Atomic Structure And Isotopes

Democritus

Can divide a sample of matter only so many times, eventually you would end up with a particle that cannot be split any further

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

Used symbols to represent atoms of different elements and developed the first table of atomic masses

Most of his predictions still hold true today

Carried out a series of experiments using cathode rays

The electron

Cathode rays are emitted from the negative electrode inside cathode ray tubes

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

Cathode rays were actually electrons so must have come from within the atoms of the electrodes themselves

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

The plum pudding model/atom

Rutherford in 1909

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

An atom’s positive charge must be concentrated into a volume much smaller than the size of an atom

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

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

A link between x-ray frequencies and elements’’ atomic numbers but he could not explain this

The proton and we now know that the atomic number tells us the number of protons in an element atom

Democritus Dalton Thompson Rutherford Bohr Mosely Rutherford

The mass of an atom on an isotope compared with 1/12 of the mass of an atom of carbon-12

Weighted mean mass of an atom of an element compared with 1/12 of the mass of an atom of carbon-12