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Timeline of the Atomic Theory
Transcript of Timeline of the Atomic Theory
The purpose of this timeline is to display how the atomic theory developed over time, as scientists and philosophers added to our knowledge of the atom through deductions made upon completion of various experiments.
Even in the present day, scientists continue to add to our knowledge of the atom.
Many scientists have already contributed to today’s knowledge of atoms and the atomic theory.
The atomic theory suggests that all matter is made up of atoms.
This idea was first introduced around 400 BC by a Greek philosopher named Democritus, but was largely ignored until the early 1800’s when John Dalton proposed his own atomic theory.
Our knowledge of the atom and the atomic theory has vastly grown since then, thanks to the work of a large number of scientists.
Around 400 BC (5th Century BC) a Greek philosopher named Democritus first proposed that all matter is composed of miniscule, indivisible particles which are in constant motion.
He named these particles atoms, which means indivisible or uncuttable in Greek.
He also proposed that these atoms cannot be changed, but are instead able to combine in different ways in order to form everything that we can see.
Due to the rejection of one of the most influential philosophers in Greece, Democritus’ theory was ignored for almost 2,000 years.
In 1750, a Croatian-born scientist named Rudjer Boscovich suggested that Democritus’ idea of atoms being indivisible may have been incorrect.
Instead, Boscovich believed that atoms contained even smaller parts, when then held even smaller parts again and so on until eventually you would have the fundamental building blocks of matter that held no size at all.
In the early 1800’s John Dalton (an English chemist) proposed an atomic theory.
This theory suggested that;
All matter consists of tiny particles called atoms – tiny, solid spheres in various stages of motion,
that atoms are indestructible and unable to be changed,
that elements are defined by the weight of their atoms (therefore all atoms of the same element have identical weights),
that during chemical reactions atoms combine in precise whole-number ratios,
and that when elements react, their atoms may combine in multiple whole-number ratios.
Over time there have been multiple changes and exceptions to Dalton’s theory.
Around the 1850’s evidence from the discovery of electrically charged particles and radioactivity led to the idea that atoms may not be solid and indestructible after all.
In the 1830’s Michael Faraday (a British physicist) made a significant discovery which eventually led to the idea that atoms may have an electrical component.
This discovery was made upon observing an experiment of his in which he placed two opposite electrodes in a solution of water and a dissolved compound.
One of the elements that composed the dissolved compound gathered on one of the electrodes and the remaining element gathered on the opposite electrode.
From this Faraday deduced that electrical forces were responsible for atoms combining in compounds.
In 1897, an English scientist named Joseph John Thomson (or J. J. Thomson) discovered electrons during an experiment with cathode rays, when he determined that the rays consisted of negatively charged particles of a light weight which must be a part of the atom.
He then proposed the “plum-pudding” model of an atom.
In this model the negatively-charged electrons represented the raisins which are dotted throughout a pudding and the dough represented the positively charged sphere of the rest of the atom.
In 1911, Ernest Rutherford (a previous student of J.J. Thompson) proved the plum-pudding model to be incorrect.
He achieved this by firing particles at solid substances such as gold foil and then recording the location in which these particles struck as they passed through the foil.
Most of the particles passed through the foil as had been predicted, but a small percentage of the particles ricocheted straight back or off on an angle.
From this Rutherford concluded that the atom must consist of mostly empty space with a tiny, dense nucleus which contained positively-charged protons in the centre.
He also concluded that most of the mass of an atom is contained within the nucleus, which has the negatively-charged electrons orbiting it.
Finally, Rutherford proposed that these electrons would eventually lose their energy and spiral into the nucleus, as a result of radiating electromagnetic energy during their orbit.
In 1913, a Danish physicist named Niels Bohr suggested that electrons were only able to travel in a certain set of orbits surrounding the nucleus.
The Bohr model of the atom suggested that;
electrons are only able to occupy specific, stable orbits surrounding the nucleus,
that each orbit has an energy level associated with it,
and that energy is absorbed when an electron moves to a higher orbit (and therefore a higher energy level) from a lower orbit and that energy is emitted when an electron moves from a higher orbit to a lower orbit.
Although Bohr’s model of the atom showed why electrons did not spiral towards the nucleus, it did not explain many other characteristics of atoms.
In 1926, Erwin Schrödinger (an Austrian physicist) introduced a model of atom that contained sub-energy levels.
This was a result of his work with applying mathematical equations to calculate the likelihood of finding an electron in a specific location.
In 1932, James Chadwick discovered that within the nucleus of an atom were particles which he named neutrons.
He found that they had no electric charge and a mass relatively the same as a proton.
By 1964, research showed what the particles that compose protons, electrons and neutrons may be like.
It has been found that protons, electrons and neutrons may be subdivided into over 50 unstable particles.
One of these particles was named quarks by American physicist Murray Gell-Mann.
The atomic theory has come a long way since its beginnings with Democritus in Ancient Greece.
After John Dalton’s atomic theory was proposed in the early 1800’s, many new developments in science have led to the development of the atomic theory and our understanding of the atom.
These are developments such as the discovery of particles with an electric charge,
atoms being affected by electrical forces when combining in compounds,
the proposition of J. J. Thomson’s plum-pudding atomic model,
Rutherford’s revelation that atoms must consist of mostly empty space with electrons orbiting a dense nucleus which contains positively-charged protons,
Bohr’s atomic model suggesting that each orbit surrounding the nucleus has an energy level associated with it that electrons can move to by gaining or losing energy,
the discovery of neutrons by Chadwick in 1932,
and the most recent proposition that protons, electrons and neutrons are composed of over 50 particles, one of which is the quark.
I predict that there will be many new developments in our knowledge of the atom in the future.
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Atomic Theory n.d., Uoregon, accessed 19 February 2013, <http://abyss.uoregon.edu/~js/ast123/lectures/lec04.html>.
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Saffman, M 2013, 'Atom' , World Book Student, World Book, Chicago, viewed 18 February 2013,
The "plum-pudding" atom model.
J. J. Thomson
Rutherford's Atom Model
Bohr's Atomic Model
Chadwick's Atomic Model