Atomic Structure Flashcards
On the Periodic Table which is the
a) Mass Number
b) Atomic Number?
a) the big number - it is the total number of protons and neutrons
b) the little number - it is the number protons (sometimes known as the Proton Number)
What is an isotope?
An atom of the same element with the same number of protons but a different number of neutrons
Why do isotopes of the same element have the same chemical properties?
They have the same number of outer electrons
Do isotopes of the same element have the same physical properties?
Why?
No - they have different physical properties because they have a different number of neutrons and therefore different masses so boiling/ melting points are different
Describe the electron configuration of an atom
Electrons are held in Principal Energy Levels, which are given the numbers 1,2,3…
Principal Energy Levels are divided into sub-levels, which are assigned the letters s,p,d,f
Sub-Levels are further divided into orbitals and each orbital holds a maximum of 2 electrons
List the sub-Levels in order of Energy, from lowest to highest
s -> p -> d -> f
How many orbitals does an s sub level have
1
How many orbitals can a p sub level hold?
3
How many orbitals can a d sub level hold
5
How many orbitals can a f sub level hold
7
What is the maximum number of electrons in principal energy level:
a) 1
b) 2
c) 3
d) 4
Which rule applies to this?
a) 2
b) 8
c) 18
d) 32
The 2n squared rule
What is the order in which the energy levels are filled?
1s2, 2s2, 2p6, 3s2, 3p6, 4s2, 3d10, 4p6, 5s2, 4d10, 5p6
What rule applies to 4s?
It fills before 3d as it is lower in energy but, when ionised, electrons are removed from 4s before 3d
True or false:
Electrons fill the orbital of lowest energy first
True
What is the electron configuration of Chromium? (24 electrons)
Why is it different?
1s2, 2s2, 2p6, 3s2, 3p6, 4s1, 3d5
This is a more stable arrangement and is of lower energy
What is the electronic configuration of Copper? (29 electrons)
Why is this different?
1s2, 2s2, 2p6, 3s2, 3p6, 4s1, 3d10
The structure is of lower energy and represents a more stable arrangement
What do paired electrons in an orbital have
Opposite spin
Which is block
s
p
d
f
On the periodic table
s= group 1&2 p= group 3-0 d= transition metals f = the bottom two
How to determine the electronic configuration using the table
The period number is the PEL number, d-block elements PEL number is “period minus 1”
The block letter becomes the sub-level
The column it is in within that block is the ‘power of’ number
Define First Ionisation Energy
The minimum amount of energy required to remove one electron from a gaseous atom
What factors affect the amount of energy needed to ionise an atom?
Size of atom - number of PELs and number of protons
Attraction between nucleus and outer electron -affected by size, shielding and number of protons
Repulsion between paired electrons
As you travel down a group what happens to the size of each atom?
Why?
What does this mean for the outer electron?
What happens to ionisation as you go down?
It increases because there are more PELs
It is further from the nucleus so there is less attraction and there will be more shielding
First Ionisation energy decreases
Generally what happens to first ionisation energy as you go across a period?
Why?
It increases
Same shielding, increasing number of protons- increased nuclear charge and stronger attraction between nucleus and outer electron.
Atomic radius decreases
More energy needed to remove outer electron
Why does ionisation energy decrease for group 3 elements across a period?
Group 3 is in the p block
P is higher energy than S so group 3 elements have electrons which are further from the nucleus and so there is less attraction
Why is first ionisation energy if group 6 less than that of group 5?
There is a pair of electrons in a p-orbital so there is more repulsion, meaning less energy is needed to remove an electron
Why does the amount of energy needed to remove the next electron increase?
The ion is getting increasingly positive so the attraction between nucleus and outer electron increases. This decreases the radius so the outer electron becomes closer to the nucleus
When there is a very large increase in ionisation energy what does it mean?
The electron is being removed from a PEL which is much closer to the nucleus
Define First Ionisation Energy
The minimum amount of energy required to remove one electron from a gaseous atom
What happens to the First Ionisation Energy when you go down a group?
Why?
Ionisation energy decreases
Electron removed from higher PEL and is therefore further from nucleus. There is more shielding. This means less energy is required to remove the electron i.e. there is a weaker attraction
What are the 5 stages of a TOF mass spectrometer
- Ionisation
- Acceleration
- Ion Drift
- Ion Detection
- Data analysis
What two methods can be used for ionisation in a TOF mass spectrometer
Electro spray ionisation Electron impact (electron gun)
Explain electro spray ionisation
Sample dissolved in a volatile solvent
Injected through a fine hypodermic needle to give fine mist
Tip of needle has a high voltage
Each particle is ionised by gaining a proton
The solvent evaporates and ions are attracted towards a negative plate where they are accelerated
Show a generic equation for electro spray ionisation
X(g) + H+ = XH+(g)
What kind of molecule is used for electro spray ionisation
Large molecules (eg biological molecules)
Describe electron impact ionisation
High energy electrons are shot from an electron gun at the sample and knock off an electron to create a 1+ ion
What is a generic equation for electron impact ionisation
X(g) —> X+(g) + e-
What kind of ionisation would you use for a small molecule (eg an element)?
Electron impact
What happens in stage two of a TOF mass spectrometer
Acceleration
Positive ions are the accelerated by an electric field to a constant kinetic energy.
The rate of acceleration depends on the ion’s mass: lower mass ions will travel faster
Higher mass ions will have a lower velocity
What is Stage 3 of a TOF mass spectrometer
Ion Drift
Ions are separated.
They drift through a flight tube and the time that it takes for the ion to reach the detector at a known distance is measured.
Lighter ions reach the detector first so the mass can be determined from the time taken for the ion to reach the detector
What is Stage 4 of TOF mass spectrometry
Ion Detection
Positive ions hit the detector (a negatively charged plate) and pick up electrons.
A current is generated when the ions gain an electron
The greater the abundance of the ions, the bigger the current produced
Describe Stage 5 of TOF mass spectrometry
Data Analysis
Detector is linked to an amplifier and then to a recorder, which converts the current into a peak and that is shown in a mass spectrum
The height of the peak is proportional to the size of the current generated and therefore proportional to the abundance of the ion
Flight times are analyses and recorded as abundance against mass/charge ratio (m/z)
As ions are usually 1+, m/z =mass
What is used as a standard to measure the relative atomic mass of all other elements?
Carbon-12
What is Relative Atomic Mass (Ar) defined as?
Ar=
average mass of one atom of an element x 12
—————————-
What does the m/z of an ion show?
The mass of each isotope as the ion has a charge of 1+
Is there are two peaks on a graph what does it show?
There are two isotopes
Why may the Ar you calculate be different to the Ar on the periodic table?
The abundance in the sample may be different
How to work out Ar using the data from the mass spectrometer
Ar=
Sum of (m/z of a peak x abundance of that peak) ————————————
Total abundance
What is fragmentation?
When covalently bonded molecules have their bonds broken and break into fragments during electron impact ionisation by the electron gun
Does fragmentation occur to all of the molecules ?
No
Some will remain intact and just gain a positive charge. These unfragmented, positive molecules will give the peak with the greatest m/z value. This is called the molecular ion peak and shows the Mr of the molecule
Why may there be many peaks when electron impact ionisation is used?
Is this the same for electro spray ionisation?
Fragmentation
Nope - very little fragmentation occurs here
How do you find the Mr of a molecule when it has been ionised using electro spray ionisation?
Find the m/z value of the peak and minus one from this value.
You do this because you must minus the mass of the proton gained during ionisation
What is the equation used to show electro spray ionisation
X(g) + H+ -> XH+(g)
For electro spray ionisation what is The Mr equal to?
The peak with the greatest m/z value minus one
Sometimes a very small peak will appear at a m/z of 1 greater than the molecular ion peak.
For which technique does this occur?
What causes this?
What do we do with this peak?
Both techniques
The molecules that contain either Carbon-13 or Hydrogen-12
We ignore these peaks
What are the isotopes of Br and at what ratio do they exist?
Br-79 and Br-81
1:1
What are Chlorine’s isotopes and what is their ratio?
Cl-35 and Cl-37
3:1
What does the presence of different isotopes result in the mass spectrometer?
Multiple molecular ion peaks
How to work out mass of one atom
Mass = Mr/ L
