[3.1.1] Atomic Structure

Question 1

What is the relative charge and relative mass of the three sub-atomic particles?

Answer 1

PROTON

• Relative charge = +1
• Relative mass = 1

NEUTRON

• Relative charge = 0
• Relative mass = 1

ELECTRON

• Relative charge = -1
• Relative mass = 1/1840

Question 2

What is the mass number and which letter represents it?

Answer 2

• The mass number is the total number of protons and neutrons in the atom.
• It is represented by the letter A.

Question 3

What is the atomic number and which letter represents it?

Answer 3

• The atomic number is the total number of protons in the nucleus.
• It is represented by the letter Z.

Question 4

How would you calculate the number of neutrons in an atom?

Answer 4

Number of neutrons = mass number - atomic number

Question 5

What are isotopes?

Answer 5

• Isotopes are atoms with the same number of protons, but different numbers of neutrons.

Question 6

Describe the principles of time of flight mass spectrometry.

Answer 6

• The mass spectrometer can be used to determine all the isotopes present in a sample of an element and to therefore identify elements.
• It needs to be under a vacuum otherwise air particles would ionise and register on the detector.

IONISATION

Sample can be ionised in two ways:

1. ELECTRON IMPACT

• Vaporised sample is injected at low pressure.
• An electron gun fires high-energy electrons at the sample.
• This knocks out an outer electron.
• Forming positive ions with different charges
  
  • e.g. Ti (g) -> Ti+ (g) + e⁻
• This method is used for elements and substances with low formula mass, electron impact can cause larger organic molecules to fragment.

2. ELECTROSPRAY IONISATION

• The sample is dissolved in a volatile, polar solvent.
• It is injected through a fine needle giving a fine mist or aerosol.
• The tip of the needle has a high voltage.
• At the tip of the needle the sample molecule, M, gains a proton, H⁺, from the solvent forming MH⁺.
• This method is used for larger organic substances because the ‘softer’ conditions mean fragmentation does not occur.

ACCELERATION

• Positive ions are accelerated by an electric field to a constant kinetic energy.
• Given that all the particles have the same KE, the velocity of each particle depends on its mass.
  
  • Lighter particles have a faster velocity and heavier particles have a slower velocity.

ION DRIFT

• The positive ions with smaller m/z values will have the same kinetic energy as those with larger m/z but will move faster.
• Heavier ions will take longer to move through the drift area as they’re moving at a lower speed.
• The ions are distinguished by different flight times.

DETECTION

• The ions reach the detector and generate a small current, which is fed to a computer for analysis.
  
  • The current is produced by electrons transferring from the detector to the positive ions.
• The size of current is proportional to the abundance of the species.
• For each isotope, the mass spectrometer can measure an m/z (mass/charge ratio) and an abundance.

Question 7

Answer 7

Question 8

How do you calculate relative atomic mass?

Answer 8

• R.A.M = ∑(isotopic mass x % abundance) / 100

If relative abundance is used instead of percentage abundance, use this equation:

• R.A.M = ∑(isotopic mass x % abundance) / total relative abundance

Question 9

Answer 9

Question 10

Answer 10

Question 11

How would you determine the Mr of a molecule that’s undergone electron impact ionisation from a mass spectrum?

Answer 11

• If a molecule is put through a mass spectrometer with an electron impact ionisation stage, it will often break up and give a series of peaks caused by fragments.
• The peak with the largest m/z, however, will be due to the complete molecule and will be equal to the relative molecular mass, Mr, of the molecule.
• This peak is called the parent ion or molecular ion.

Question 12

How would you determine the Mr of a molecule that’s undergone electro-spay ionisation from a mass spectrum?

Answer 12

• If a molecule is put through a mass spectrometer with electro-spray ionisation then fragmentation will not occur.
• There will be one peak that will equal the mass of the MH⁺ ion.
  
  • It will therefore be necessary to subtract 1 to get the Mr of the molecule.
  • So if a peak at 521.1 is for MH⁺, the relative molecular mass of the molecule is 520.1.

Question 13

Describe how electrons are arranged in an atom.

Answer 13

ELECTRONS ARE ARRANGED ON…

1. Principle energy levels numbered 1, 2, 3, 4…
   
   • 1 is closest to the nucleus.

SPLIT INTO ->

2. Sub energy levels labelled s, p, d & f.
   
   • s holds up to 2 electrons.
   • p holds up to 6 electrons.
   • d holds up to 10 electrons.
   • f holds up to 14 electrons.

SPLIT INTO ->

3. Orbitals which hold up to 2 electrons of opposite spin.

• This means that:
  
  • s sub energy levels have 1 orbital.
  • p sub energy levels have 3 orbitals.
  • d sub energy levels have 5 orbitals.
  • f sub energy levels have 7 orbitals.

Question 14

Describe the order in which an atom fills its subshell.

Answer 14

IN ORDER OF INCREASING ENERGY

1s -> 2s -> 2p -> 3s -> 3p -> 4s -> 3d -> 4p -> 5s -> 4d -> 5p

• 3d is higher in energy than 4s and so gets filled after the 4s.

Question 15

What are orbitals? What shape are the s and p orbitals?

Answer 15

• Orbitals represent the mathematical probabilities of finding an electron at any point within certain spatial distributions around the nucleus.
• Each orbital has its own approximate, 3D shape.
  • S orbitals are shaped like spherical.
  • P orbitals are shaped like dumbbells.

Question 16

Describe how you would write the electronic structure for oxygen using letters and numbers.

Answer 16

Question 17

Describe how you would use a spin diagram to represent fluorine’s electronic structure.

Answer 17

Question 18

What are s, p and d block elements?

Answer 18

THE PERIODIC TABLE IS SPLIT INTO BLOCKS…

• s block elements are ones whose outer electron is filling a s-sub shell.
• p block elements are ones whose outer electron is filling a p-sub shell.
• d block elements are ones whose outer electron is filling a d-sub shell.

Question 19

What is the electronic structure of chromium (Cr)?

Answer 19

Cr = 1s² 2s² 2p⁶ 3s² 3p⁶ 4s¹ 3d⁵

Question 20

What is the electronic structure of copper (Cu)?

Answer 20

Cu = 1s² 2s² 2p⁶ 3s² 3p⁶ 4s¹ 3d¹⁰

Question 21

What happens when d-block elements form ions?

Answer 21

• They lose their 4s electrons first.

Question 22

Define first ionisation energy.

Answer 22

The first ionisation energy is the enthalpy change when one mole of gaseous atoms forms one mole of gaseous ions with a single positive charge.

Question 23

What is the equation for first ionisation energy?

Answer 23

H (g) -> H⁺ (g) + e⁻

• The equation for 1st ionisation energy allows follows the same pattern.
• It does not matter if the atom does not normally form a +1 ion or is not gaseous.

Question 24

Define second ionisation energy.

Answer 24

The second ionisation energy is the enthalpy change when one mole of gaseous ions with a single positive charge forms one mole of gaseous ions with a double positive charge.

Question 25

What is the equation for second ionisation energy?

Answer 25

Ti⁺ (g) -> Ti²⁺ (g) + e⁻

Question 26

What factors affect ionisation energy?

Answer 26

THERE ARE THREE MAIN FACTORS…

1. THE ATTRACTION OF THE NUCLEUS
   
   • The more protons in the nucleus, the greater the attraction.
2. THE DISTANCE OF THE ELECTRONS FROM THE NUCLEUS
   
   • The bigger the atom, the further the outer electrons from the nucleus and the weaker the attraction to the nucleus.
3. SHIELDING OF THE ATTRACTION OF THE NUCLEUS
   
   • An electron in an outer shell is repelled by electrons in complete inner shells, weakening the attraction of the nucleus.

Question 27

Why are successive ionisation energies always larger?

Answer 27

• The second ionisation energy is always bigger than the first ionisation energy.
• When the first electron is removed a positive ion is formed.
• The ion increases the attraction on the remaining electrons and so the energy required to remove the next electron is larger.

Question 28

Explain the big jump between 4 & 5.

Answer 28

Question 29

Answer 29

Question 30

Why has helium got the largest first ionisation energy?

Answer 30

• Its first electron is in the first shell closest to the nucleus and has no shielding effects from inner shells.
• Helium has a bigger first ionisation energy than hydrogen as it has one more proton

Question 31

Why do first ionisation energies decrease down a group?

Answer 31

• As one goes down the group, the outer electrons are found in shells further from the nucleus and are more shielded so the attraction of the nucleus becomes smaller.

Question 32

Why is there a general increase in first ionisation energy across a period?

Answer 32

• As one goes across a period, the electrons are being added to the same shell which has the same distance from the nucleus and same shielding effect.
• However, the number of protons increases making the effective attraction of the nucleus greater.

Question 33

Why has Na have a much lower first ionisation energy than Ne?

Answer 33

• This is because sodium will have its outer electron in a 3s shell further from the nucleus is more shielded.
• So sodium’s outer electron is easier to remove and has a lower ionisation energy.

Question 34

Why is there a small drop in ionisation energy from Mg to Al?

Answer 34

• Al is starting to fill a 3p subshell, whereas Mg has its outer electrons in the 3s subshell.
• The electrons in the 3p subshell are slightly easier to remove because the 3p electrons are higher in energy and also slightly shielded by the 3s electrons.

Question 35

Why is there a small drop in ionisation energy from P to S?

Answer 35

• With sulfur there are 4 electrons in the 3p subshell and the 4th is starting to doubly fill the first 3p oribital.
• When the second electron is added to the 3p orbital there is a slight repulsion between two negatively charged electrons which makes the second electron easier to remove.