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All AQA A level chemistry notes
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Fundamental particlesWednesday, October 19, 2022 7:23 PM
• Our current model shows electrons in different energy levels compared to models before
Mass number - totals number of protons and neutrons in the nucleus of an atom
Atomic number - number of protons in nucleus
History of the atom
John Dalton - He thought that atoms were spherical and that all the atoms in one element had the same mass and where indivisible
J.J Thompson -
• Discovered and measured the mass of an electron
• As electrons had a negative charge, there had to be some source of positive charge inside the atom as well
• As electrons were much lighter than whole atoms, there had to be something to account for the rest of the mass
He proposed the plum pudding model. The atom was viewed to be a large positive sphere which embedded smaller negatively charged electrons in circular
arrays like plums in a pudding. However now we know:
• Electrons are arranged in energy levels
• Nucleus contains protons and neutrons
Ernest Rutherford -
• Fired alpha particles at a thin film of gold
• If the plum pudding model was true then the particles would have been deflected by the Positive sphere; instead most passed s traight through the film
• Atom is mostly empty space with a positive nucleus and orbiting electrons - nuclear model
Niels Bohr -
• Problem with nuclear model - electron would spiral into the nucleus and the atom would collapse
• Bohr therefore assigned electrons to fixed orbits of defined energy
atomic structure Page 1
,atomic structure Page 2
,Mass number and isotopes
13 January 2023 20:48
Mass number (A) - totals number of protons and neutrons in the nucleus of an atom
Atomic number (Z) - number of protons in nucleus
Isotope - Different atomic forms of the same element containing the same number of protons, different number of neutrons and different relative atomic
masses
• Atomic radius is the same as each isotope has same number of protons
• Isotopes of an element have the same chemical properties as they have the same electron configuration
What might cause the RAM of a sample to be different from the RAM given in the periodic table?
• Different amounts of isotopes
atomic structure Page 3
, Mass spec
Wednesday, October 19, 2022 7:33 PM
https://www.chemguide.co.uk/analysis/masspecmenu.html#top
Electrospray ionisation
1. The sample is dissolved in a volatile solvent
2. The sample is injected through a fine hypodermic needle to give a fine mist
3. The needle has a high voltage
4. Particles are ionised by gaining a proton from the solvent as they leave the needle producing XH+ ions
5. The solvent evaporates away while the XH+ ions are attracted towards a negative place where they are accelerated
X (g) + H+ → XH+ (g)
Electron impact ionisation
1. The sample is vaporised
2. High energy electrons are fired from an electron gun at the sample
3. This knocks off an electron from each particle forming a 1+ ion
X (g) → X+ (g) + e-
Acceleration
1. The positive ions are accelerated towards the negatively charged electric plate using an electric field so they all have the same kinetic energy
2. Velocity of particles depend on mass - lighter particles have quicker velocity
Ion drift
1. Positive ions travel through a hole in the negatively charged plate to form a beam
2. They then travel along the flight tube to a detector - lighter ions reach the detector in less time
Detection
1. Positive ions hit a negatively charged electron plate
2. When they hit the detector the positive ions gain electrons from the electrons; electrons are transferred to the ions
3. This generates an electric current
4. The size of the current is proportional to the abundance of those ions hitting the plate and gaining an electron
X+ + e- = X
atomic structure Page 4
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