Atomic Structure Flashcards
What force holds the protons and neutrons
Strong nuclear force
What holds electrons and protons together in an atom
Electrostatic forces of attraction
Why is a nuclear force stronger than an electrostatic force?
It overcomes repulsion between protons in nucleus
What decides the chemical properties of an element
The number and arrangement of electrons
Why do isotopes have the same chemical properties?
They have the same electron configuration
Define the relative atomic mass (Ar) of an element
Average mass of an atom of an element on a scale where an atom of carbon-12 is 12
Define relative isotopic mass
Mass of an atom of an isotope of an element on a scale where an atom of carbon-12 is 12
Define relative molecular mass (Mr)
Average mass of a molecule on a scale where an atom of carbon-12 is 12
What does a mass spectrometer do and how?
- It determines the mass of separate atoms (or molecules)
- It works by forming ions from a sample and them separating them according to the ration charge to their mass
What 6 things happen when a sample is squirted into time of flight (TOF) mass spectrometer?
- Vacuum
- Ionistion
- Acceleration
- Ion drift
- Detection
- Data analysis
Describe the step vacuum in mass spectrometry (TOF)
Whole apparatus is kept under a vacuum to prevent ions produced from colliding with air molecules
Name the two ways you can ionise your sample in mass spectrometry (TOF)
- Electrospray ionisation
- Electron impact ionisation
Describe the method of electrospray ionisation
- A high voltage is applied to a sample in a polar solvent
- Sample molecule, M, gains a proton forming MH+
Describe the method of electron impact ionisation
- Sample is bombarded by high-energy electrons
- Sample molecule loses an electron becoming + ions
Describe the step acceleration in mass spectrometry (TOF)
Positively charged ions are accelerated by an electric field (attracted to a negatively charged plate) so they all have the same kinetic energy
Describe the step ion drift in mass spectrometry (TOF)
- Ions enter regions with no electric field so they just drift through it
- Lighter ions will drift faster than heavier ions
Describe the step detection in mass spectrometry (TOF) & state how abundance is measured
- Lighter ions travel at higher speeds so reach detector in less time than heavier ions
- Positive ions collected at detector (metal plate)
- Causing current to flow electrically (+ ions gain e- from metal plate causing current)
- Abundance is measured by the strength of current, stonger current means more ions hitting the plate at one time
Describe the step data analysis in mass spectrometry (TOF)
Signal from detector is sent to a computer which generates a mass spectrum
What does the height of each peak give on the mass spectrum?
Relative isotopic abundance
If the sample is an element, what does each line represent on the mass spectrum?
A different isotope of the element
What does the y-axis of mass spectrum represent?
Abundance of ions
What does the x-axis on the mass spectrum represent?
‘mass/charge’ ratio (m/z)
Describe how to work out the relative atomic mass from mass spectrum (4)
- Spectrum gives % relative abundance (of isotopes) & m/z (mass/charge ratio)
- Multiply m/z by % relative abundance for each isotope
- Sum these values
- Divide by the sum of the % relative isotopic abundances (usually 100)
Describe how you can use mass spectrometry to identify elements
You can see if the sample being analysed has the same relative abundances of isotopes
