Changes

: 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 ancient Greek word for 'uncuttable' was [[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 responsible for the apparent random motion of small visible [[particle]]s such as [[pollen]].

: 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 2000 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 him, in 1904, to propose the [[Plum Pudding Model]] of the [[atom]] in which [[atom]]s were believed to be a solid ball of [[positive]] [[Electrical Charge|charge]] with [[electron]]s stuck inside to give an overall [[neutral]] [[Electrical Charge|charge]] to 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 [[Positive Charge|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. This was known as [[Rutherford's Alpha Scattering Experiment]].

: 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 [[Positive Charge|positively charged]]. This gave [[Ernest Rutherford|Rutherford]] [[Scientific Evidence|evidence]] to develop a new model of the [[atom]] which he proposed in 1911 called the [[Nuclear Model]] in which a very small [[Positive Charge|positively charged]] [[Atomic Nucleus|nucleus]] is surrounded by [[orbit]]ing [[electron]]s.

: In 1913 [[Neils Bohr]] added to the [[Nuclear Model]] when he proposed that [[electron]]s could only [[orbit]] the [[Atomic Nucleus|nucleus]] in specific [[Electron Orbital|electron orbitals]] or 'shells' creating the [[Bohr Model]] of the [[atom]].

=====The Addition of the Neutron=====

: In 1932 [[James Chadwick]] discovered a [[Neutral (Charge)|neutral]] [[Subatomic Particle|subatomic particle]] which he named a [[Neutron]]. This [[neutron]] explained the existence of [[isotope]]s and completed out modern understanding of the [[atom]].