cap 1 - atomic structure and line spectra

Question 1

atoms

Answer 1

smallest building block of matter that can exist stably and independently, uniquely defining a chemical element

Question 2

atomic forces

Answer 2

• coulomb’s law (electrostatic attraction)
• electrostatic repulsion
• strong nuclear force

Question 3

subatomic particles

Answer 3

proton
- charge: 1+
- location: nucleus
- mass: 1 amu

neutron
- charge: 0
- location: nucleus
- mass: 1 amu

electron
- charge: 1-
- location: probabilistic electron cloud of energy levels
- mass: 1/1836 amu (almost negligable)

Question 4

isotopes

Answer 4

variations of the same element with different numbers of neutrons
=> same chemical properties e.g. reactivity (same p+ and e-)
=> different physical properties e.g. density, mass, stability

Question 5

relative atomic mass

Answer 5

weighted average of the masses of all isotopes (i.e. calculated based off relative isotopic abundance multiplied by relative isotopic mass)

same atomic mass as displayed on the periodic table (decimals!)

differs to mass number!! mass number is the mass of one specific isotope of an element, generally in whole numbers

example:
relative atomic mass of copper = 63.55amu
mass number of copper (isotope) = 65amu

Question 6

mass spectrometry

Answer 6

accurate instrumental technique used to measure the relative isotopic mass and relative abundance within a sample

Question 7

common uses of mass spectrometry

Answer 7

• carbon14 dating
• detecting illegal drugs
• forensic sciences
• space exploration (research)
• medical applications
• quality control
• geology

Question 8

steps of mass spectrometry

Answer 8

1. vaporisation
   sample enters the spectrometer in a gaseous form, after being vaporised in a vacuum chamber
2. ionisation
   the vapour is passed through a high-energy electron beam, where collisions with the beam result in loss of one (or sometimes two) electron/s, thus forming cations
3. acceleration
   the resulting cations are accelerated by an electric field to form a high speed beam of positive ions
4. deflection
   the high speed beam of positive ions are directed through a strong magnetic field, perpendicular to the ion’s path, as generated by the electromagnet, where ions are deflected into circular paths of different radii based on mass.
   lower mass (lighter) => more deflection => smaller radius
5. detection
   ions are collected with the current measured (no. of ions to the mass)
   graphed as relative abundance (y) over m/z mass to charge ratio (x)

Question 9

absorption spectra

Answer 9

element-specific frequencies of electromagnetic radiation (light) at which energy is absorbed when transitioning up from a ground to an excited state

difference between energy levels corresponds to energy of photon (different quanta)

electron promoted to a higher level

continuous spectrum of light has specific frequencies (black lines) of light missing => gaps between the energy levels (absorbed by atom)

Question 10

emission spectra

Answer 10

element-specific frequencies of electromagnetic radiation (light) at which energy is emitted when transitioning down from an excited to a ground state

difference between energy levels corresponds to energy of photon (different quanta)

only specific frequencies of light emitted as coloured lines on a black background => frequencies shown = energy emitted

Question 11

basic emission spectroscopy (flame tests)

Answer 11

• qualitative data
• electrons excited through heat
• absorb energy to transition from ground to excited state
• when returning to ground state, emit photons at characteristic frequencies of light; combinations of photon frequencies produce visible colour light => passed through monochromator

Question 12

spectroscopy

Answer 12

study of interactions between matter and electromagnetic radiation

basis of several qualitative and quantitative analytical techniques

Question 13

strengths of flame tests

Answer 13

• quick and easy test for metal atoms
• convenient (easy to access materials)

Question 14

weaknesses of flame tests

Answer 14

• qualitative data only (subjective)
• only a small range of metals are detectable with a flame test (emissions may not be on the visible light spectrum)
• metals in low concentrations may be difficult to observe
• mixtures of metals will produce confusing results
• used a standard flame
  => luminous with orangish hue that may obscure emitted colours
  => perhaps not hot enough to achieve proper excitation of metal

Question 15

advantages of atomic absorption spectroscopy over flame test

Answer 15

• quantitative data (comparable, easy to analyse)
• can handle/process mixtures of many metals
• highly selective for one metal to be tested
• can test larger range of elements
• very sensitive to low concentrations

Question 16

emission examples (flame tests)

Answer 16

copper (green)
sodium (yellow)
potassium (lilac)
strontium (scarlet)