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The development of the modern concept of the atom, the size and nature of an atom and the ideas on sub-atomic particles

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The development of the modern concept of the atom, the size and nature of an atom and the ideas on sub-atomic particles


Throughout the ages the study of what we call today chemistry has evolved into a highly developed point of study. One distinct element of chemistry is atomic theory. Throughout the ages atomictheory has been developed and extended by many different men who were all well-known chemists and physicists in their day. They developed the study of atoms from pure conjecture into known facts. The five atomic theories of the past two centuries represent the sudden advancement of science in modern times. Beginning with a basic theory on the behavior of atoms to the current model, some changes have been made, and some ideas are still the same.

Ancient Greek philosophers believed that everything was made up of invisible particles called atmos. Since then the theory of atoms did not progress until 1803. John Dalton was the first scientist to compose a theory of matter base on atoms. Dalton’s atomic theory is based on four concepts. He stated: “1. All elements are composed of atoms, which are indivisible and indestructible particles. 2. All atoms of the same element are exactly alike; in particular, they have the same mass. 3. Atoms of different elements are different; in particular, they have different masses. 4. Compounds are formed by the joining of atoms of two or more elements.” 1 All of Dalton’s ideas account for the laws of definite and multiple proportions and the law of conservation of mass. Some of Dalton’s points are still thought to be true, but over time this original theory has been modified. The first of these modifications came in 1897 when J.J. Thomson discovered the electron. Based on the work of William Crookes and his “Crookes tube” (Cathode-ray tube), Thomson discovered a negative charged particle was the cause of the light produced by the cathode-ray tube. He also discovered that these particles are present in all elements. These cathode-ray particles are now known as electrons. Soon after the discovery of electrons the proton was discovered. This led Thomson to conclude that there were an equal number of both particles present in the atom.

Twelve years later Lord Ernest Rutherford was experimenting with alpha particles. He shot a stream of them at a piece of gold foil surrounded by zinc-sulfide. When an alpha particle strikes ZnS it produces a flash of light. The particles mostly stayed in a constant stream through the foil, but a few were deflected. This led Rutherford to believe that there must be a small, dense cluster of protons in the middle of the atoms to deflect the small number of particles. Neils Bohr was the next physicist to advance the atomic theory. He explained what Rutherford could not about how the electron could stay in orbit around the nucleus. When the electron has little energy it is closer to the nucleus, when it absorbs more energy it travels farther from the nucleus.  There is a definite number of electrons that can be in the same orbit. When the orbits closest to the nucleus are filled the atom is at a ground state. When the electrons become charged they move into a higher orbit and are then at an exited state. When the electrons move into a closer orbit they release a photon of radiation. This model for the atom became insufficient. The charged-cloud model was born. It was the research of many scientists that led to this. Instead of showing the position and orbit of the electron, it shows the most probable locations.

The orbits from the Bohr model are divided into sub-orbits, but there is no way of showing the infinite amount of sub-orbits or which orbit the electron would be in so the most dense place in the cloud is the most probable location of an electron. The Atomic theory has gone through many changes since Dalton’s time, but two of his ideas are still true. What we know about atoms today could be proved entirely wrong tomorrow just like Dalton. The atomic theory is still just a theory and some day it might be proved or disproved.



Democritus of Abdera, a Greek philosopher around 460 BC – ca. 370 BC was the first scientist to propose that everything is made up of atoms.  He came to this conclusion when he asked himself a question.  If you break a piece of matter in half, and then break it in half again, how many breaks will you have to make before you can break it no further? Democritus thought that it ended at some point, a smallest possible bit of matter. He called these basic matter particles, atoms.

He believed that these atoms were physically, but not geometrically, indivisible, and that empty space lies within these atoms. According to Democritus’ theory, atoms are indestructible; have always been, and always will be. His theory further went on to expound that in motion; there are an infinite number of atoms and kinds of atoms, which differ in shape, and size. image01.jpg”The more any indivisible exceeds, the heavier it is as this would have an impact on the atom’s mass. He was instrumental in proposing the earliest views on the shapes and connectivity of atoms. He articulated that the solidness of the material corresponded to the shape of the atoms involved. In explaining this notion, he used several analogies that were linked to the human senses. These analogies include iron atoms are which is solid and strong with hooks that lock them into a solid; water atoms he explained are smooth and slippery; salt atoms, because of their taste, are sharp and pointed; and air atoms are light and whirling. By using these analogies he gave an image of an atom that distinguished them from each other by their shape, size, and the arrangement of their parts.

John Dalton


John Dalton was born at Eaglesfield, near Cockermouth in Cumberland, England, on September 6, 1766, the son of a devout Quaker family. He was such a brilliant youth that he became a teacher when barely 12 years old. Dalton's theory was based on three important propositions. The first was that all matter is composed of extremely small, indivisible, and indestructible particles called atoms. The second was that the atoms of one element are all exactly alike in every respect including weight but are different from the atoms of every other element. The last was that when elements combine to form compounds their atoms combine in simple numerical proportions such as one to one, two to one, and four to three.


The ideas of atoms had been suggested centuries earlier by Greek philosopher Democritus, so the concept was not entirely unfamiliar to Dalton's contemporaries. But Dalton's complete formulation of a consistent theory was a breakthrough. One of the most important features of the theory was its proposal that atoms differed from each other by weight. This was something measurable, making Dalton's the first quantitative atomic theory ever advanced. Chemists had long puzzled over why a substance such as copper carbonate, however prepared, always contained the same proportions by weight of copper (five parts), oxygen (four parts), and carbon (one part). Dalton's theory that elements combine atom by atom in simple numerical proportions explained it, because if all atoms of a particular element have the same weight, they must have definite combining weights.Dalton tried to work out the relative weights of different atoms from the proportions by weight of the elements in certain compounds, so becoming the first to prepare a table of atomic weights. He also drew up a system of notations to represent elements, discarding the obscure drawings that had been handed down from the alchemists of ancient times. He created clear sysmbols to stand for the atoms of different elements, and used them in drawings that showed what took place during chemical reactions. For example, molecules were shown as groups of atom symbols linked together.

 J. J.Thomson


Sir Joseph John "J. J." Thomson (18 December 1856 – 30 August 1940) was a British physicist and Novel laureate. He is credited for the discovery of the electron and of isotopes, and the invention of the mass spectrum. Thomson was awarded the Novel Prize in Physics 1906 for the discovery of the electron and for his work on the conduction of electricity in gases.

Several scientists, such as Willam Prout and Norman Lockyer had suggested that atoms were built up from a more fundamental unit, but they envisaged this unit to be the size of the smallest atom, hydrogen. Thomson, in 1897, was the first to suggest that the fundamental unit was over 1000 times smaller than an atom, suggesting the sub-atomic particles now known as electrons. Thomson discovered this through his explorations on the properties of cathode rays. Thomson made his suggestion on 30 April 1897 following his discovery that Lenard rays could travel much further through air than expected for an atomic-sized particle. He estimated the mass of cathode rays by measuring the heat generated when the rays hit a thermal junction and comparing this with the magnetic deflection of the rays. His experiments suggested not only that cathode rays were over 1000 times lighter than the hydrogen atom, but also that their mass was the same whatever type of atom they came from. He concluded that the rays were composed of very light, negatively charged particles which were a universal building block of atoms. He called the particles "corpuscles", but later scientists preferred the name electron which had been suggested by George Johnstone in 1894, prior to Thomson's actual discovery. In April 1897 Thomson had only early indications that the cathode rays could be deflected electrically. A month after Thomson's announcement of the corpuscle he found that he could deflect the rays reliably by electric fields if he evacuated the discharge tubes to very low pressures. By comparing the deflection of a beam of cathode rays by electric and magnetic fields he was then able to get more robust measurements of the mass to charge ratio that confirmed his previous estimates. This became the classic means of measuring the charge and mass of the electron. Thomson believed that the corpuscles emerged from the atoms of the trace gas inside his Cathode ray tube. He thus concluded that atoms were divisible, and that the corpuscles were their building blocks. 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—the electrons were embedded in the positive charge like plums in a plum pudding (although in Thomson's model they were not stationary, but orbiting rapidly).


Eugene Goldstein


Eugene Goldstein (1850-1930) a German psychiatrist observed a cathode-ray tube and found rays travelling in the direction opposite to that of cathode rays. He concluded that these were composed of positive particles. The positive particle was later called as protons. In the year 1886, Goldstein who was the scientist that had given cathode rays their name, did some experimenting with a perforated cathode in an evacuated tube. When cathode rays were given off in one direction toward the anode, other rays found their way through the holes in the cathode and sped off in the opposite direction.

Since these other rays travelled in the direction opposite to the negatively charged cathode rays, it seemed that they must be composed of positively charged particles. This hypothesis was confirmed when the manner in which they were deflected in a magnetic field was studied.

Ernest Rutherford


Ernest Rutherford performed a series of experiments with radioactive alpha particles  Rutherford fired tiny alpha particles at solid objects such as gold foil.  He found that while most of the alpha particles passed right through the gold foil, a small number of alpha particles passed through at an angle (as if they had bumped up against something) and some bounced straight back like a tennis ball hitting a wall.  Rutherford's experiments suggested that gold foil, and matter in general, had holes in it.  These holes allowed most of the alpha particles to pass directly through, while a small number ricocheted off or bounced straight back because they hit a solid object.

In 1911, Rutherford proposed a revolutionary view of the atom. His atomic theory 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 and the rest of the atom was mostly space. The nucleus was so dense that the alpha particles would bounce off of it, but the electrons were so tiny, and spread out at such great distances, that the alpha particles would pass right through this area of the atom. Approximately 1 in 8000 was deflected leading him to his theory that most of the atom was made up of 'empty space'. Rutherford's atom resembled a tiny solar system with the positively charged nucleus always at the centre and the electrons revolving around the nucleus.


Interpreting Rutherford's Gold Foil Experiment

The positively charged particles in the nucleus of the atom were called protons.  Protons carry an equal, but opposite, charge to electrons, but protons are much larger and heavier than electrons.  


The Rutherford atomic model has been alternatively called the nuclear atom, or the planetary model of the atom

Sir James Chadwick
James Chadwick(above) created one of the most descriptive models of an atom.

Biography of James Chadwick

James Chadwick was born in Cheshire, England, on October 20, 1891. He was the son of John Joseph Chadwick and Anne Mary Knowles. He attended Manchester High School prior to entering Manchester University in 1908. He later graduated from the Honors School of Physics in 1911. He then spent two years working in a Physical Laboratory in Manchester, where he worked on various radioactivity problems also gaining is Masters Degree in 1913. That same year he was awarded the 1851 Exhibition Scholarship to proceed to Berlin to continue his work. While in Berlin he worked with Hans Geiger at the Technical University of Berlin.


Chadwick’s Discovery

In 1932 James Chadwick made one of the greatest discovers for nuclear science.James discovered the neutron in the atom; he was able to do this due to the fact that it has no electrical charge. This turned out to be an extremely useful tool for bombarding atomic nuclei.The discovery of the neutron made it possible for physicist to create elements that were heavier than uranium in the lab, and also lead to other great discoveries.Due to this great discovery James Chadwick was awarded with the Hughes Medal of the Royal Society in 1932, and in 1935 he was awarded with the Nobel Prize for physics.
Chadwick’s Experiment:

Chadwick took smashed beryllium particles.He allowed the radiation that was released to hit wax paper.When the beryllium atoms and the hydrogen atoms in the wax paper hit together they released neutral particlesresulting in the discovery of the neutron.


Neils Bohr

In 1913 Neils Bohr proposed his model of atom which superseded Rutherford's atomic model. According to classical theories this atom should collapse. It also failed to explain the discrete spectral lines of elements. Bohr's model of atom could successfully explain the stability of atom by introducing Quantization. It could also explain the Hydrogen spectra. Bohr obtained the value of radius of hydrogen atom and its energy, both of which agree well with experimental results. Bohr's atomic theory formed the basis for the old Quantum theory. Bohr's Postulates:

(a) The electron revolves in circular orbits around the nucleus which are restricted by the quantization of angular momentum that is they revolve in orbits where the angular momentum of electron is an integral multiple of h/2π, where h is Planck's constant.

  mvr = nh/2π

In these orbits of special radius electron does not radiate energy as expected from Maxwell's laws. These orbits are called stationary states. This is called as Bohr's quantization rule.


(b) The energy of the atom has a definite value in a stationary orbit. The electron can jump from one stationary orbit to another. If it jumps from an orbit of higher energy E2 to an orbit of lower energy E1, it emits a photon. The energy of the photon is E2-E1.The wavelength of the emitted radiation is given by the Einstein - Planck equation.

E2-E1= hν = hc/λ

The electron can also absorb energy from some source and jump from a lower energy level to a higher energy level. After that the electron emits the energy and comes back to the first orbit. image06.pngimage07.jpghydrogen spectrum

The figure above (left) shows the various ways of how an electron can reach ground level after being excited to the third energy level n=3.The total energy that the electron emits as photon is hv30=hv32+hv10=hv32+hv20=hv32+hv32+hv21+hv10.







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