3.1.1 - Atomic Structure

Question 1

What is mass spectrometry?

Answer 1

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.

Question 2

What are the two types of ionisation that can happen in TOF mass spec?

Answer 2

Electrospray Ionisation
Electron Impact (preferred method)

Question 3

Outline the 4 steps in TOF mass spectrometry.

Answer 3

1) Ionisation
2) Acceleration
3) Ion Drift
4) Detection

Question 4

Describe the steps involved in ElectroSpray Ionisation

Answer 4

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

Question 5

Describe Electron Impact Ionisation (4)

Answer 5

Vaporise the sample
Sample is bombarded by high energy electrons
An electron will be removed from each particle
This will create +1 ions

Question 6

Describe the Acceleration phase of TOF Mass Spec

Answer 6

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.

Question 7

Describe the Ion Drift phase of Mass Spec

Answer 7

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

Question 8

Describe Detection

Answer 8

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.

Question 9

When do we use electronspray ionisation over electron impact?

Answer 9

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.

Question 10

What can happen to molecules during an electron impact ionisation stage? What will this show on the mass spectra?

Answer 10

Molecules will often break up and give a series of peaks caused by the fragments.

Question 11

What do we mean by molecular ion / parent ion?

Answer 11

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.

Question 12

What must we consider when working out Mr from a mass spectra, in which Electrospray Ionisation was used?

Answer 12

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

Question 13

Briefly describe the A-Level model for Electronic Configuration

Answer 13

• 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

Question 14

State the order in which sub shells fill up

Answer 14

1s - 2s - 2p - 3s - 3p - 4s - 3d - 4p - 5s - 4d - 5p

Question 15

When filling up orbitals, what must you do?

Answer 15

Fill each orbital singly before starting to pair up electrons.

Question 16

What is the shape of a s sub shell?

Answer 16

Spherical

Question 17

What are the shape of p sub levels?

Answer 17

• Shaped like dumbbells

Question 18

Describe how the periodic table is split into blocks.

Answer 18

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.

Question 19

Why does the TOF mass spectometer need to be under a vaccuum?

Answer 19

To prevent air particles ionising and registering on the detector

Question 20

What is the equation of time of flight?

Answer 20

t = d x root m/2KE

Question 21

What are the exceptions to the electronic configuration rule that 4s is filled before 3d?

State their electronic configurations and suggest why

Answer 21

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)

Question 22

What’s important to note about the electronic structure of d-block ions?

Answer 22

They lose 4s electrons before their 3d electrons

Question 23

Define first ionisation energy

Answer 23

The first ionisation energy is the energy required to remove 1 mole of electrons from 1 mole of gaseous atoms

Question 24

Define second ionsation energy

Answer 24

The energy required to remove 1 mol of electrons from 1 mole of gaseous ions with a single positive charge

Question 25

Represent first ionisation energy in an equation

Answer 25

H(g) -> H^+ (g) + e^- (always gaseous, don’t forget the state symbols!)

Question 26

What are the three main factors affecting ionisation energy?

Answer 26

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)

Question 27

Why are successive ionisation energies always larger?

Answer 27

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.

Question 28

How can we identify what group an unknown element is in using it’s successive ionisation energies?

Answer 28

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.

Question 29

Why has Helium got the largest first ionisation energy?

Answer 29

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

Question 30

Why do first ionisation energies decrease down a group?

Answer 30

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

Question 31

Why is there a general increase in first ionisation energy across a period?

Answer 31

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.

Question 32

Why has Na got a much lower first ionisation energy than Neon?

Answer 32

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.

Question 33

Why is there a small drop from Mg to Al in 1st i.e?

Answer 33

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

Question 34

Why is there a small drop from P to S in ionisation energy?

Answer 34

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

Question 35

What element has the highest second ionisation energy?

Answer 35

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.

Question 36

What’s the equation for TOF if you know two masses and a time?

Answer 36

Ma x Tb^2 = Mb x T^2a

Question 37

What was John Dalton’s model of the atom at the start of the 19th century?

Answer 37

He said solid spheres made up different elements

Question 38

What was J.J. Thomson’s atomic model?

Answer 38

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’

Question 39

What was the gold foil atomic experiment?

Answer 39

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

Question 40

What atomic model was developed after the gold foil experiment?

Answer 40

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

Question 41

How did Niels Bohr develop Rutherford’s nuclear model further?

Answer 41

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

Question 42

Why is the Bohr model not fully correct?

Answer 42

The lecterns in each shell have slightly different energies
The model now includes sub-shells
These explain experimental ionisation energy trends

Question 43

State and explain the trend in the first ionisation energies of the elements in Group 2 from magnesium to barium: MS [3]

Answer 43

Decrease
Ions get bigger with more shells
Weaker attraction of the ion to the lost electron

Question 44

State the element in Period 3 that has the highest melting point. Explain your answer: MS [3]

Answer 44

Silicon
Covalent bonds
Lots of them have to be broken which requires a lot of energy to achieve

Question 45

State the element in Period 3 that has the highest first ionisation energy. Explain your answer: MS [3]

Answer 45

Argon
Largest number of protons and the highest nuclear charge
There is the same amount of shielding

Question 46

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]

Answer 46

Paired electrons in the 3p orbital
Repel

Question 47

State the meaning of the term first ionisation energy: MS [2]

Answer 47

Enthalpy needed to remove one mole of electrons from one mole of atoms
In gaseous state

Question 48

Explain why the second ionisation energy of sodium is greater than the second ionisation energy of magnesium: MS [3]

Answer 48

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

Question 49

Define the term relative atomic mass of an element: MS [2]

Answer 49

The mean/average mass of an atom relative to the mass of 1/12th of the mass of an atom of Carbon-12

Question 50

State the meaning of the term mass number of an isotope: MS [1]

Answer 50

The total number of protons and neutrons in the nucleus of an atom