Atomic Structure - HQ

Question 1

Emission spectra

Answer 1

The spectrum of frequencies of light (electromagnetic radiation) emitted when electrons return from a high-energy state to a lower one after absorbing energy, often through heating, lighting, or applying voltage

Question 2

Flame tests

Answer 2

A qualitative analytical technique used to identify the presence of certain metal ions in a sample by observing the colors emitted when the ions are heated in a flame.

Question 3

Why are flame tests not that good?

Answer 3

Only a limited number of metal ions can be distinguished in this way as:
* A Bunsen flame is not enough to excite the electrons of many atoms
* It is difficult to distinguish between very similar colours. E.g. Reddish colours are produced by calcium, strontium and lithium compounds

Question 4

How can flame tests be more accurate?

Answer 4

• The use of reference flames of known compounds (burnt under the same conditions)
• Observation of the flames in a darkened room using a spectroscope

Question 5

Atomic emission spectrometry (AES)

Answer 5

A quantitative analysis to identify elements in a flame

Question 6

Steps of atomic emission spectrometry

Answer 6

• The samples are heated, but at much higher temperatures.
• The light is passed through a prism or diffraction grating (similar to a spectroscope)
• Instead of being all the spectra as a whole, a device called the monochromator allows only single wavelengths at a time to pass through
• The spectra can then be detected and recorded in different ways.
• The intensity of each spectral line can be recorded using a spectograph or spectrometer.
  (Hence AES provides a more quantitative (hence practical) method for identifying elements)

Question 7

Spectograph

Answer 7

The darkness on the negative for a particular wavelength indicates:
* Intensity
* Hence, abundance of the element causing it

Question 8

Spectrometer

Answer 8

The detected light from the monochromator is converted to an electrical current and then displayed digitally/graphically.

Question 9

Absorption spectra

Answer 9

When a beam of white light is passed through a cool sample of a gas, an absorption spectrum is created
* Instead of giving a continuous spectrum, this shows that particular frequencies of light are missing
* I.e. There are black spaces in locations throughout the spectrum, indicating that some frequencies have been removed by the gas
* Like the emission spectrum, the absorption spectrum is once again unique to each type of atom, hence identifying the gaseous element through which the light has passed
* For a given sample, the dark lines (i.e. missing lines) in its absorption spectrum correspond exactly to bright lines in its emission spectrum

Question 10

Atomic absorption spectrometry

Answer 10

An analytical technique to determine the concentration of elements in a sample by analyzing the amount of light absorbed as it passes through the sample.

Question 11

Atomic absorption spectrometry steps

Answer 11

The light absorbed is light emitted by the element
* A hollow cathode lamp is coated with the metal being investigated. Hence the cathode light which passes through the atomised sample contains the exact wavelengths that can be absorbed by the metal being analysed
* The amount of cathode light absorbed by the metal atoms gives a measure of their concentration
* Importantly, the cathode light is pulsed (i.e. on/off). This allows for the detector to distinguish between:
* The cathode light left over after absorbance
* The light naturally emitted by the atomised sample in the flame

Question 12

Mass spectrometry

Answer 12

Used to measure masses and relative concentrations of atoms and molecules
E.g. It can be used to determine:
* The isotopic masses of naturally occurring elements
* Their relative abundances
* Hence, the element’s relative atomic mass

Question 13

Components of mass spectrometer

Answer 13

• Ioniser
• Particle accelerator
• Deflector
• Detector and recorder

Question 14

Mass spectrometer

Ioniser

Answer 14

Converts a portion of the sample into ions

Question 15

Mass spectrometer

Particle accelerator

Answer 15

The resulting ions are accelerated by an electric field and passed through a velocity selector

Question 16

Mass spectrometer

Deflector

Answer 16

A magnetic field causes the ions to be deflected. The amount of deflection depends on the ion’s:
* Mass
* Charge
Any difference in charge would give distinctly different results compared to small differences in mass and could be easily accounted for

Question 17

Mass spectrometer

Detector and recorder

Answer 17

The mass/charge ratio and relative concentrations are detected and recorded

Question 18

Mass spectrometer

Relative atomic masses

Answer 18

The mass spectrum of an element shows characteristic peaks for each isotope
* The position of these peaks indicates the relative mass of each isotope
* The height of these peaks indicates the relative abundance of each isotope. The heights can be considered as a fraction or percentage of the total height