Over the span of hundreds of years science has advanced tremendously improving our understanding of what makes up the world we live in. The atom is one of sciences important findings and has had a changing history of new discoveries, always altering the way we see things.
In the early 1800s a man called John Dalton made an atomic theory with predictions about atoms. He stated that atoms are tiny particles that make up elements and that they are indivisible. We now know that there are in fact smaller particles inside the atom, but at this stage it was assumed that the atom was the smallest. He also said that all atoms of a given element are the same and atoms of one element are different from those in every other element. This meant that every different element had its own unique type of atoms. (Dalton also developed the first table of atomic masses)
As the years went on a scientist called JJ Thomson discovered the electron by using cathode ray tubes. This discovery proved that there was an even smaller particle inside the atom itself, disproving one of Dalton’s theories and improving our understanding about elements and atoms. JJ Thomson created the famous plum pudding atom model which shows an atom made up of negative electrons moving about in a ‘sea’ of positive charge, thus making the atom neutral with no overall charge. His discovery of the electron advanced atomic theories massively.
In 1909 the scientist, Ernest Rutherford conducted an experiment in which he fired alpha particles (positive) at a thin piece of gold film in a vacuum; from this test he proposed the nuclear atom. He found out that a few particles were actually deflected meaning there is a positive charge in the nucleus. Rutherford’s findings disproved the plum pudding model and the nuclear atom was accepted.
In 1918 Rutherford discovered the proton after linking and comparing his ideas with other scientists, this meant that there was yet another particle smaller than the atom. Eventually in 1932 James Chadwick discovered the uncharged particles called neutrons, subsequent to studies of a new type of radiation emitted after bombarding light elements with high energy and particles. The neutron doesn’t really affect the properties of the atom, it just adds to the mass.
So, the atom was thought of as being indivisible when studies began but as time went on and science progressed, 3 new particles: electrons, protons and neutrons were discovered. Different theories had been disproved along the way but some are still believed to this day. Further developments may sprout another discovery in the future, altering our model of the atom once again. It is thought that protons and neutrons could be made up of smaller particles named quarks but this is just a theory!