atomic structure [P1] PAPER 1

Question 1

define the term ‘relative atomic mass’

Answer 1

average mass of one atom of an element × 12 / mass of one atom of 12C

Question 2

explain why atoms of the same element may have different relative atomic masses

Answer 2

they have different numbers of neutrons

Question 3

what is an isotope?

Answer 3

atoms of the same element with different numbers of neutrons, but the same number of protons and electrons

Question 4

explain why isotopes of the same element have the same chemical properties

Answer 4

• they all have the same electron configuration

* the number of electrons and their configuration determine chemical properties

Question 5

explain the meaning of the term ‘first ionisation energy’

Answer 5

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

Question 6

what are the stages of time of flight mass spectrometry?

Answer 6

1. ionisation
2. acceleration
3. ion drift
4. detection

Question 7

what are the relative charges and masses of protons, neutrons and electrons?

Answer 7

Question 8

what generally happens to the ionisation energy of an element as you go along a period of the periodic table and why?

Answer 8

• increases
• same shielding
• atomic radius decreases
• nuclear charge increases
• stronger electrostatic forces of attraction between the outer electron and nucleus
[so more energy is required to lose the electron]

Question 9

how is relative atomic mass calculated?

Answer 9

• (sum of all m/z × associated relative abundances) / (sum of all relative abundances)
• (sum of all m/z × associated % of abundances) / 100

Question 10

why may the relative atomic mass of this sample be different to the relative atomic mass given in the periodic table?

Answer 10

other isotopes present / some isotopes are absent / different abundances of isotopes

Question 11

describe how an ion is formed in a TOF mass spectrometer

Answer 11

elements/low Mr compounds (electron gun):
• electron gun fires high energy electrons at gaseous sample
• electrons are knocked out to form positive ions
• e.g. Mg (g) -> Mg+ (g) + e-

High Mr compounds (electrospray):
• aqueous solution of the sample is sprayed out of a high voltage needle
• each molecule gains a proton to form a positive ion
• X (aq) + H+ -> XH+ (aq)

Question 12

why is it necessary to ionise molecules when measuring their masses in a TOF mass spectrometer?

Answer 12

• ions, not molecules, will be attracted to, and be accelerated by an electric field
• only ions will produce a current when hitting the detector - this current is proportional to the molecule’s abundance

Question 13

state the meaning of the term ‘mass number’

Answer 13

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

Question 14

what two measurements are recorded for each isotope on the mass spectrum in a mass spectrometer?

Answer 14

• m/z (mass to charge ratio)
• relative abundance

Question 15

how is a current generated in a TOF mass spectrometer?

Answer 15

electrons are transferred at the detector from the detector plate to the positive ion

Question 16

how is a TOF mass spectrometer able to separate different molecules to give multiple peaks?

Answer 16

• positive ions are accelerated by an electric field to a constant kinetic energy
• the lighter ions move faster than the heavier ions with the same kinetic energy, so have a shorter time of flight
• the ions reach the detector at different times

Question 17

why is the second ionisation energy of sodium higher than the first ionisation energy of sodium?

Answer 17

electron being removed is closer to the nucleus (since it’s in a lower energy level)

Question 18

how do you calculate the mass of one ion?

Answer 18

• take mass number
• divide by 1000 to convert to kg as mass number is g of one mole
• divide by L (Avogadro’s constant) - 6.022×10^23
[same as doing mass number/1000L]

e.g.
mass of Li+ ion:
• mass number of Li+ = 7 
• 7/1000 = 0.007
• 0.007/L = 1.16×10^-26
mass of Li+ ion = 1.16×10^-26 kg

Question 19

how would you calculate speed, given you have kinetic energy and mass?

Answer 19

Question 20

how would you calculate the time it took for an ion to travel through the flight tube in a TOF mass spectrometer, given you have the kinetic energy, distance of the tube in metres, and have worked out the mass of the ion in kg?

Answer 20

Question 21

how would you calculate the distance of a flight tube of a TOF mass spectrometer, given you have the time it took in s, kinetic energy, and have worked out the mass of the ion in kg?

Answer 21

Question 22

if it took the 81Br+ ‘x’ seconds to travel along the flight tube in a TOF mass spectrometer, how long would it take the 79Br+ ion to travel down the same flight tube under the same conditions?

Answer 22

• KE of 81Br+ = KE of 79Br+ since all ions have a constant kinetic energy in a TOF MS
• mass numbers of each ion don’t need to be converted into kg as the constants used to do this would end up cancelling out
• rearrange and solve for t:

Question 23

what happens to the ionisation energy of an element as you go down a group and why?

Answer 23

• decreases
• (nuclear charge increases, but) atomic radius increases
• more shielding
• weaker forces of electrostatic attraction between the nucleus and outer electron
[so less energy is required to lose the electron]

Question 24

why does the ionisation energy of an element decrease as you go along from group 2 to 3 in period 3?

Answer 24

• Mg outer electron is in 3s orbital
• Al outer electron is in 3p orbital
• p orbital is a higher energy level
[so larger atomic radius, therefore weaker forces of electrostatic attraction between nucleus and outer electron, so less energy required to lose the electron]

Question 25

why does the ionisation energy of an element decrease as you go along from group 5 to 6 in period 3?

Answer 25

• outer electron from P is in 3p orbital with one electron • outer electron from S is in 3p orbital with a pair of electrons • electron pair repulsion in S [so weaker forces of electrostatic attraction between nucleus and outer electron, so less energy required to lose the electron]

Question 26

what do large increases in successive ionisation energies indicate?

Answer 26

* where electrons are being removed from the next principal energy level * can be used to determine the group number of an element (biggest difference’s number of ionisation = number of shells = group number) * e.g.

Question 27

identify the s, p and d blocks on the periodic table

Answer 27

Question 28

how do you write the shorthand electron configuration of an element?

Answer 28

full shell element [i.e. group 0 element] + extra e.g.

Question 29

what are the electron configurations of chromium (Cr) and copper (Cu)?

Answer 29

they only have 1 electron in the 4s orbital:

Question 30

for transition metals, how are ions formed?

Answer 30

electrons lost from s orbital first | e.g.

Question 31

chlorine exists as two isotopes, 35Cl and 37Cl, in the ratio 3:1. in what ratio and what m/z are the peaks present in the mass spectrum of Cl2?

Answer 31

peaks at m/z = 70, 72 and 74 in the ratio 9:6:1

Question 32

* sample of strontium has a relative atomic mass of 87.7 and consists of 3 isotopes, 86Sr, 87Sr and 88Sr * 86Sr:87Sr = 1:1 * calculate the % abundance of the 88Sr isotope in this sample

Answer 32

``` • 86Sr = x% • 87Sr = x% • 88Sr = (100-2x)% • [86x + 87x + 88(100-2x)] / 100 = 87.7 ⇒ x = 10 • % abundance of 88Sr = 100 - 2(10) ⇒ % abundance of 88Sr = 80% ```

Question 33

* a sample of Pb contains isotopes with mass numbers 206, 207 and 208 in a 1:1:2 ratio of abundance * calculate the relative atomic mass of R, giving your answer to one decimal place

Answer 33

• 1:1:2 = 25% : 25% : 50% • relative atomic mass = [206×25 + 207×25 + 208×50] / 100 ⇒ relative atomic mass of R = 207.3 (1 d.p.)

Question 34

what must you ensure when doing TOF calculations?

Answer 34

units ⚠️ • distance is in metres • mass is in kg (convert from mass number) • time in s

Question 35

define the term ‘relative molecular mass’

Answer 35

average mass of one molecule × 12 / mass of one atom of 12C

Question 36

why do isotopes of the same element have slightly different physical properties?

Answer 36

different numbers of neutrons

Question 37

identifying an element from a graph/table of ionisation energies for a specific period and explain how you’ve done this?

Answer 37

* see where the largest jump is - group number of the element * explanation: large jump in ionisation energy for the __th ionisation energy - this is when the electron is being removed from a lower energy level

Question 38

A student does an experiment to determine the relative molecular mass (Mr) of liquid A. The student injects a sample of A into a gas syringe in an oven. At the temperature of the oven, liquid A vaporises. The student noticed that some of the liquid injected into the gas syringe did not vaporise. Explain the effect that this has on the Mr calculated by the student.

Answer 38

Calculated Mr would be greater than the actual since a lower volume would’ve been recorded

Question 39

What is the importance of taking the percentage yield of a reaction into account?

Answer 39

to maximise the mass of reactants that end up in the desired product, and minimising the amount of by-products

Question 40

The first ionisation energies of the elements in Period 2 change as the atomic number increases. Explain the pattern in the first ionisation energies of the elements from lithium to neon. [6 marks]

Answer 40

Question 41

What does Avogadro’s constant represent?

Answer 41

It is the number of atoms/molecules in one mole of a substance

Question 42

State two differences between the plum pudding model of the atom and the model of atomic structure used today.

Answer 42

* central nucleus contains protons and neutrons in the model used today * electrons are arranged in orbitals in the model used today

Question 43

Explain how ions are detected and relative abundance is measured in a TOF mass spectrometer.

Answer 43

* ion hits the negative plate at the detector, gaining an electron and a current is generated * the relative abundance is proportional to the size of the current

Question 44

Explain why certain elements in the Periodic Table are classified as __ block elements.

Answer 44

Elements in the __ block have their outer electrons in __ orbitals

Question 45

Explain why the second ionisation energy of calcium is lower than the second ionisation energy of potassium.

Answer 45

• Ca+ loses an electron from a higher energy level (so, outer electron is further from the nucleus) • More shielding in Ca+ [so, weaker forces of attraction between the outer electron and nucleus]

Question 46

Answer 46

Question 47

Outline how the TOF mass spectrometer is able to separate these two species to give two peaks.

Answer 47