AQA A-Level Chemistry atomic structure topic. I covered all specification points and went through mark schemes to find wording they liked. I am a student myself and these are my own notes I wrote as revision.
Mechanisms Map Summary AQA Chemistry: A Level - Unit 3.3.14 - Organic synthesis
AQA A - level Chemistry Atomic Structure notes
AQA A Level Chemistry Summary Notes - AS Equilibria
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Atomic Structure:
Models of the atom:
- Solid sphere model (Dalton) 1803: suggested atoms were indivisible but were
different in size from element to element and could combine to form new
substances.
- Plum pudding model (J.J. Thomson) 1904: discovered electrons, model
contained negative electrons randomly spread through a solid sphere/cloud.
- Nuclear model (Rutherford) 1911: gold foil experiment (alpha scattering),
discovered the nucleus and that most of the electron was empty space. Knew
electrons surrounded the nucleus, but didn’t know what paths they took.
- Bohr model (Bohr) 1913: suggested the existence of shells and different energy
levels.
- Rutherford model (Rutherford) 1920: discovered the neutron existed and was in
the nucleus.
- Quantum model: orbitals and particle-wave duality. Orbitals are regions where
there’s a high probability that you’ll find an electron.
Protons: relative mass of 1, relative charge of +1
Neutrons: relative mass of 1, relative charge of 0 (neutral).
Electrons: relative mass of 1/2000 (actually closer to 1/1840), relative charge of -1.
The number of electrons in a shell can be calculated with 2n2 where n is the
energy level/number of the shell.
An atom consists of a nucleus containing protons and neutrons surrounded by
electrons. Mass number (A) and atomic (proton) number (Z).
Mass number and isotopes
Relative atomic mass (Ar) is the weighted mean mass of an atom of an element
compared with 1/12th of the mass of an atom of Carbon-12.
Relative molecular mass (Mr) is the weighted average of the mass of a molecule
compared with 1/12th of the mass of an atom of Carbon-12.
Relative isotopic mass is the mass of a particular isotope of an atom compared
to 1/12th of an atom of carbon-12, for example 35Cl would have a relative isotopic mass
of 35.
, Isotopes are atoms with the same number of protons, but different numbers of
neutrons. They have the same chemical properties as their electron configurations are
the same, but they have different physical properties (melting/boiling points etc) and
some may have unstable nuclei, leading to radioactive decay due to their different
masses.
Mass spectrometry:
A mass spectrum is a graph showing the relative abundance (often as a
percentage) on the y-axis and the mass: charge (m/z) ratio on the x-axis.
The whole apparatus is kept under vacuum to avoid collisions between air
particles and ions and to avoid ionising air particles which would cause them to be
detected by the detector.
The mass spectrometer gives accurate information about relative isotopic mass
and also about the relative abundance of isotopes. It can be used to identify elements
and to determine relative molecular mass.
Ionisation: the process of forming 1+ gaseous ions, there are two ways of doing this:
Electron impact: used mainly for samples with lower formula masses as it can
cause fragmentation (the process in which a molecular ion breaks into smaller ions,
radicals, and/or neutral molecules).
- A vaporised (in the gaseous state) sample is injected at low pressure.
- An electron gun fires high speed electrons at the sample.
- These electrons knock out the outer electron, forming positively charged ions.
- 2+ ions can be formed, these will form peaks with an m/z value half the size of
the mass as mass is divided by charge.
- X(g)→X+(g)+e-
Electrospray ionisation: Electrospray ionisation is more often used for larger
organic molecules, as it does not cause as much fragmentation.
- Sample is dissolved in a volatile (easily evaporated at normal temperatures) and
polar solvent.
- The sample is injected through a fine (maybe hypodermic) needle to form a fine
mist or aerosol.
- A high voltage passes through the tip of the needle, making the molecule gain a
proton (H+) from the solvent (why it must be polar).
- The solvent evaporates and the ions move towards the negative plate.
- You must remove 1 from the m/z value, as it has gained a proton.
- X(g)+H+→XH+(g)
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