A brief summary of the developing ideas of the atom – Chemistry AS level

Robert Boyle (1661)

He proposed that there were some substances that couldn’t be made simpler. We now know them as the chemical elements.


John Dalton (1803)

Dalton suggested that elements were composed of indivisible (inseparable) atoms. All the atoms of a particular element had the same mass but atoms of different elements had different masses. Atoms couldn’t be broken down.


Henri Becquerel (1896)

Becquerel discovered radioactivity – this showed that particles could come from inside the atom and therefore the atom wasn’t indivisible as Dalton had suggested.


JJ Thomson (1897)

He discovered the electron (the first sub-atomic particle to be discovered). He showed that electrons were negatively charged and electrons from all elements were the same. The inside of the atom having some source of negative charge meant that there had to be a positive charge too and he suggested that electrons were located within the atom in circular arrays.


Ernest Rutherford (1911)

He found that most of the mass and all the positive charge of the atom were in the nucleus.


Niels Bohr (1922)

Bohr explained what happens inside an atom and developed a picture of atomic structure. The Bohr model shows the atom as a small, positively charged nucleus surrounded by orbiting electrons. Bohr discovered that electrons travel in separate orbits around the nucleus and that the number of electrons in the outer orbit determines the properties of an element.


Erwin Schrodinger (1926)

He generated the quantum mechanical model (this predicts the odds of the location of the electron).


James Chadwick (1932)

He proved the existence of neutrons in the atom and that the atomic number is determined by the number of protons in an atom.


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