Atomic Structure (Physical Chem)

define relative isotopic mass

• mass of a single isotope of an element compared to 1/12 the mass of one atom of carbon 12

define relative atomic mass

• average mass of an atom of element (weighted for isotopes) compared to 1/12 of the mass of one atom of carbon 12

1/12th of the mass of one atom of carbon-12 is ?

1

relative isotopic mass is the same as?

• mass number

what is Mass spectrometry used for?

• find the abundance and mass of each isotope in an element allowing us to determine its relative atomic mass

• find the relative molecular mass of substances made of molecules.

what are the different stages of how a mass spectrometer works?

• Ionisation (electrospray or electron impact)

• acceleration

• flight tube/ion drift

• detection

Q: How does the time of flight of an ion depend on its mass and charge?

.

The greater the mass, the longer the time of flight.

Therefore, time of flight is inversely proportional to the m/z ratio.

Q: What are the two ionisation techniques used in TOF mass spectrometry?

Electron impact ionisation

Electrospray ionisation

Q: Describe electron impact ionisation.

• The sample is vaporised.

• high-energy electrons (come from an electron gun) are fired at the gaseous sample.

• An electron is knocked off each particle.

• This forms 1+ ions.

Equation:

X(g) → X+(g) + e-

• AQA says electron impact is used for elements and substances with low formula mass.

Q: What is a molecular ion?

• The 1+ ion formed from a molecule by electron impact ionisation.

• It represents the molecule with one electron removed.

Q: Describe electrospray ionisation.

The sample is dissolved in a volatile solvent.

It is injected through a fine hypodermic needle to produce a fine mist.

A high voltage is applied to the needle.

Molecules gain a proton (H+).

This forms XH+ ions.

The solvent evaporates.

AQA describes electrospray as a soft ionisation technique, commonly used for substances with higher

molecular masses, including biological molecules such as proteins.

Q: What is the difference between electron impact and electrospray ionisation?

Electron impact

• Sample is vaporised

• Loses an electron

• Produces X+

• Used for lower-mass substances

• Fragmentation can occur

Electrospray

• Sample is dissolved in a volatile solvent

• Gains H+

• Produces XH+

• Used for higher-mass substances

• Fragmentation rarely occurs

Q: What happens during acceleration in a TOF mass spectrometer?

• The positive ions are accelerated by an electric field.

• All the ions are given the same kinetic energy.

• Lighter ions have a higher velocity

• Heavier ions have a lower velocity

Q: Why do lighter ions travel faster than heavier ions?

All ions have the same kinetic energy.

Therefore, lighter ions have a higher velocity than heavier ions.

describe the Flight tube / lon drift stage

• The positive ions leave the electric field and travel through the flight tube.

• All ions have the same kinetic energy.

• lons with a lower m/z travel faster.

• lons with a lower m/z reach the detector sooner.

• lons with a higher m/z travel slower and take longer to reach the detector.

• Therefore, the time of flight depends on the mass of the ion.

describe the Detection stage

• The positive ions hit a negatively charged detector plate.

• The ions gain electrons from the plate and are discharged.

• This causes a movement of electrons, producing an electrical current.

• The size of the current indicates the number/relative abundance of ions reaching the detector.

• A greater abundance → larger current → larger peak on the mass spectrum.

• The time taken to reach the detector is used to determine the m/z ratio.

what is the molecular ion peak.

• the peak with the highest m/z ratio which comes from the unbroken molecular ion

• the m/z ratio of this peak = molecular mass of the molecule.

Q: What is fragmentation in mass spectrometry?

Fragmentation is when molecules break up into smaller pieces inside the mass spectrometer.

This happens because the conditions inside the mass spectrometer are very extreme.

Q: In electrospray ionisation, how do you calculate the relative molecular mass (Mr)?

Q: What are principal quantum numbers and how do they relate to electron shells?

A: Each electron shell is given a principal quantum number. The further the shell is from the nucleus, the

bigger the principal quantum number. The principal quantum numbers are 1, 2, 3 and 4.

what are shells divided into?

sub shells called s,p,d and f

define an orbital

a region or space where an electron is most likely in.

what does each subshell consist of

electron orbitals

how many electrons can each orbital hold

2

describe the different rules for writing electronic configuration

Rule 1 (Aufbau principle) : electrons enter the lowest energy orbital available

Rule 2 (Hund’s rule) : electrons prefer to occupy orbitals on their own because they repel each other. they only pair up when no empty orbitals of the same energy are available .

rule 3 - for ions, electrons are added or removed from the highest occupied sub shell. for transition metals, electrons are removed from the 4s orbital before 3d and electrons occupy 4s before 3d.

what is one thing to note about 3d and 4s subshell

4s subshell has a lower energy level than 3d so 4s fills up first but it is conventional to write 3d before 4s.

define particles that are isoelectronic

• particles with the same electronic configuration. e.g neon and oxide

what is the shorthand electronic configuration

noble gas notation

define 1^st ionisation energy

the energy required to remove one electron from each atom in a mole of gaseous atoms producing one mole of 1+ gaseous ions (units are kJ mol^-1)

how would you write an equation for the first and second ionisation energy of sodium

Na_(g) → Na⁺ + e^-

Na+_(g) → Na²⁺_(g) + e^-

define 2nd ionisation energy and why it is bigger than the 1st ionisation energy

• The second ionisation energy is the energy needed to remove one mole of electrons from one mole of 1+ ions in their gaseous state to form one mole of 2+ ions (also in their gaseous state).

• 2nd ionisation energies are higher than 1st because the electron is being removed from a positive ion.

• removing a negative electron from a positive ion requires a lot of energy

what does a high ionisation energy mean

there is a strong attraction between the electron and the nucleus so more energy is needed to remove the electron.

explain the factors that affect ionisation energy

• distance between the nucleus and the outermost electrons (atomic radius) : As the atomic radius increases, there is a weaker electrostatic force of attraction between the positive nucleus and the negative outermost electron and vice versa.

• nuclear charge (no of protons) : the greater the number of protons, the stronger the electrostatic force of attraction is between the positive nucleus and the negative outermost electron.

• shielding : electrons in the outer shell are repelled by electrons in the inner shell which creates a shielding effect that reduces the electrostatic force of attraction between the negative outer electrons and the positive nucleus.

explain why the second ionisation energy of lithium is higher than the first

• the second electron is being removed from a 1s sub shell which is closer to the nucleus so there is a stronger eletrostatic force of attraction between the positive nucleus and the negative outermost electron which means a lot of energy is required to remove/lose the electron.

• the second electron is also being removed from Li⁺ ion so requires more energy.

• there is less shielding because there are no inner electron shells between the 1s electron and the nucleus.

what is succesive ionisation energy

• The successive removal of electrons from the same atom, one electron at a time.

Q: Why does successive ionisation energy gradually increase when electrons are removed from the same

outer shell?

• Remaining electrons experience less electron-electron repulsion.

• They are pulled slightly closer to the nucleus.

• Electrostatic attraction between the nucleus and remaining electrons increases.

• Therefore, more energy is needed to remove each successive electron.

• This causes a gradual increase in successive ionisation energies.

period number

number of electron shells

Q: How can successive ionisation energies be used to determine the group of an element?

• Look for a large jump in ionisation energy.

• Count the number of electrons removed before the large jump.

• This shows the number of electrons in the outer shell.

• Therefore, it indicates the element's group number (for main-group elements).

general trend of the 1st ionisation energy down a group

The 1st ionisation energy decreases down the group

1. atomic radius increases which means that there is a weaker electrostatic force of attrcation between the positive nucleus and the outermost negative electron so less energy is required to remove the electron.

2. there is greater shielding because the number of internal energy levels increase so there is more inner electrons to repel the negative outermost electron creating a weaker electrostatic force of attrcation between the positive nucleus and the outermost negative electron so less energy is required to remove the electron.

3. nuclear charge (no of protons) increases down the group . This would increase the attraction between the nucleus and outermost electron. However, the effects of increased atomic radius and increased shielding outweigh this.

explain the general trend of the 1st ionisation energy across a period

1st ionisation energy increases across a period.

1. as we move across a period the nuclear charge increases (no of protons) so there is stronger electrostatic force of attraction between the positive nucleus and the negative outermost electron. The stronger attraction pulls the outermost electron closer to the nucleus which leads to decreased atomic radius (smaller atom). more energy required to remove an electron

2. shielding is similar and the distance from the nucleus is marginally decreased.

explain the first ionisation energy from group 2 to 3 and 5 to 6

• first ionisation energy decrease from Group 2 to Group 3

• 
  

• Group 2 outermost electron is in an s subshell.

• Group 3 outermost electron is in a p subshell.

• The p subshell is higher in energy and slightly further from the nucleus.

• Therefore, the outermost electron experiences weaker electrostatic attraction to the positive nucleus.

• Less energy is required to remove the electron, so first ionisation energy decreases.

the first ionisation energy decrease from group 5 to 6.

• Group 5: electron removed from a p orbital containing one electron.

• Group 6: electron removed from a p orbital containing a pair of electrons.

• The paired electrons experience electron-electron repulsion.

• This makes an electron easier to remove.

• Therefore, less energy is required to remove the electron, so first ionisation energy decreases.