3.1.1.2 Mass Number and Isotopes

Question 1

Mass number symbol:

Answer 1

A

Question 2

Atomic (proton) number symbol:

Answer 2

Z

Question 3

Relative atomic mass (Ar):

Answer 3

• the mean mass of an element divided by 1/12th of the mean mass of an atom of the carbon-12 isotope
• this takes the relative abundances of the different isotopes of a element into account

Question 4

Isotopes:

Answer 4

atoms of the same element with the same atomic number (as they have the same number of protons) but with different number of neutrons resulting in a different mass number

Question 5

How will neutral atoms of isotopes react?

Answer 5

• neutral atoms of isotopes will react chemically in the same way as their proton number and electron configuration is the same
• the sharing and transfer of electrons is unaffected

Question 6

How will neutral atoms of isotopes’ physical properties differ?

Answer 6

different mass number means they may have different physical properties

Question 7

How to calculate relative atomic mass?

Answer 7

Question 8

Tricky relative atomic mass maths question:

Answer 8

A sample of naturally occurring silicon consists of Si-28, Si-29 and Si-30. If the atomic mass of silicone is 28.085 and the natural abundance of Si-29 is 4.67%, what are the natural abundances of Si-28 and Si-30

[(28 x y) + (29 x 4.67) + (30 x (95.33-))] / 100 = 28.055

etc.

Question 9

What is mass spectrometry used to find?

Answer 9

Gives accurate info about:

• relative abundance and mass if isotopes in an element allowing us to determine relative atomic mass
• relative molecular mass of substances made of molecules

Question 10

What takes place in ToF mass spectrometry?

Answer 10

• particles of the substance are ionised to form positive ions which are accelerated so that they all have the same kinetic energy
• the time taken to travel a fixed distance is then used to find the mass of each ion in the sample

Question 11

Diagram of ToF mass spectrometer:

Answer 11

Question 12

Why is the interior of the mass spectrometer a vacuum?

Answer 12

• it needs to be under a vacuum otherwise air particles would ionise and register on the detector
• • to prevent ions from colliding with molecules in the air and affecting the time the ion takes to reach the detector

Question 13

Stages of ToF mass spectrometry:

Answer 13

1. Ionisation - electro spray ionisation / electron impact/bombardment
2. Acceleration
3. Ion Drift/Flight Tube
4. Ion Detection
5. Data Analysis