atomic structure [P1] PAPER 1 Flashcards

1
Q

define the term ‘relative atomic mass’

A

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

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2
Q

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

A

they have different numbers of neutrons

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3
Q

what is an isotope?

A

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

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4
Q

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

A
  • they all have the same electron configuration

* the number of electrons and their configuration determine chemical properties

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5
Q

explain the meaning of the term ‘first ionisation energy’

A

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

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6
Q

what are the stages of time of flight mass spectrometry?

A
  1. ionisation
  2. acceleration
  3. ion drift
  4. detection
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7
Q

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

A
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8
Q

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

A

• 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]

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9
Q

how is relative atomic mass calculated?

A
  • (sum of all m/z × associated relative abundances) / (sum of all relative abundances)
  • (sum of all m/z × associated % of abundances) / 100
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10
Q

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

A

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

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11
Q

describe how an ion is formed in a TOF mass spectrometer

A

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)

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12
Q

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

A
  • 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
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13
Q

state the meaning of the term ‘mass number’

A

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

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14
Q

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

A

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

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15
Q

how is a current generated in a TOF mass spectrometer?

A

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

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16
Q

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

A
  • 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
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17
Q

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

A

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

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18
Q

how do you calculate the mass of one ion?

A

• 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
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19
Q

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

A
20
Q

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?

A
21
Q

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?

A
22
Q

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?

A
  • 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:
23
Q

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

A

• 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]

24
Q

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

A

• 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]

25
Q

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

A

• 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]

26
Q

what do large increases in successive ionisation energies indicate?

A
  • 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.
27
Q

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

A
28
Q

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

A

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

e.g.

29
Q

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

A

they only have 1 electron in the 4s orbital:

30
Q

for transition metals, how are ions formed?

A

electrons lost from s orbital first

e.g.

31
Q

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?

A

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

32
Q
  • 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
A
• 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%
33
Q
  • 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
A

• 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.)

34
Q

what must you ensure when doing TOF calculations?

A

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

35
Q

define the term ‘relative molecular mass’

A

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

36
Q

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

A

different numbers of neutrons

37
Q

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

A
  • 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
38
Q

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.

A

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

39
Q

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

A

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

40
Q

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]

A
41
Q

What does Avogadro’s constant represent?

A

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

42
Q

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

A
  • central nucleus contains protons and neutrons in the model used today
  • electrons are arranged in orbitals in the model used today
43
Q

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

A
  • 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
44
Q

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

A

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

45
Q

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

A

• 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]

46
Q
A
47
Q

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

A