Atomic structure Flashcards
What is the relative charge of a proton
+1
What is the relative charge of a neutron
0
What is the relative charge of an electron
-1
What is the relative mass of a proton
1
What is the relative mass of a neutron
1
What is the relative mass of an electron
1/2000
What is an isotope
An element with the same number of protons but different number of neutrons
State briefly the history of the atom
Dalton - Atoms are spheres
JJ Thompson - Plum pudding model
Rutherford - Gold leaf experiment (alpha particles fired at gold leaf some went through some deflected some reflected)
Bohr - Electrons fixed in energy shells (Experiment to prove this was when EM radiation was absorbed electrons moved between shells and they emit this radiation when electrons move down to lower energy shells)
Today - Electrons have subshells (they don’t have the same energy in shells)
What are the 5 stages in TOF mass spectrometry
Vaporisation (for electron gun)
Ionisation
Acceleration
Flight tube
Detection
Explain how electron impact ionisation works in TOF mass spectrometry
*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
What is the general equation for electron impact ionisation
X(g) —> X⁺(g) + e⁻
Explain how electrospray ionisation works in TOF mass spectrometry
- The sample is dissolved in a volatile, polar solvent
- injected through a fine needle giving a fine mist or aerosol
- the tip of needle has high voltage (tip is attached to a high voltage power supply)
- At the tip of the needle the sample molecule, M, gains a proton, H+ from the solvent forming MH+
- The solvent evaporates away while the MH+
ions move towards a negative plate
When is electron impact ionisation more likely to be used over electrospray ionisation
Used for elements and substances with a low formula mass as can cause larger organic molecules to fragment
When is electrospray ionisation more likely to be used over electron impact ionisation
Electro spray ionisation is used preferably for larger organic molecules. The ‘softer’ conditions of this technique mean fragmentation does not occur
What is the general equation for electrospray ionisation
M(g) + H+ —> MH+(g)
How are ions detected in TOF mass spectrometry
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
Describe the velocity of ions with different m/z values during the 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.
What is the definition of relative atomic mass (Ar)
The average mass of an atom of an element when measured on a scale on which the mass of an atom of C12 is exactly 12
What is the definition of relative molecular mass (Mr)
The average mass of a molecule when measured on a scale on which the mass of an atom of C12 is exactly 12
What is the definition of relative isotopic mass
The mass of an atom of an isotope of an element measured on a scale on which the mass of an atom of C12 is exactly 12
What is the equation for calculating relative atomic mass from abundance
(%abundance of A x m/z of A) + (%abundance of B x m/z of B)
What is the M+1 or molecular ion peak
If a molecule is put through a mass spectrometer with an electron impact ionisation stage it will often break
up and give a series of peaks caused by the
fragments. The peak with the largest m/z will be due to the complete molecule and will be equal to the relative molecular mass , Mr ,of the molecule. This peak is called the parent ion or molecular ion
Why does ionisation energy decrease down a group
Atomic radius increases, increase in shielding, therefore the electrostatic force of attraction between the nucleus and outer electron is weaker meaning the energy required to remove the outer electron decreases
What is the trend in ionisation energy across a period
Generally increases due to an increase in protons with the same shielding leading to a greater electrostatic force of attraction between outer electron and nucleus
State and explain the exceptions to the trend in ionisation energy across period 3
Al - The outer electron in Al sits in a higher energy 3p subshell which is slightly further from the nucleus and therefore has a lower first ionisation energy compared to Mg, whose outer electron sits in the 3s subshell
S - The outer electron in S sits in an orbital with 2 electrons in it. Thus, as opposite charges repel, less energy is required to remove an electron from an orbital with 2 electrons compared to an orbital with 1 electron such as in P
What is the definition of first ionisation energy
The enthalpy change when one mole of gaseous
atoms forms one mole of gaseous ions with a single positive charge
or
Enthalpy change for
X(g) —> X+(g) + e-
