Atomic Structure

Question 1

What does an atom consist of?

Answer 1

An atom consists of a nucleus containing protons and neutrons surrounded by electrons

Question 2

What is the relative charge and mass of a proton ?

Answer 2

Relative mass is 1
Relative charge is +1

Question 3

What is the relative mass and charge of a neutron?

Answer 3

Relative mass is 1
Relative charge is 0

Question 4

What is the relative mass and charge of an electron ?

Answer 4

Relative mass is 1/1840
Relative charge -1

Question 5

What are isotopes?

Answer 5

Isotopes are atoms with the same number of protons and different number of neutrons

Question 6

Why do isotopes have similar chemical properties ?

Answer 6

Isotopes have similar chemical properties because they have the same electronic structure. They may have slightly varying physical properties because they have different masses.

Question 7

What can the mass spectrometer be used to determine ?

Answer 7

The mass spectrometer can be used to determine all the isotopes present in a sample of an element and to therefore identify elements

Question 8

Why does the mass spectrometer have to be under a vacuum?

Answer 8

It needs to be under a vacuum otherwise air particles would ionise and register in the detector

Question 9

What are the four steps of a mass spectrometer?

Answer 9

1.ionisation
2.acceleration
3.flight tube/ion drift
4.detection

Question 10

What are the two ways a sample can be ionised?

Answer 10

Electron impact
Electro spray ionisation

Question 11

Describe electron impact

Answer 11

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-

Question 12

Describe electro spray ionisation

Answer 12

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 towards a negative plate

Question 13

What is electron impact used for?

Answer 13

Electron impact is used for elements and substances with low formula mass . Electron impact can cause larger organic molecules to fragment

Question 14

What is electro spray ionisation used for?

Answer 14

Electro spray ionisation is used preferably for larger organic molecules.The softer conditions of this technique mean fragmentation doesn’t occur

Question 15

Describe acceleration in a mass spectrometer

Answer 15

Positive ions are accelerated by an electric field to a constant kinetic energy

KE=1/2mv^2

Question 16

What does the velocity of each particle depend on ?

Answer 16

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

Question 17

Describe the flight tube

Answer 17

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

t=d/v

Question 18

Describe detection

Answer 18

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

Question 19

What can the mass spectrometer measure for each isotope?

Answer 19

For each isotope the mass spectrometer can measure a M/z (mass /charge ratio) and an abundance

Question 20

How do you give the species for a peak in a mass spectrum?

Answer 20

If asked to give the species for a peak in a mass spectrum then give charge and mass number eg 24Mg+

Question 21

Equation for R.A.M

Answer 21

R.A.M=