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AQA
A Level Chemistry
Unit 3.1.1 - Atomic structure
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Summary notes for AQA A-Level Chemistry Unit 3.1.1 - Atomic Structure
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Unit 3.1.1 - Atomic structure
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A-Level Chemistry
Summary notes for AQA A-Level Chemistry Unit 3.1.1 - Atomic Structure by an Imperial College London MSci Chemistry graduate. Notes divided into the following sections: Fundamental Particles, Mass Numbers and Isotopes, Time-of-flight (TOF) mass spectrometer, Electron Configuration, Ionisation Energi...
Atomic Structure
Fundamental Particles
Atom: the smallest particle of an element which has the characteristic properties of that element.
Relative mass of subatomic particles
Atoms are made of smaller particles of 3 types: protons, neutrons + electrons
Atoms of different elements contain different combinations…
Particle Symbol Relative Mass Relative Charge Where it’s found
Protons p 1 +1 nucleus
Neutrons n 1 0 nucleus
Electrons e 1/1840 -1 energy levels
• As the relative mass of an electron is negligible, most of the mass of an atom is concentrated in the
nucleus. The nucleus occupies only a small fraction of the total vol. of the atom.
• The incredibly high density w/in the nucleus suggests that the particles w/in it are drawn very close
together by extremely powerful forces, which overcome the repulsion protons have for each other as
they are +vely charged.
Mass Number and Isotopes
Atomic number: equal to the no. of protons in the nucleus of an atom.
Mass number: total no. of protons + neutrons in the nucleus of an atom.
Isotopes: atoms w/ different no. of neutrons but same no. of protons.
Relative isotopic mass: the mass of a single isotope of an element relative
to 1/12 mass of an atom of carbon-12.
Time-of- ight (TOF) mass spectrometer
It is used to identify elements and to determine relative molecular mass. It gives accurate information
about relative isotopic mass and also about relative abundance of isotopes.
1) Ionisation:
All the ions in the simple treatment of TOF mass spectrometry = mononuclear ions (w/ a single +ve
charge).
• Electrospray ionisation: The sample is dissolved in a volatile solvent and injected through a ne
hypodermic needle giving a ne mist. A high voltage is applied to the tip of the needle, causing
each particle to gain a proton as it leaves the needle. The solvent evaporates (∴ removed) to
produce gaseous positive ions.
• Electron impact ionisation: The sample is vaporised and an ‘electron gun’ is used to re high
energy electrons at it. This knocks one electron off each particle, so they become +1 ions.
2) Acceleration:
An electric eld is applied to give all the ions w/ the same charge a constant KE. As KE = 0.5mv2, it
depends on the mass of the particles and their velocity. As all particles are given the same KE,
heavier particles (larger Mr) move more slowly than lighter particles.
3) Ion drift:
The ions enter a region w/ no electric eld called the ight tube. Here the ions are separated based
on their different velocities. The smaller fast ions travel through the ight tube much more rapidly
+ arrive at the detector rst.
4) Ion detection:
The detector records the different ight times of the ions. The +vely charged ions arrive at the
detector and cause a small electric current because of their charge.
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5) Data analysis:
The ight times are analysed and recorded as a mass spectrum by the data analyser. The mass
spectrum obtained is a plot of relative abundance against mass to charge ratio (m/z).
General information about a mass spectrum
It is a series of peaks on a graph where the vertical axis is relative abundance (which is the same as
the electric current from the detector) + the horizontal axis is the m/z (or sometimes m/e).
The relative abundance is a measure of how many of each ion is present. Often the highest peak
(known as the base peak) is given a value of 100 and the other peaks are worked out relative to this
value.
The m/z for a single charge ion = mass of the ion, as mass divided by 1 = mass. The m/z values for
isotopes of an element are the relative isotopic masses for these isotopes.
Mass spectrum of an element: e.g. Magnesium
The peaks in the spectrum are caused by the isotopes of Mg.
The mass spectrum tells us several things:
1) Mg = 3 peaks = 3 isotopes.
2) The relative isotopic masses of these are 24, 25 + 26 = the m/
z values of each of the isotopes.
3) The most abundant isotope of Mg = relative isotopic mass of
24. This is the m/z value for the base peak.
4) The ion responsible for the peak at: 24 is 24Mg+; 25 is 25Mg+;
at 26 is 26Mg+.
5) The relative atomic mass (Ar) of Mg can be calculated using
info. in the mass spectrum. Simply nd the mean of all the
atoms of the different isotopes.
- E.g. relative atomic mass = ((100x24) + (8.9x25) + (10.9x26))/119.8 = 24.26 = 24.3 (1 d.p.)
Mass spectrum of a molecular element: e.g. Chlorine
Cl exists as diatomic molecules. In the mass spectrometer the molecules of an element like Cl can form
ions w/ a single +ve charge but the molecules can also be broken up into atoms, which each form single
+vely charged ions.
Cl has 2 isotopes: 35Cl and 37Cl.
There are 5 different possible ions which should be
detected in the mass spectrometer:
• 35Cl+
• 37Cl+
• (35Cl-35Cl)+
• (37Cl-37Cl)+
• (35Cl-37Cl)+
∴ there should be peaks seen at m/z values of 35, 37, 70,
72 and 74 on a mass spectrum of molecular Cl.
Mass spectrum of a compound: e.g. Ethanol
For a compound, the last major peak at the highest m/z
base peak →
value is the molecular ion ∴ the m/z value of this
peak is the relative molecular mass (Mr) of the
compound. ∴ in this case, for ethanol, as the molecular
ion peak (or M+) is an m/z value of 46, Mr = 46. Mr of
- The base peak is usually not the molecular ion peak. compound→
- There is a small peak at 47 and this is caused by the
presence of one 13C atom.
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