ATOMIC THEORY

ATOMIC THEORY

In chemistry and physics, atomic theory is a theory of the nature of matter, which states that matter is composed of discrete units called atoms, as opposed to the obsolete notion that matter could be divided into any arbitrarily small quantity. It began as a philosophical concept in ancient Greece (Democritus) and India and entered the scientific mainstream in the early 19th century when discoveries in the field of chemistry showed that matter did indeed behave as if it were made up of particles. The word "atom" (from the ancient Greek adjective atomos, 'indivisible') was applied to the basic particle that constituted a chemical element, because the chemists of the era believed that these were the fundamental particles of matter. However, around the turn of the 20th century, through various experiments with electromagnetism and radioactivity, physicists discovered that the so-called "indivisible atom" was actually a conglomerate of various subatomic particles (chiefly, electrons, protons and neutrons) which can exist separately from each other. In fact, in certain extreme environments such as neutron stars, extreme temperature and pressure prevents atoms from existing at all. Since atoms were found to be actually divisible, physicists later invented the term "elementary particles" to describe indivisible particles. The field of science which studies subatomic particles is particle physics, and it is in this field that physicists hope to discover the true fundamental nature of matter.

 

History of the Atomic Theory in Ancient Times

Chemistry, as we all know, is the science of matter. You should already be familiar with concepts like substances, elements, compounds, molecules, and other such lovely terms.  Ever wondered though, what are all these things made of?  Let's use Lego® as an example. I have a nice house made of Lego® pieces. Now, if I break down this house, I'll have walls and windows and a door. I can break down these further, and even further, until I get an individual, smallest piece of Lego® that still retains the properties of the house (in this case, colour and texture).

Introducing-Atomic Theory:

Atomic theory is a theory that attempts to answer that question. It states that all matter is composed out of extremely small particles called atoms. An
atom
is the smallest particle that retains the properties of an element (so a carbon atom would still have the properties of carbon, but if you break the atom down these properties will disappear).

This theory, although it sounds simple, is in reality extremely complex. It eliminated the notion that matter like water would never stop being water no matter how small a quantity of it you have. Although it's still only a theory, nowadays, it is widely accepted and supported by virtually everyone.

And like most theories, the atomic theory has great story and history behind it.

Meet Democritus: Like most modern science, the atomic theory started in Ancient Greece (which explains the stupid names of things). The guy who thought it all up was known as Democritus, a materialist philosopher, in the 5th century BCE . He thought very hard about matter and the universe, and came up with an idea. Democritus stated that all matter is made up of units that move around in a void. Democritus believed that these units are indivisible and unchangeable. In fact, he decided to call them atomos , which means uncuttable in Greek. Although these atomos are all made up of the same matter, their shape, size, and orientation in space explains the existence of different substances and materials.

While most of Democrtius's ideas aren't acceptable today (atoms are divisible into smaller units, and they are in cnostant change), the revolutionary thought that matter is composed of tiny particles moving in a void is accepted today as the basis for modern atomic theory.

Meet Kanada: The Greek weren't the only smart guys around. In India, during the 6 century BCE, was a guy named Kanada. He founded the Vaisheshika school of philosophical thought, which came up with its own atomic theory independently of Democritus. The idea was that matter was made of a finite number of paramāṇus, which are basically atoms, indivisible and eternal. Each type of atom has its own unique individuality. Atoms make up the 4 bhūtas- water, fire, earth and air, which make up all other matter. They had various theories about how these atoms combine, and believed the interactions are ultimately caused by the will of Ishvara, a Hindu god. Again, while some of these theories aren't generally accepted today, the general ideas are there for the tiny particles that make up matter.

What happened next?

For a long time, nothing. People had more important things to do than think about matter (mostly fight wars and try to survive the hard life of the Middle Ages). Although some alchemists existed, trying to turn stuff to gold and create an immortality potion, they weren't too concerned about atoms.

And so, for hundreds of years, nobody cared, and atomic theory wasn't explored. People either accepted the notion that matter can be divided into infinitely small amounts without a change in properties, or that there's something that makes up matter, but since nobody could really check it made no difference.

Until one day, in the early 1800s, an English guy name John Dalton changed everything.

 

Modern atomic theory

Earliest empirical evidence

Near the end of the 18th century, two laws about chemical reactions emerged without referring to the notion of an atomic theory. The first was the law of conservation of mass, formulated by Antoine Lavoisier in 1789, which states that the total mass in a chemical reaction remains constant (that is, the reactants have the same mass as the products). The second was the law of definite proportions. First proven by the French chemist Joseph Louis Proust in 1799, this law states that if a compound is broken down into its constituent elements, then the masses of the constituents will always have the same proportions, regardless of the quantity or source of the original substance.
John Dalton studied and expanded upon this previous work and developed the law of multiple proportions: if two elements came together form more than one compound, then the ratios of the masses of the second element which combine with a fixed mass of the first element will be ratios of small integers. For instance, Proust had studied tin oxides and found that their masses were either 88.1% tin and 11.9% oxygen or 78.7% tin and 21.3% oxygen (these were tin(II) oxide and tin dioxide respectively). Dalton noted from these percentages that 100g of tin will combine either with 13.5g or 27g of oxygen; 13.5 and 27 form a ratio of 1:2. Dalton found an atomic theory of matter could elegantly explain this common pattern in chemistry - in the case of Proust's tin oxides, one tin atom will combine with either one or two oxygen atoms.
Dalton also believed atomic theory could explain why water absorbed different gases in different proportions: for example, he found that water absorbed carbon dioxide far better than it absorbed nitrogen. Dalton hypothesized this was due to the differences in mass and complexity of the gases' respective particles. Indeed, carbon dioxide molecules (CO₂) are heavier and larger than nitrogen molecules (N₂).
Dalton proposed that each chemical element is composed of atoms of a single, unique type, and though they cannot be altered or destroyed by chemical means, they can combine to form more complex structures (chemical compounds). This marked the first truly scientific theory of the atom, since Dalton reached his conclusions by experimentation and examination of the results in an empirical fashion.

Various atoms and molecules as depicted in John Dalton's A New System of Chemical Philosophy (1808).

In 1803 Dalton orally presented his first list of relative atomic weights for a number of substances. This paper was published in 1805, but he did not discuss there exactly how he obtained these figures. The method was first revealed in 1807 by his acquaintance Thomas Thomson, in the third edition of Thomson's textbook, A System of Chemistry. Finally, Dalton published a full account in his own textbook, A New System of Chemical Philosophy, 1808 and 1810.
Dalton estimated the atomic weights according to the mass ratios in which they combined, with hydrogen being the basic unit. However, Dalton did not conceive that with some elements atoms exist in molecules — e.g. pure oxygen exists as O₂. He also 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). The flaw in Dalton's theory was corrected in 1811 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's law allowed him to deduce the diatomic nature of numerous gases by studying the volumes at which they reacted. For instance: since two liters of hydrogen will react with just one liter of oxygen to produce two liters of water vapor (at constant pressure and temperature), it meant a single oxygen molecule splits in two in order to form two particles of water. Thus, 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.
In 1827, the British botanist Robert Brown observed that dust particles inside pollen grains floating in water constantly jiggled about for no apparent reason. In 1905, Albert Einstein theorized that this Brownian motion was caused by the water molecules continuously knocking the grains about, and developed a hypothetical mathematical model to describe it. This model was validated experimentally in 1908 by French physicist Jean Perrin, thus providing additional validation for particle theory (and by extension atomic theory).

Discovery of subatomic particles

Main articles: Electron and Plum pudding model

Thomson's illustration of the Crookes tube by which he proved the particle nature of cathode rays. Cathode rays were emitted from the cathode C, sharpened to a beam by slits A and B, then passed through the electric field generated between plates D and E.

When the cathode ray (blue line) passed through the electric field (yellow), it was deflected. Atoms were thought to be the smallest possible division of matter until 1897 when J.J. Thomson discovered the electron through his work on cathode rays. A Crookes tube is a sealed glass container in which two electrodes are separated by a vacuum. When a voltage is applied across the electrodes, cathode rays are generated, creating a glowing patch where they strike the glass at the opposite end of the tube. Through experimentation, Thomson discovered that the rays could be deflected by an electric field (in addition to magnetic fields, which was already known). He concluded that these rays, rather than being a form of light, were composed of very light negatively charged particles he called "corpuscles" (they would later be renamed electrons by other scientists).

Thomson believed that the corpuscles emerged from the molecules of gas around the cathode. 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 as the electrons were embedded in the positive charge like plums in a plum pudding (although in Thomson's model they were not stationary). Thomson's illustration of the Crookes tube by which he proved the particle nature of cathode rays. Cathode rays were emitted from the cathode C, sharpened to a beam by slits A and B, then passed through the electric field generated between plates D and E. When the cathode ray (blue line) passed through the electric field (yellow), it was

John Dalton’s Atomic Model

Dalton’s atomic model is one of the fundamentals of physics and chemistry. This theory of atomic composition was hypothesized and partially confirmed by the English chemist and Physicist John Dalton. Dalton came with his Atomic theory as a result of his research into gases. He discovered that certain gases only could be combined in certain proportions even if two different compounds shared the same common element or group of elements. Through deductive reasoning and experimentation, he made an interesting discovery. His findings led him to hypothesize that elements combine at the atomic level in fixed ratios. This ratio would naturally differ in compounds due to the unique atomic weights of the elements being combined. This was a revolutionary idea but further experimentation by himself and others confirmed his theory. The findings became the basis of Dalton’s Atomic Laws or Model. These laws focus on five basic theorems. First, Pure Elements consist of particles called atoms. Second, atoms of an element are all the same for that element. That means gold is gold and oxygen is oxygen down to the last atom. Third, atoms of different elements can be told apart by their atomic weights. Fourth, atoms of elements unite to form chemical compounds. Finally, atoms can neither be created nor destroyed in chemical reaction. The grouping only changes.  The last of Dalton’s Atom Model were at the time considered true for all reactions involving atoms. This was later corrected with the discovery of nuclear fission and fusion. So we now know that this only holds true for chemical reactions. Like other scientific theories, Dalton’s atomic model has been perfected over time with the research and discoveries of other scientists. We now know that the atom can be divided into even smaller particles and we have even discovered the actual internal atom structure, even able to view it through modern technology. We now know that atomic weight is a product of the structure of the atoms as well. This atomic theory made possible modern chemistry and physics. Up until Dalton’s time the atom was only considered to a philosophical construct passed down by the ancient Greeks. Dalton’s ground breaking work made theory reality. This understanding the atom helped to fuel many other discoveries such as the fundamental forces and Einstein’s theory of relativity. It is especially is important when one goes into Quantum physics a discipline that looks at physics at the atomic and subatomic levels.

Rutherford's model

The Rutherford model or planetary model is a model of the atom devised by Ernest Rutherford. Rutherford directed the famous Geiger-Marsden experiment in 1909, which suggested on Rutherford's 1911 analysis that the so-called "plum pudding model" of J. J. Thomson of the atom was incorrect. Rutherford's new model^[1] for the atom, based on the experimental results, had the new features of a relatively high central charge concentrated into a very small volume in comparison to the rest of the atom and containing the bulk of the atomic mass (the nucleus of the atom). Rutherford's model did not make any new headway in explaining the electron-structure of the atom; in this regard Rutherford merely mentioned earlier atomic models in which a number of tiny electrons circled the nucleus like planets around the sun, or a ring around a planet (such as Saturn). However, by implication, Rutherford's concentration of most of the atom's mass into a very small core made a planetary model an even more likely metaphor than before; as such a core would contain most of the atom's mass, in an analogous way to the Sun containing most of the solar system's mass. Rutherford's model was later corrected by Niels Bohr.


source : wikipedia