Atom Theory

Invented by : John Dalton

Invented in year : 1803

The Atom is a basic unit of matter consisting of a dense, central nucleus surrounded by a cloud of negatively charged electrons. Simply put - it is the smallest component of an element having the chemical properties of the element. The Atomic nucleus contains a mix of positively charged protons and electrically neutral neutrons (except in the case of hydrogen-1, which is the only stable nuclide with no neutron). The electrons of an Atom are bound to the nucleus by the electromagnetic force. Likewise, a group of Atoms can remain bound to each other, forming a molecule. An Atom containing an equal number of protons and electrons is electrically neutral, otherwise it has a positive or negative charge and is an ion. An Atom is classified according to the number of protons and neutrons in its nucleus: the number of protons determines the chemical element, and the number of neutrons determine the isotope of the element. Atoms can only be observed individually using special instruments such as the scanning tunnelling microscope.

History of the Invention

The concept of an Atom was first proposed by early Indian and Greek philosophers. In the 17th and 18th centuries, chemists provided a physical basis for this idea by showing that certain substances could not be further broken down by chemical methods. The earliest references to the concept of Atoms date back to ancient India in the 6th century BCE, appearing first in Jainism. The Nyaya and Vaisheshika schools developed elaborate theories of how Atoms combined into more complex objects. In the West, the references to Atoms emerged a century later from Leucippus, whose student, Democritus, systematized his views.

John Dalton, was a British chemist and physicist who came up with the Theory Of Atomistics. Dalton's research studies on the properties of atmosphere and gases were the basis for his Atomic Theory. His theory gave evidence on the smallest particles in the universe. He first announced his theory at a Royal Institution in a lecture in 1803. According to his Theory of Atoms, there are different types of Atoms, these are called elements. Hydrogen, oxygen and nitrogen are examples of elements. Each of these elements is different from one another only by its weight. Atoms of the same element all have the same mass. Atoms of different elements are different in mass. Dalton used symbols to represent the elements. He imagined Atoms to be tiny spheres, thus to draw Atoms would be to draw circles. He was of the opinion that Atoms of different elements had different mass; as mass unit he took the mass of one Atom of hydrogen. He laid the foundation of modern Atomistics and he described world's microstructure explaining most of occurrences known those days

Development in the Invention of Atom

Dalton mistakenly believed that the simplest compound between any two elements is always one Atom of each (so he thought water was HO, not H2O). This, in addition to the crudity of his equipment, resulted in his table being highly flawed. For instance, he believed oxygen Atoms were 5.5 times heavier than hydrogen Atoms, because in water he measured 5.5 grams of oxygen for every 1 gram of hydrogen and believed the formula for water was HO (an oxygen Atom is actually 16 times heavier than a hydrogen Atom).

In 1811, the flaw in Dalton's theory was corrected by Amedeo Avogadro. Avogadro had proposed that equal volumes of any two gases, at equal temperature and pressure, contain equal numbers of molecules (in other words, the mass of a gas's particles does not affect its volume). Avogadro was able to offer more accurate estimates of the Atomic mass of oxygen and various other elements, and firmly established the distinction between molecules and Atoms.

Until 1897, Atoms were thought to be the smallest possible division of matter. But J.J. Thomson discovered Electron through his work on cathode rays. To explain the overall neutral charge of the Atom, he proposed that the corpuscles were distributed in a uniform sea of positive charge; this was the Plum Pudding Model as the electrons were embedded in the positive charge like plums in a Plum Pudding.

In 1909, Thomson's Plum Pudding Model was disproved by one of his former students, Ernest Rutherford, who discovered that most of the mass and positive charge of an Atom is concentrated in a very small fraction of its volume, which he assumed to be at the very centre. In 1911, Rutherford proposed a revolutionary view of the Atom. He suggested that the Atom consisted of a small, dense core of positively charged particles in the centre (or nucleus) of the Atom, surrounded by a swirling ring of electrons.

In 1913, Niels Bohr applied the Quantum Theory developed by Max Planck and Albert Einstein, which further added to the theory of Atomic structure. The development of Quantum Mechanics during the 1920s resulted in a satisfactory explanation for all phenomena related to the role of electrons in Atoms and all aspects of their associated spectra.

In 1932, James Chadwick discovered a third type of subatomic particle, which he named the Neutron. Neutrons helped stabilize the protons in the Atom's nucleus. With the discovery of the neutron in 1932 the modern picture of the Atom was complete.

Role of the Discovery of Atom in the Improvement of Human Life

• Dalton's works there were enough proofs of Atoms' subsistence that it became a public admitted scientific conception.

• Study of Atom gave a better understanding of Physics and Chemistry.

• Further study of Atoms led to the discovery of electrons, protons, neutrons, photons and others. These discoveries further helped in developing technologies which revolutionised communications, electronic instrumentation etc.