3.1.1 - Atomic Structure Flashcards
What is mass spectrometry?
A method of analysis, which helps us to determine relative atomic masses of unknown substances. It also helps us to determine the abundances of specific isotopes.
What are the two types of ionisation that can happen in TOF mass spec?
Electrospray Ionisation
Electron Impact (preferred method)
Outline the 4 steps in TOF mass spectrometry.
1) Ionisation
2) Acceleration
3) Ion Drift
4) Detection
Describe the steps involved in ElectroSpray Ionisation
Electrospray Ionisation:
Dissolve the sample in a polar solvent
Push it through a tiny nozzle at high pressure
Apply a high voltage
Each particle will gain a proton
The sample will be a gaseous sample of positive ions
Describe Electron Impact Ionisation (4)
Vaporise the sample
Sample is bombarded by high energy electrons
An electron will be removed from each particle
This will create +1 ions
Describe the Acceleration phase of TOF Mass Spec
The positive ions move through a negative electric field, accelerating toward the negative plate. Lighter ions and ions with a higher charge achieve a higher speed here.
Describe the Ion Drift phase of Mass Spec
During ion drift, the different ions will move across the flight tube. Positive ions with smaller m/z values will have the same KE as those with a large m/z values and will move faster.
The ions are distinguished by different flight times
Describe Detection
Assuming they have the same charge, lighter ions will reach the detector first. When these positive ions hit the detection plate, they gain an electron, producing a flow of charge. The greater the abundance, the greater the current produced.
When do we use electronspray ionisation over electron impact?
Electron impact can cause larger, organic molecules to fragment. Hence, we use electrospray ionisation for larger, organic molecules - the ‘‘softer’’ conditions of this technique means fragmentatio won’t occur.
What can happen to molecules during an electron impact ionisation stage? What will this show on the mass spectra?
Molecules will often break up and give a series of peaks caused by the fragments.
What do we mean by molecular ion / parent ion?
The largest peak. This is when the complete molecule passes through the mass spectrometer - it has the largest m/z and will be equal to the relative molecular mass.
What must we consider when working out Mr from a mass spectra, in which Electrospray Ionisation was used?
The peak will be equal to MH+. To get M, the molecule, we must subtract 1 - the mass of a H+. If the peak is 521.1, there Mr is 520.1
Briefly describe the A-Level model for Electronic Configuration
- Principle Energy Levels (numbered 1, 2, 3, 4) with 1 being closest to the nucleus
SPLIT INTO
- Sub energy levels, (s, p, d, f)
S - 2
P- 6
D - 10
F- 14
SPLIT INTO
- Orbitals which hold up to 2 electrons of opposite spin
State the order in which sub shells fill up
1s - 2s - 2p - 3s - 3p - 4s - 3d - 4p - 5s - 4d - 5p
When filling up orbitals, what must you do?
Fill each orbital singly before starting to pair up electrons.
What is the shape of a s sub shell?
Spherical
What are the shape of p sub levels?
- Shaped like dumbbells
Describe how the periodic table is split into blocks.
A s block element (groups 1,2) has their outer electron filling a s sub-shell
A p block element is one whose outer electron is filling a p-sub shell
A d block element is one whose outer electron is filling a d-sub shell.
Why does the TOF mass spectometer need to be under a vaccuum?
To prevent air particles ionising and registering on the detector
What is the equation of time of flight?
t = d x root m/2KE
What are the exceptions to the electronic configuration rule that 4s is filled before 3d?
State their electronic configurations and suggest why
Chromium (Cr) - 1s2 2s2 2p6 3s2 3p6 4s1 3d5
(it is more stable to have a half full 3d shell)
Copper (Cu) - 1s2 2s2 2p6 3s2 3p6 4s1 3d10
(it is more stable to have a full 3d shell)
What’s important to note about the electronic structure of d-block ions?
They lose 4s electrons before their 3d electrons
Define first ionisation energy
The first ionisation energy is the energy required to remove 1 mole of electrons from 1 mole of gaseous atoms
Define second ionsation energy
The energy required to remove 1 mol of electrons from 1 mole of gaseous ions with a single positive charge
Represent first ionisation energy in an equation
H(g) -> H^+ (g) + e^- (always gaseous, don’t forget the state symbols!)
What are the three main factors affecting ionisation energy?
1) The attraction of the nucleus
(more protons in the nucleus, means greater 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 attractive force)
3) Shielding of the Attraction from the Nucleus
(a greater number of electrons in inner shells will repel those in the outer shell, weakening the attraction of the nucleus)
Why are successive ionisation energies always larger?
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.
How can we identify what group an unknown element is in using it’s successive ionisation energies?
We can look for a big jump, which shows us that the next electron is removed from an inner shell. This indicates how many electrons were in the outer shell.
Why has Helium got 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 got 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.
Why is there a small drop from Mg to Al in 1st i.e?
Al is starting to fill a 3p sub shell, whereas Mg has its outer electrons in the 3s sub shell. The electrons in the 3p subshell are slightly easier to remove because the 3p electrons are higher in energy and are also slightly shielded by the 3s electrons
Why is there a small drop from P to S in ionisation energy?
With sulphur there are 4 electrons in the 3p sub shell and the 4th is starting to doubly fill the first 3p orbital. When the second electron is added to a 3p orbital there is a slight repulsion between the two negatively charged electrons which makes the second electron easier to remove
What element has the highest second ionisation energy?
Lithium - it’s first electron would be removed from the 2s shell. it’s second electron is removed from the 1st shell, closest to the nucleus, which has no shielding effects from inner shells. It has a bigger second ionsation energy than He due to a higher nuclear charge.
What’s the equation for TOF if you know two masses and a time?
Ma x Tb^2 = Mb x T^2a
What was John Dalton’s model of the atom at the start of the 19th century?
He said solid spheres made up different elements
What was J.J. Thomson’s atomic model?
He discovered the electron in 1897
This disproved the previous model thinking atoms were solid and inseparable
This was the plum pudding model
Electrons were on the surface
Of a positively charged ‘pudding’
What was the gold foil atomic experiment?
Alpha particles were fired at a thin sheet of gold foil
Most passed straight through
Only a few were deflected straight backwards
This disproved the plum pudding model, which would have expected most to be slightly deflected
What atomic model was developed after the gold foil experiment?
The nuclear model of the atom
A tiny positively charged nucleus
Surrounded by a cloud of negative electrons
Most of the atom was empty space
How did Niels Bohr develop Rutherford’s nuclear model further?
He proposed that electrons existed in shells
These were orbits with fixed energy
When the electrons moved between shells, electromagnetic radiation with a fixed energy was emitted or absorbed
This made sense of experimental data that detected radiation
Why is the Bohr model not fully correct?
The lecterns in each shell have slightly different energies
The model now includes sub-shells
These explain experimental ionisation energy trends
State and explain the trend in the first ionisation energies of the elements in Group 2 from magnesium to barium: MS [3]
Decrease
Ions get bigger with more shells
Weaker attraction of the ion to the lost electron
State the element in Period 3 that has the highest melting point. Explain your answer: MS [3]
Silicon
Covalent bonds
Lots of them have to be broken which requires a lot of energy to achieve
State the element in Period 3 that has the highest first ionisation energy. Explain your answer: MS [3]
Argon
Largest number of protons and the highest nuclear charge
There is the same amount of shielding
Explain why the value of the first ionisation energy of sulfur is less than the value of the first ionisation energy of phosphorus: MS [2]
Paired electrons in the 3p orbital
Repel
State the meaning of the term first ionisation energy: MS [2]
Enthalpy needed to remove one mole of electrons from one mole of atoms
In gaseous state
Explain why the second ionisation energy of sodium is greater than the second ionisation energy of magnesium: MS [3]
Na2+ has an electron lost from a 2p orbital
Mg2+ requires a loss of electron from a 3s orbital
There is less shielding in Na
Define the term relative atomic mass of an element: MS [2]
The mean/average mass of an atom relative to the mass of 1/12th of the mass of an atom of Carbon-12
State the meaning of the term mass number of an isotope: MS [1]
The total number of protons and neutrons in the nucleus of an atom
