Unit 1: Section 1 - Atomic Structure

Question 2

Relative atomic mass definition

Answer 2

The average mass of an atom of an element on a scale where an atom of carbon-12 is 12 (an average, so not usually a whole number)

Question 3

Relative isotopic mass

Answer 3

The mass of an atom of an isotope of an element on a scale where an atom of carbon-12 is 12 (usually a whole number)

Question 4

Relative molecular mass

Answer 4

The average mass of a molecule on a scale where an atom of carbon-12 is 12

Question 5

What can a mass spectrometer be used to find?

Answer 5

Relative atomic mass, relative molecular mass, an relative isotopic abundance

Question 6

What is the process when a sample is squirted into a TOF mass spectrometer?

Answer 6

1. Ionisation
2. Acceleration
3. Ion drift
4. Detection

Question 7

What are the 2 ways of ionising the sample in TOF mass spectrometry?

Answer 7

1. Electrospray ionisation
2. Electron impact ionisation

Question 8

What is electrospray ionisation

Answer 8

1. The sample is dissolved and pushed through a small nozzle at high pressure
2. A high voltage is applied to it, causing each particle to gain an H+ ion
3. The sample is turned into a gas made up of positive ions

Question 9

What is electron impact ionisation?

Answer 9

1. The sample is vaporised and an ‘electron gun’ is used to fire high energy electrons at it
2. This knocks one electron off each particle, so they become +1 ions

Question 10

What is acceleration in TOF spectrometry?

Answer 10

The positively charged ions are accelerated by an electric field so that they all have the same kinetic energy (this means that the lighter ions will end up moving faster than the heavier ions)

Question 11

What is ion drift in TOF spectrometry

Answer 11

The ions enter a region with no electric field, so they just drift through it (lighter ions will drift through faster than heavier ions)

Question 12

What is detection in TOF spectrometry?

Answer 12

1. Because lighter ions travel at higher speeds in the drift region, they reach the detector in less time than heavier ions.
2. The detector detects charged particles, an electrical current is produced in the detector when a charged particle hits it
3. A mass spectrum is produced

Question 13

What is a mass spectrum?

Answer 13

1. mass/charge against abundance (often as a percentage)
2. If the sample is an element, each line will represent a different isotope of that element
3. If the spectrum was produced using electron impact ionisation, then you have to take away one from the m/z ratio to get the relative mass of each isotope

Question 14

How to work out relative atomic mass from a mass spectrum

Answer 14

1. For each peak read the % relative isotopic abundance from the y- axis and the relative isotopic mass from the x-axis, and multiply them together to get the total mass for each isotope
2. Add up these totals
3. Divide by 100 (percentages were used)
4. If the relative abundance is not given as a percentage, divide by the sum of the relative abundances instead of 100

Question 15

How do chromium and copper’s electron configuration differ?

Answer 15

They donate one of their 4s electrons to the 3d sub-shell, because they favour a more stable full or half filled d sub-shell

Question 16

How do transition metals behave unusually when it comes to electronic configuration?

Answer 16

When they become ions, they lose their 4s electrons before their 3d electrons

Question 17

What is the definition of the first ionisation energy?

Answer 17

The energy needed to remove 1 electron from each atom in 1 mole of gaseous atoms to form 1 mole of gaseous 1+ ions (you have to put energy in to ionise an atom or molecule so it’s an endothermic process)

Question 18

What are the important things you have to remember about ionisation energies?

Answer 18

1. You must use the gas state symbol because ionisation energies are measured for gaseous atoms
2. Always refer to 1 mole of atoms rather than 1 singular atom
3. The lower the ionisation energy, the easier it is to form an ion

Question 19

What are the factors affecting ionisation energy?

Answer 19

1. Nuclear charge - the more protons there are in the nucleus, the more positively charged the nucleus is and the stronger the attraction for the electrons
2. Distance from nucleus - weaker attraction
3. Shielding - as the number of electrons between the outer electrons between the outer electrons and the nucleus increases, the outer electrons feel less attraction towards the nuclear charge

Question 20

What is the definition for the second ionisation energy?

Answer 20

The energy needed to remove 1 electron from each ion in 1 mole of gaseous 1+ions to form 1 mole of gaseous 2+ ions

Question 21

How do successive ionisation energies change?

Answer 21

1. Within each shell, successive ionisation energies increase, this is because electrons are being removed from an increasingly positive ion - there’s less repulsion amongst the remaining electrons, so they’re held more strongly by the nucleus
2. The big jumps in ionisation energy happen when a new shell is broken into - an electron is being removed from a shell closer to the nucleus

Question 22

What are the trends in 1st ionisation energies?

Answer 22

1. Down a group they decrease
2. Across a period they generally increase

Question 23

How does ionisation energy decrease down group 2?

Answer 23

1. Each element down group 2 has an extra electron shell compared to the one above, so the extra inner shells will shield the outer electrons from the attraction of the nucleus
2. The extra shell means the outer electrons are further away from the nucleus, so the attraction to the nucleus will be greatly reduced

Question 24

How does ionisation energy increase across period 3?

Answer 24

1. The number of protons is increasing, which means a stronger nuclear attraction
2. All the outer electrons are at roughly the same energy level even if they’re in different orbitals
3. There’s generally little shielding effect or extra distance to lessen the attraction from the nucleus

Question 25

Why is there a drop in 1st ionisation energy between groups 2 and 3?

Answer 25

1. Aluminium’s outer electron is in a 3p orbital rather than a 3s, the 3p orbital has a slightly higher energy than the 3s orbital, so the electron on average is found to be further from the nucleus
2. The 3p orbital has additional shielding provided by the 3s2 electrons
3. Both of these factors override the effect of the increased nuclear charge
4. This pattern provides evidence for the theory of electron sub-shells

Question 26

Why is there a drop in 1st ionisation energy between groups 5 and 6?

Answer 26

1. The shielding is identical in the phosphorus and sulfur atoms, and the electron is being removed from an identical orbit
2. In phosphorus’ case, the electron is being removed from a singly-occupied orbital, but in sulfur’s case the electron is being removed from an orbital containing 2 electrons
3. The repulsion between 2 electrons in an orbital means that electrons are easier to remove from shared orbitals