Spectroscopy & The Structure of Atoms

Question 1

What is the relationship between electromagnetic radiation & the energy gap between an atom’s energy levels (ΔE) ?

Answer 1

• Electrons release energy (electromagnetic radiation) when dropping to a lower level, and they absorb energy when jumping to a higher level.
• The frequency of electromagnetic radiation emitted is proportional to ΔE (change in energy)

Question 2

Why can we identify elements from the electromagnetic radiation emitted?

Answer 2

Each atom has a unique energy level (ΔE). It will only absorb/emit electromagnetic radiation at specific frequencies that matches the difference between energy levels (ΔE).

Question 3

How does Absorption Spectroscopy work? (4)

Answer 3

1) Atom is at ground state initially
2) When light source passes through sample, electromagnetic radiation of the right frequency is absorbed by the atom
3) Electron is promoted to a higher energy level, only when electromagnetic frequency matches the energy gap between two energy levels. Atom goes into excited state
4) Since ΔE is characteristic of each element (unique), when seeing absorption of this particular frequency, we can identify the element

Question 4

How does Emission Spectroscopy work? (5)

Answer 4

1) Supply energy to atoms in sample
2) Energy excites some of the atoms - electrons move up to higher energy levels
3) When electrons drop back to its ground state, energy is given out in the form of electromagnetic radiation
4) Detected by detector and meter
5) Frequency of radiation given out = energy gap between energy levels. Element can be identified. Concentrated can also be determined by measuring the intensity

Question 5

What are Farunhofer lines?

Answer 5

Black lines in the solar (colour from sun) spectrum due to absorption by hydrogen/helium other gases

Question 6

How do we use a prism to obtain the spectrum of an element? (3)

Answer 6

1) Use electricity to get atoms to excited state
2) When atoms drop back to ground state, they give out light
3) The beam of light passes through slits, then into the prism. The prism breaks down the light into its components. Each ray of light emitted has different colours and hence different frequencies. This can be detected

Question 7

What is excited state and ground state?

Answer 7

excited state: when electrons rise from a lower energy level to a higher energy level
ground state: when electrons drop from a higher energy level to a lower energy level

Question 8

How does the flame test for metals work? (2)

Answer 8

1) Take the salt of a metal and introduce it to a flame
2) flame will change colour according to the metal present

Question 9

Why does the flame test for metals work?

Answer 9

The heat of the flame will excite the electrons. After dropping back to ground state, it emits electromagnetic radiation [that matches the energy difference], hence giving out a coloured light.

Question 10

What is the limitation of a flame test, and what is its alternative?

Answer 10

Limitation: qualitative - only know which element is present, but not its amount
Alternative: Flame photometer/Atomic Absorption Spectrometer (measuring intensity, which is proportional to concentration of the element)

Question 11

What are some common colours of flames of metal?

Answer 11

Sodium: orange
Barium: green
Calcium: red
Lithium: red
Copper: green
Potassium: lilac

Question 12

How does the Atomic Absorption Spectrometer (AAS) work? (5)

Answer 12

1) A sample dissolved in acid will pass into the flame. flame becomes atomised (atoms present in flame)
2) next to the flame, there is a lamp that generates light specifically for the element of interest (matching energy gap in the element)
3) atoms in the flame absorb light
4) when passed through the monochromator & detector, there is a loss of light intensity
5) loss of light intensity is proportional to the amount of atoms of interest in the flame

Question 13

What are the advantages of the AAS? (5)

Answer 13

1) Fast
2) Specific
3) Sensitive
4) Small sample size needed; can detect trace amounts
5) quantitative

Question 14

What are the disadvantages of the AAS? (2)

Answer 14

1) Destructive (but only small amount destroyed)
2) Must be done one element at a time

Question 15

How does the Energy Dispersive X-Ray Fluorescence (EDX) work? (4)

Answer 15

1) an electron beam is fired at the sample
2) it hits an electron in the core energy level and ejects it, creating a vacancy
3) a high energy energy electron drops down to fill the vacancy, emitting a characteristic (unique) x-ray
4) intensity is proportionate to concentration of elements in the sample

Question 16

Why is an X-Ray emitted for EDX?

Answer 16

The drop is quite big between two energy levels

Question 17

What are 3 advantages of EDX?

Answer 17

1) non-destructive
2) quantitative
3) can be used when sample is very small and cannot be seen with visible light

Question 18

When is the Scanning Electron Microscope (SEM)-EDX used?

Answer 18

When the sample is very small and cannot be seen with visible light. Useful for gunshot residue, which diameter is very small

Question 19

How does the Neutron Activation Analysis work? (6)

Answer 19

1) Use neutron to hit nucleus
2) Nucleus absorbs neutron and becomes a compound nucleus (excited state), which decays
3) This will give out a prompt gamma ray, and nucleus will convert itself into a radioactive nucleus
4) Radioactive nucleus will give out a Beta particle & delayed gamma ray
5) Nucleus will end up as product nucleus
6) Gamma rays are forms of electromagnetic radiation, characteristic (unique) to each element

Question 20

What are the advantages of neutron activation analysis? (3)

Answer 20

1) non destructive
2) both qualitative & quantitative
3) 70% of elements can be studied

Question 21

What are the disadvantages of neutron activation analysis? (2)

Answer 21

1) needs a nuclear reactor
2) sample may be radioactive afterwards