1.1 Atomic Structure
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Atomic number
Number of protons in the nucleus of an atom
Mass number
Number of protons and neutrons in the nucleus of an atom
Isotopes
Atoms of the same element (same number of protons) with a different number of neutrons
What is a mass spectrometer used for?
Calculating the relative atomic mass of an element or the relative molecular mass of a compound.
What are the 4 stages of a TOF mass spectrometer?
• Ionisation • Acceleration • Separation of ions • Detection
Why is the spectrometer kept in a vacuum?
To prevent ions that are made from colliding with molecules in the air.
Mass spectrometer What are the two methods of ionisation?
• Electron impact • Electrospray ionisation
Mass spectrometer How does electron impact ionisation work?
• Sample is vapourised and injected into spectrometer • High energy electrons fired from an electron gun • Electron is knocked from each particle • Positive ions formed
Mass spectrometer What is an electron gun?
A hot wire filament that releases high energy electrons.
Mass spectrometer What is the charge of the ions formed?
1+
Mass spectrometer How are the positive ions accelerated?
With an electric field
Mass spectrometer How are the ions separated?
Lighter ions travel through the spectrometer faster than heavier ions
Mass spectrometer How are the ions detected?
Each ion gains an electron - this generates a current
Mass spectrometer How is the abundance of each ion determined?
It is proportional to the current produced. The greater the abundance, the greater the current.
Mass spectrometer Why is the sample ionised?
So it can be accelerated and detected
Relative isotopic mass
The mass of an isotope relative to 1/12th the mass of an atom of carbon-12
Relative atomic mass (Ar)
Weighted average mass of all isotopes of an element / 1/12 mass of one atom of carbon-12
What order do the energy levels fill up?
1s–>2s–>2p–>3s–>3p–>4s–>3d–>4p
Electron configuration for chromium (Cr)?
1s²2s²2p⁶3s²3p⁶3d⁵4s¹
Electron configuration for copper (Cu)?
1s²2s²2p⁶3s²3p⁶3d¹⁰4s¹
(First) Ionisation energy (chemistry)
The energy needed to remove one mole of electrons from one mole of atoms in the gaseous state.
Why do successive ionisation energies increase?
• Attraction of the outer electrons and nucleus increases (same number of protons for fewer electrons) • Large increase where outer electron is in a new main shell (closer to nucleus with less shielding)
Which shell are electrons lost from first in period 4 transition metals?
4s
Mass spectrometer How does electrospray ionisation work?
• Sample dissolved in a volatile polar solvent • Injected through a hypodermic needle which gives a fine mist • Tip of needle is attached to positive terminal of a high voltage supply • Particles gain a protons from the solvent