atomic structure [P1] PAPER 1 Flashcards
define the term ‘relative atomic mass’
average mass of one atom of an element × 12 / mass of one atom of 12C
explain why atoms of the same element may have different relative atomic masses
they have different numbers of neutrons
what is an isotope?
atoms of the same element with different numbers of neutrons, but the same number of protons and electrons
explain why isotopes of the same element have the same chemical properties
- they all have the same electron configuration
* the number of electrons and their configuration determine chemical properties
explain the meaning of the term ‘first ionisation energy’
the energy required to remove one mole of electrons from one mole of gaseous atoms to form one mole of +1 ions
what are the stages of time of flight mass spectrometry?
- ionisation
- acceleration
- ion drift
- detection
what are the relative charges and masses of protons, neutrons and electrons?
what generally happens to the ionisation energy of an element as you go along a period of the periodic table and why?
• 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]
how is relative atomic mass calculated?
- (sum of all m/z × associated relative abundances) / (sum of all relative abundances)
- (sum of all m/z × associated % of abundances) / 100
why may the relative atomic mass of this sample be different to the relative atomic mass given in the periodic table?
other isotopes present / some isotopes are absent / different abundances of isotopes
describe how an ion is formed in a TOF mass spectrometer
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)
why is it necessary to ionise molecules when measuring their masses in a TOF mass spectrometer?
- 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
state the meaning of the term ‘mass number’
total number of protons and neutrons in the nucleus of an atom
what two measurements are recorded for each isotope on the mass spectrum in a mass spectrometer?
• m/z (mass to charge ratio)
• relative abundance
how is a current generated in a TOF mass spectrometer?
electrons are transferred at the detector from the detector plate to the positive ion
how is a TOF mass spectrometer able to separate different molecules to give multiple peaks?
- 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
why is the second ionisation energy of sodium higher than the first ionisation energy of sodium?
electron being removed is closer to the nucleus (since it’s in a lower energy level)
how do you calculate the mass of one ion?
• 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
how would you calculate speed, given you have kinetic energy and mass?
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?
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?
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?
- 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:
what happens to the ionisation energy of an element as you go down a group and why?
• 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]
why does the ionisation energy of an element decrease as you go along from group 2 to 3 in period 3?
• 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]
why does the ionisation energy of an element decrease as you go along from group 5 to 6 in period 3?
• 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]
what do large increases in successive ionisation energies indicate?
- 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.
identify the s, p and d blocks on the periodic table
how do you write the shorthand electron configuration of an element?
full shell element [i.e. group 0 element] + extra
e.g.
what are the electron configurations of chromium (Cr) and copper (Cu)?
they only have 1 electron in the 4s orbital:
for transition metals, how are ions formed?
electrons lost from s orbital first
e.g.
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?
peaks at m/z = 70, 72 and 74 in the ratio 9:6:1
- 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
• 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%
- 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
• 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.)
what must you ensure when doing TOF calculations?
units ⚠️
• distance is in metres
• mass is in kg (convert from mass number)
• time in s
define the term ‘relative molecular mass’
average mass of one molecule × 12 / mass of one atom of 12C
why do isotopes of the same element have slightly different physical properties?
different numbers of neutrons
identifying an element from a graph/table of ionisation energies for a specific period and explain how you’ve done this?
- 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
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.
Calculated Mr would be greater than the actual since a lower volume would’ve been recorded
What is the importance of taking the percentage yield of a reaction into account?
to maximise the mass of reactants that end up in the desired product, and minimising the amount of by-products
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]
What does Avogadro’s constant represent?
It is the number of atoms/molecules in one mole of a substance
State two differences between the plum pudding model of the atom and the model of atomic structure used today.
- central nucleus contains protons and neutrons in the model used today
- electrons are arranged in orbitals in the model used today
Explain how ions are detected and relative abundance is measured in a TOF mass spectrometer.
- 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
Explain why certain elements in the Periodic Table are classified as __ block elements.
Elements in the __ block have their outer electrons in __ orbitals
Explain why the second ionisation energy of calcium is lower than the second ionisation energy of potassium.
• 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]
Outline how the TOF mass spectrometer is able to separate these two species to give two peaks.
