Atomic structure

Question 1

State the meaning of the term isotope

Answer 1

atoms with the same protons, but have a different number of neutrons

Question 2

Describe how ions are formed in a time of flight (TOF) mass spectrometer. ( 2 marks)

Answer 2

For electron impact

the sample is bombarded by high energy electrons
the sample molecule loses an electron forming M+

For electrospray

A high voltage is applied to a sample in a polar solvent
the sample molecule, M, gains a proton forming MH+

Question 3

A TOF mass spectrometer can be used to determine the relative molecular mass of molecular substances.

Explain why it is necessary to ionise molecules when measuring their mass in a TOF mass spectrometer ( 3 marks)

Answer 3

Ions, will interact with and be accelerated by an electric field

and only ions will create a current when hitting the detector

Question 4

Define the term relative atomic mass.

Answer 4

The average mass of 1 atom of an element compared

1/12 mass of one atom of 12C

Question 5

Define the term relative atomic mass. ( 2 marks)

Answer 5

The average mass of 1 atom of an element compared

1/12 mass of one atom of 12C

Question 6

In a mass spectrometer, the relative abundance of each isotope is proportional to the current generated by that isotope at the detector.

Explain how this current is generated. (2 marks)

Answer 6

Electron(s) transferred / flow (at the detector)
(From detector / plate) to the (+) ion

Do not mention the charged plate

Question 7

State why isotopes of an element have the same chemical properties. ( 1 mark)

Answer 7

They have the same electron configuration

Question 8

Suggest one reason why particles with the same mass and velocity can be deflected by different amounts in the same magnetic field. ( 1 marks)

Answer 8

Size of the charge (on the ion) / different charges / different m/z

Question 9

Explain why chromium is placed in the d block in the Periodic Table.

Answer 9

The highest energy electron is in the d subshell

Question 10

State the relative charge and relative mass of a proton, of a neutron and of an electron.

In terms of particles, explain the relationship between two isotopes of the same element.

Explain why these isotopes have identical chemical properties. ( 7 marks)

Answer 10

1. Neutrons Electrons Proton
   - charge: 0 -1 +1
   - relative mass is 1 1/2000 1

The relationship between two isotopes is that they have the same number of electrons and protons but a different number of neutrons

Have identical chemical properties because they have same electron configuration

Question 11

Define the term relative atomic mass. An element exists as a mixture of three isotopes.

Explain, in detail, how the relative atomic mass of this element can be calculated from data obtained from the mass spectrum of the element. ( 7 marks)

Answer 11

Relative atomic mass is the mean mass of an atom divided by the mean mass of 1/12 of carbon 12

Spectrum gives (relative) abundance
And m/z
Multiply m/z by relative abundance for each isotope
Sum these values
Divide by the sum of the relative abundances

Question 12

Define the first ionisation energy ( 1 mark)

Answer 12

amount of energy required to remove one mole of electrons from one mole of gaseous atoms to form one mole of 1+ ions

Question 13

How do you work out relative atomic mass

Answer 13

the sum of isotopic mass x % abundance divide 100

and if you don’t have % then do

the sum of isotopic mass x % abundance divide total relative abundance

Question 14

What is the trend in ionisation energies across a period

Answer 14

1. They increase
2. nuclear charge increases
3. Shielding stays the same

Question 15

What is the trend in ionisation down a group

Answer 15

1. decreases
2. more shielding
3. larger atomic radius

Question 16

Why are successive ionisation energies larger

Answer 16

When the first electron is removed from the atom a positive ion is formed
this means that there is now a greater attraction to the nucleus than before