Atomic Structure And Isotopes Flashcards
Who developed the first idea of the atom?
Democritus
What did Democritus suggest?
Can divide a sample of matter only so many times, eventually you would end up with a particle that cannot be split any further
What were John Dalton’s 3 predictions about atomic theory?
1) Atoms are tiny particles that make up elements
2) Atoms cannot be divided
3) Atoms of an element are different to those of every other element
What did John Dalton do?
Used symbols to represent atoms of different elements and developed the first table of atomic masses
What is a positive about John Dalton?
Most of his predictions still hold true today
What did Thompson do?
Carried out a series of experiments using cathode rays
What did Thompson discover?
The electron
How did the cathode rays experiments work?
Cathode rays are emitted from the negative electrode inside cathode ray tubes
What specifically did the cathode rays experiment show?
Cathode rays were the stream of particles (electrons) and each particle had the following properties:
1) It had a negative charge
2) It could be deflected by both a magnet and an electric field
3) It had a very small mass
What did Thompson conclude?
Cathode rays were actually electrons so must have come from within the atoms of the electrodes themselves
What did Thompson propose?
An atom should be thought of as being made up of negative electrons moving around in a sea of positive charge - overall negative charge is the same as the overall positive charge meaning that the atom is neutral with no overall charge
What did Thompson create?
The plum pudding model/atom
Who brought about the downfall of the plum pudding model?
Rutherford in 1909
What did Rutherford do?
Carried out an experiment in which they directed alpha particles towards a sheet of very thin gold foil and measured any deflection of the particles - calculated plum pudding model would hardly deflect any alpha particles at all - they found most were not deflected, only a small % were deflected through large angles with a very few particles being deflected back towards the source
What did Rutherford conclude?
An atom’s positive charge must be concentrated into a volume much smaller than the size of an atom
What did Rutherford propose?
In 1911, he proposed the nuclear atom:
1) Positive charge and most of its mass is concentrated in the nucleus at the centre
2) Negative electrons orbit this nucleus
3) Most of the atom’s volume would be in the space between the tiny nucleus and the orbiting electrons
4) Overall positive and negative charges must balance
What did Bohr do?
In 1913, altered Rutherford’s model to allow electrons to follow only certain paths otherwise electrons would spiral into the nucleus and this was called the planetary atom in which the electrons orbit a central nuclear sun in shells
What did Mosely discover in 1913?
A link between x-ray frequencies and elements’’ atomic numbers but he could not explain this
What did Rutherford discover in 1918?
The proton and we now know that the atomic number tells us the number of protons in an element atom
Outline the order of the discovery of atom
Democritus Dalton Thompson Rutherford Bohr Mosely Rutherford
What is relative isotopic mass?
The mass of an atom on an isotope compared with 1/12 of the mass of an atom of carbon-12
What is relative atomic mass?
Weighted mean mass of an atom of an element compared with 1/12 of the mass of an atom of carbon-12
