Detailed notes for AQA AS level chemistry unit 3.1.1 (atomic structure). Covers: history of the atom, relative atomic mass, time of flight (TOF) mass spectrometry and calculations, electronic structure and ionisation energies. Includes diagrams and example calculations for RAM and TOF. Also explain...
History of the Atom
Start of the 19th Century (John Dalton):
• John Dalton described atoms as solid spheres.
• He believed different spheres made up different elements.
1897 (JJ Thompson):
• JJ Thompson’s experiments of charge and mass showed that an
atom must contain smaller, negatively charged particles, i.e.
electrons.
• This showed that atoms weren’t solid and indivisible.
• The new model was known as the plum pudding model.
• The plum pudding model presents the atom as a ball of positive
charge with electrons stuck in it.
1909 (Ernest Rutherford):
• Ernest Rutherford and his students Hans Geiger and Ernest Marsden conducted the gold foil experiment.
• Positively charged alpha particles were fired at an extremely thin sheet of gold.
• Based on the plum pudding model, they expected most
particles to pass straight through the sheet (completely
missing the electrons) with only a few particles getting
slightly deflected, as the positive charge was very spread
out.
Results of the Gold Foil Experiment
• Most particles passed straight through the sheet.
• This means the atom is mostly empty space.
• Some particles were slightly deflected.
• This means the atom has a tiny but strong positively
charged region.
• Rarely, some particles were deflected backwards.
• This meant that most of the atom is empty space with the
positive charge being concentrated together.
The Nuclear Model
• In the nuclear model of the atom, there is a tiny positively charged nucleus at the centre, where most of the
mass is concentrated.
• A ‘cloud’ of negatively charged electrons surrounds this nucleus (meaning most of the atom is empty space).
The Bohr Model:
• Scientists realised that electrons in a ‘cloud’ around the nucleus would
quickly spiral down into the nucleus, causing the atom to collapse.
• Niels Bohr proposed a new model of an atom where electrons exist in
shells or orbits of fixed energy.
• When electrons move between shells, electromagnetic radiation (with
fixed energy or frequency) is either emitted or absorbed.
• The Bohr Model fitted experimental observations of radiation emitted
and absorbed by atoms.
The Current Model:
• Scientists later discovered that not all electrons in a shell have the
same energy.
• The model was refined to include sub-shells.
, Atomic Structure
Sub-Atomic Particle Relative Mass Relative Charge
Proton 1 1+
Neutron 1 0
Electron 1/1836 1-
Isotopes
Isotopes: Atoms with the same number of protons but a different number of neutrons. This means they are atoms of
the same element with the same atomic number but a different mass number.
Relative Atomic Mass
Relative Atomic Mass (Ar): The average mass of an atom of an element (taking into account all of its isotopes)
relative to 1/12 of the mass of a 12C atom.
Ar = Σ (Isotopic Abundance x Isotopic Mass Number)
Σ Isotopic Abundance
Example 1: Calculate the RAM of chromium given the data below:
Ar (Cr) = (4.3% x 50) + (83.8% x 52) + (9.5% x 53) + (2.4% x 54)
= 52.1 (1dp)
Example 2: The relative atomic mass of gallium is 69.72. It consists of two isotopes; 69Ga and 71Ga. Find the
percentage composition by mass of these two isotopes in gallium.
3. 69x + 71y = 6972 4. x + y = 100
– 69x + 69y = 6900 x + 36 = 100
2y = 72 x = 74
y = 36
69 71
Ga = 74% Ga = 36%
The benefits of buying summaries with Stuvia:
Guaranteed quality through customer reviews
Stuvia customers have reviewed more than 700,000 summaries. This how you know that you are buying the best documents.
Quick and easy check-out
You can quickly pay through credit card for the summaries. There is no membership needed.
Focus on what matters
Your fellow students write the study notes themselves, which is why the documents are always reliable and up-to-date. This ensures you quickly get to the core!
Frequently asked questions
What do I get when I buy this document?
You get a PDF, available immediately after your purchase. The purchased document is accessible anytime, anywhere and indefinitely through your profile.
Satisfaction guarantee: how does it work?
Our satisfaction guarantee ensures that you always find a study document that suits you well. You fill out a form, and our customer service team takes care of the rest.
Who am I buying these notes from?
Stuvia is a marketplace, so you are not buying this document from us, but from seller JunaidAli025. Stuvia facilitates payment to the seller.
Will I be stuck with a subscription?
No, you only buy these notes for £2.99. You're not tied to anything after your purchase.
