Atomic Structure

Question 1

What is an atomic number?

Answer 1

The number of protons in the nucleus of an atom.

Question 2

What is the mass number?

Answer 2

The number of protons and neutrons in the nucleus of the atom.

Question 3

What is an isotope?

Answer 3

Atoms of the same element so they have the same number of protons with a different number of neutrons.

Question 4

Why do isotopes have the same chemical properties?

Answer 4

They have the same electronic configuration.

Question 5

How does a time of flight mass spectrometer work (ionisation)?

Answer 5

1) Electron impact ionisation:
- Sample is injected into spectrometer and vaporised into a gas.
- High energy electrons from an electron gun are fired at the sample.
- The high energy electrons knock off an electron from each particle to form positive ions.

2) Electrospray ionisation:
- Sample is dissolved in polar solvent.
- Sample is injected through a fine hypodermic needle to give a fine mist.
- The tip of the needle is attached to the positive terminal of a high voltage supple.
- Particles gain a proton (H+ ion) from the solvent to form positive ions.

A volatile solvent is used.

Question 6

How does a time of flight mass spectrometer work (acceleration)?

Answer 6

• An electric field is applied to accelerate the positive ions.
• This gives all the ions with the same charge kinetic energy.
• KE = 1/2 X m X V^2

Question 7

How does a time of flight mass spectrometer work (separation)?

Answer 7

• The positive ions travel through the mass spectrometer to the detector.
• The time of flight of each particle depends on its velocity.
• Time of flight = distance travelled / velocity
• Lighter ions travel faster and reach the detector first.
• Heavier ions travel slower and reach the detector last.

Question 8

How does a time of flight mass spectrometer work (detection)?

Answer 8

• A detector records the different ions as they arrive at the detector.
• At the detector, each ion gains an electron. This generates a current.
• The size of the current is proportional to the abundance of each ion. The greater the current, the greater the abundance.

Question 9

What are the key principles of TOF mass spectrometry?

Answer 9

• The sample must be ionised so that it can be accelerated and detected.
• The whole spectrometer is kept under vacuum to prevent ions from colliding with molecules in air which would cause them to slow down.

Question 10

How do you calculate the mass of one atom/ion?

Answer 10

Mass of one ion/atom (in kg) = relative isotopic mass X 10^-3 / avagadro’s number

Question 11

If distance travelled and kinetic energy are constant, what is the time of flight equation?

Answer 11

m1 / t1^2 = m2 / t2^2

Question 12

What is the relative isotopic mass?

Answer 12

The mass of an isotope relative to 1/12th the mass of an atom of carbon-12.

Question 13

What is the relative atomic mass?

Answer 13

The weighted average mass of all the isotopes relative to 1/12th the mass of an atom of carbon-12.

Question 14

What is the equation for relative atomic mass?

Answer 14

Sum of (mass X abundance of each isotope)
———————————————————
Total abundance