mass
Relative isotopic mass - the mass of an isotope relative to 1/12th the mass of an atom of carbon-12.
Relative atomic mass - (weighted avg mass of all the isotopes) / 1/12 mass of one atom of 12C
What are fragmentation peaks
Fragment ions which form when one bond breaks in the molecular ion.
-Only happens in electron impact ionisation as it requires high energy meaning it’s harsh.
-Fragmentation peaks usually appear below the highest isotope peak.
Why do isotope peaks appear after the highest isotope peak
Some molecules may contain 13C or 2H
Formula for max number of electrons in a shell
2n^2
- s subshell contains 2 electrons
- p subshell contains 6
- d subshell contains 10
- f subshell contains 14
Electron arrangement of Cr and Cu
They only have 1 electron in the 4s subshell
Define 1st I.E
The energy needed to remove 1 mole of electrons from 1 mole of atoms in the gaseous state.
Factors which influence 1st I.E:
-Atomic radius
-Shielding
-Number of protons
Why do successive I.E increase
Same number of protons attracting fewer electrons => stronger attraction.
