19,879
edits
Changes
Atom
,no edit summary
===History of Atoms===
====Discovery of Atoms====
: The existence and structure of [[atom]]s was not always known.
: An ancient Greek philosopher called [[Democratus]] first proposed that matter was made of [[atom]]s. He reasoned that if you keep cutting something in half eventually you will reach part of it which cannot be cut into smaller piece. The word for 'uncutable' was [[atom]].
=====Dalton Model of the Atom=====
: It wasn't until the early 1800s that there was an [[Scientific Evidene|evidence]] based [[Scientific Theory|theory]] of the existence of an [[atom]]. It was proposed by [[John Dalton]] who suggested [[atom]]s were small [[sphere|spherical]] [[object]]s.
: [[John Dalton]] had his [[Scientific Theory|theory]] backed up by the discoveries of [[Robert Brown]] who discovered [[Brownian Motion]] showing that small invisible [[particle]]s were reponsible for the apparent random motion of small visible [[particle]]s such as [[pollen]].
=====Plum Pudding Model of the Atom=====
: In 1897 [[J.J. Thompson]] discovered that there was a [[particle]] smaller than an [[atom]] which he named the [[electron]] after another [[scientist]] had [[hypothesis|hypothesised]] their existence. The [[electron]] was found to be around 1860 times less [[mass]]ive than a [[Hydrogen]] [[atom]]. It was later realised that these [[electron]]s were responsible for [[Electrical Current|electrical current]] in [[metal]]s.
: This led to the [[Plum Pudding Model]] of the [[atom]] in which [[atom]]s were believed to be a solid ball of [[positive]] charge with [[electron]]s stuck inside to give an overall [[neutral]] charge to the [[atom]].
=====Nuclear Model of the Atom=====
: In 1909 [[Ernest Rutherford]] set two students to work on an experiment to probe the structure of the [[atom]] in the hope to determine if the [[Plum Pudding Model]] was correct.
: [[Ernest Rutherford|Rutherford's]] students [[Ernest Marsden]] and [[Hans Geiger]] fired [[Alpha Particle|alpha particle]]s, which are positively charged, at a very thin sheet of [[Gold]] foil to observe how the [[Alpha Particle|alpha particles]] changed direction as they went through the foil.
: They discovered that most of the [[Alpha Particle|alpha particle]]s went through in a straight line. A significant number were [[deflect]]ed and a very small number bounced off the [[Gold]] back towards the [[Alpha Particle|alpha]] source.
: If the [[Plum Pudding Model]] were correct then nearly all of the [[Alpha Particle|alpha particles]] should have passed straight through, unaffected, since the [[Alpha Particle|alpha particle]] is positively charged wile [[atom]]s should have an even spread of [[charge]]d [[particle]]s all the way through them. There should have been no [[Electrostatic Force|electrostatic force]] to change their direction.
: This showed that the [[atom]] must be mostly empty space, that most of the [[mass]] of an [[atom]] is concentrated in the centre and that the centre is positively charged. This gave rise to the [[Nuclear Model]] in which a very small positively charged [[Atomic Nucleus|nucleus]] is surrounded by [[orbit]]ing [[electron]]s.
=====The Bohr Model of the Atom=====