Mass Number and Isotopes
Quick Notes
- Atomic number (Z): number of protons.
- In a neutral atom the number of electrons = number of protons.
- In an ion, the number of electrons is based on the charge of the ion.
- Mass number (A): number of protons + neutrons.
- Isotopes: same number of protons but different numbers of neutrons.
- Mass spectrometry determines relative isotopic mass and abundance.
- A time of flight (TOF) mass spectrometer has four main stages:
- Ionisation
- Acceleration
- Drift
- Detection
- Relative atomic mass (Ar) can be calculated using data from a mass spectrum.
Full Notes
Mass number, isotopes and mass spectrometry have been outlined in more detail at atomic structure, mass spectrometry, and TOF mass spectrometry.
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Atomic Structure Recap
- Protons: positively charged (+1), found in the nucleus.
- Neutrons: neutral (0), found in the nucleus.
- Electrons: negatively charged (−1), found in orbitals around the nucleus.
Atomic number (Z) = number of protons.
Mass number (A) = number of protons + neutrons.
Example:
For sodium-23:
- Z = 11 → 11 protons
- A = 23 → 23 − 11 = 12 neutrons
- If neutral atom → 11 electrons
For ions:
- Positive ions lose electrons (e.g. Na+ has 11 protons and 10 electrons).
- Negative ions gain electrons (e.g. O2− has 8 protons and 10 electrons).
Isotopes
Isotopes are atoms of the same element with:
- The same number of protons
- Different numbers of neutrons
This means they have different mass numbers.
Example:
Carbon contains naturally occurring isotopes including Carbon-12, Carbon-13 and Carbon-14:
- All have 6 protons.
- C-12 has 6 neutrons, C-13 has 7 neutrons, C-14 has 8 neutrons.
Isotopes have identical chemical properties (same electron structure), but different physical properties (e.g. mass, rate of diffusion).
Time of Flight (TOF) Mass Spectrometry
A more detailed explanation is given here.
Used to determine:
- The relative isotopic mass
- The relative abundance of isotopes
- The relative molecular mass (Mr) of compounds
- The identity of unknown elements
Stages in a TOF Mass Spectrometer:
- Ionisation
- Sample is turned into ions. There are two main methods used:
- Electron impact: X → X+ + e−
electron is removed from sample to make it a positively charged ion - Electrospray: X + H+ → XH+
sample is 'given' a proton, H+, to make it a positively charged ion
- Acceleration – ions are accelerated by an electric field so all ions have the same kinetic energy.
- Ion Drift – ions pass through a drift region; lighter ions travel faster than heavier ions.
- Detection – ions hit a detector and generate a current; time is used to calculate m/z; bigger current = greater abundance.
Interpreting a Mass Spectrum
- m/z values = mass-to-charge ratio.
- Peak height or area = relative abundance.
- For atoms of an element being analysed, a spectrum shows isotopes (each peak is a different isotope).
- For molecules being analysed, the peak with highest m/z = molecular ion (M+) and represents the relative molecular mass of the sample (assuming an ion charge of 1+).
Calculating Relative Atomic Mass (Ar)
We can calculate the relative atomic mass of an element using the following formula:
Ar = (Σ (isotopic mass × % abundance)) / 100
Example:Chlorine has two isotopes:
Ar = (35 × 75 + 37 × 25) / 100
Ar = (2625 + 925) / 100 = 35.5
6. Summary
- Atomic number (Z) = protons; Mass number (A) = protons + neutrons.
- Isotopes are atoms with the same number of protons, different number of neutrons.
- Ions are atoms that have lost or gained electrons.
- TOF mass spectrometry: ionisation, acceleration, drift, detection.
- A mass spectrum can be used to determine isotopic masses of an element and the relative molecular mass of a molecule.
- Relative atomic mass (Ar) calculated from isotopic data.
Ar = (Σ (isotopic mass × % abundance)) / 100