Atomic structure Flashcards
What is an isotope?
Atoms with the same number of protons but different number of neutrons.Similar chemical properties
Why is the mass spectrometer under a vacuum?
It needs to be under a vacuum
otherwise air particles would
ionise and register on the
detector
What are the 4 steps of mass spectrometry?
1.Ionisation 2.Acceleration 3.Ion drift 4.Detection
For each isotope the mass spectrometer can
measure a m/z (mass/charge ratio) and an
abundance
What occurs in ionisation?
Sample is ionised through electron spray ionisation or electron impact.
What occurs in electron spray ionisation?
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
* at the tip of the needle the sample molecule, M, gains a proton, H+
, from the
solvent forming MH+ * M(g) + H+ MH+
(g) * The solvent evaporates away while the MH+
ions move towards a negative plate
What is 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
When is electron spray ionisation used vs electron impact
Electron impact is used for elements
and substances with low formula mass.
Electron impact can cause larger organic
molecules to fragment.
Electro spray ionisation is used
preferably for larger organic
molecules. The ‘softer’ conditions
of this technique mean
fragmentation does not occur
What occurs in 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.
Explain Ion drift / 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
What occurs in 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
why have Mass spectrometers been included in planetary space probes?
Mass spectrometers have been included in planetary space probes so that elements on other planets can
be identified. Elements on other planets can have a different composition of isotopes.
Measuring the mr of a molecule using electron impact?
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, however,
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
Measuring the mr of a molecule using electron spray ionisation?
If a molecule is put through a mass spectrometer with electro spray ionisation then
fragmentation will not occur. There will be one peak that will equal the mass of the MH+
ion. It
will therefore be necessary to subtract 1 to get the Mr of the molecule. So if a peak at 521.1 is
for MH+
, the relative molecular mass of the molecule is 520.1.
How much electrons can each subshell hold?
Sub energy levels labelled s ,
p, d and f s holds up to 2 electrons
p holds up to 6 electrons
d holds up to 10 electrons
f holds up to 14 electrons
Electronic configuration of copper and chromium
Cr 1s2 2s2 2p6 3s2 3p6 4s1 3d5
Cu 1s2 2s2 2p6 3s2 3p6 4s1 3d10
What is first ionisation energy?
The enthalpy change when one mole x(g)->x^+(g) + e^-
What affects ionisation energy?
There are three main factors
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 are successive ionisation energies larger?
The second ionisation energy of an element is always bigger than the first ionisation energy.
When the first electron is removed 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.
Standard answer for why ionisation energy has increased?
The electron is in a inner
shell closer to the nucleus and
therefore attracted much more
strongly by the nucleus than the
previous electron.
It also does not have any
shielding by inner complete shells
of electron
What is periodicity?
A repeating pattern across a
period
Why has helium the largest first ionisation energy?
Its first electron is in the first shell closest to the nucleus and has no
shielding effects from inner shells. He has a bigger first ionisation
energy than H as it has one more proton
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 has Na a much lower first ionisation energy than neon?
This is because Na will have its outer electron in a 3s shell further from
the nucleus and is more shielded. So Na’s outer electron is easier to
remove and has a lower ionisation energy.
