In chemistry and physics, an atom is the smallest particle still characterizing a chemical element.
An atom consists of a dense nucleus of positively-charged protons and electrically-neutral neutrons, surrounded by a much larger electron cloud consisting of negatively-charged electrons. An atom is electrically neutral if it has the same number of protons as electrons. The number of protons in an atom defines the chemical element to which it belongs, while the number of neutrons determines the isotope of the element.
The earliest references to the concept of atoms date back to ancient India in the 6th century BCE. The references to atoms in the West emerged a century later from Leucippus, whose student, Democritus, systemized his views. In around 450 BCE, Democritus coined the term atomos, which meant â€œuncuttable.â€
Though both the Indian and Greek concepts of the atom were based purely on philosophy, modern science has retained the name coined by Democritus.
In 1803, John Dalton used the concept of atoms to explain why elements always reacted in simple proportions, and why certain gases dissolved better in water than others. He proposed that each element consists of atoms of a single, unique type, and that these atoms could join to each other, to form chemical compounds.
In 1827 a British botanist Robert Brown used a microscope to look at dust grains floating in water. He called their erratic motion â€œBrownian motion.â€
Albert Einstein would later demonstrate that this motion was due to the water molecules bombarding the grains.
In 1897, JJ Thomson, through his work on cathode rays, discovered the electron and its subatomic nature, which destroyed the concept of atoms as being indivisible units. Later, Thomson also discovered the existence of isotopes through his work on ionized gases.
Thomson believed that the electrons were distributed evenly throughout the atom, balanced by the presence of a uniform sea of positive charge. However, in 1909, the gold foil experiment was interpreted by Ernest Rutherford as suggesting that the positive charge of an atom and most of its mass was concentrated in a nucleus at the center of the atom (Rutherford model), with the electrons orbiting it like planets around a sun. In 1913, Niels Bohr added quantum mechanics into this model, which now stated that the electrons were locked or confined into clearly defined orbits, and could jump between these, but could not freely spiral inward or outward in intermediate states.