EVOLUTION AND HISTORY Submitted to : Submitted by : Mr.DINESH N KURUP HARSHITH S PGT Physics XI A Roll no. - 14

PREFACE

https://prezi.com/qxgnktlf8doe/atomic-theory-timeline-project/Why is the history of the atom so important?Edit

It is fundamental to the understanding of science that science is understood to be a process of trial and improvement and represents the best known at the time, not an unerring oracle of truth. Development of an idea and refinement through testing is shown more in the understanding of atomic structure.

The Greek TheoristsEdit

A bust of Democritus (or Democrites), who came up with the idea of indivisible atoms.The earliest known proponent of anything resembling modern atomic theory was the ancient Greek thinker Democritus. He proposed the existence of indivisible atoms as a response to the arguments of Parmenides, and the paradoxes of Zeno.

Parmenides argued against the possibility of movement, change, and plurality on the premise that something cannot come from nothing. Zeno attempted to prove Parmenides' point by a series of paradoxes based on difficulties with infinite divisibility.

In response to these ideas, Democritus posited the existence of indestructible atoms that exist in a void. Their indestructibility provided a retort to Zeno, and the void allowed him to account for plurality, change, and movement. It remained for him to account for the properties of atoms, and how they related to our experiences of objects in the world.

Democritus proposed that atoms possessed few actual properties, with size, shape, and mass being the most important. All other properties, he argued, could be explained in terms of the three primary properties. A smooth substance, for instance, might be composed of primarily smooth atoms, while a sour substance is composed of rough or sharp ones. Solid substances might be composed of atoms with numerous hooks, by which they connect to each other, while the atoms of liquid substances possess far fewer points of connection.

Democritus proposed 8 points to his theory of atoms. [1] These are:

All matter is composed of atoms, which are bits of matter too small to be seen. These atoms CANNOT be further split into smaller portions.There is a void, which is empty space between atoms.Atoms are completely solidAtoms are homogeneous, with no internal structure.Atoms are different in: their sizes, their shapes, and their weights.Atoms are the building blocks of lifeAlchemyEdit

Although alchemy was futile, the alchemists did come up with several useful methods, including distillation (shown here).

A fire, shown by Lavoisier to be a chemical reaction and not an element.Empedocles proposed that there were four elements, air, earth, water, and fire and that everything else was a mixture of these. This belief was very popular in the medieval ages and introduced the science of alchemy. Alchemy was based on the belief that since everything was made of only four elements, you could transmute a mixture into another mixture of the same type. For example, it was believed that lead could be made into gold.

Alchemy's problem was exposed by Antoine Lavoisier when he heated metallic tin in a sealed flask. A grayish ash appeared on the surface of the melting tin, which Lavoisier heated until no more ash formed. After the flask cooled, he inverted it and opened it underwater. He discovered the water rose one-fifth of the way into the glass, leading Lavoisier to conclude that air itself is a mixture, with one-fifth of it having combined with the tin, yet the other four-fifths did not, showing that air was not an element.

Lavoisier repeated the experiment again, substituting mercury for tin, and found that the same happened. Yet after heating gently, he found that the ash released the air, showing that the experiment could be reversed. He concluded that the ash was a compound of the metal and oxygen, which he proved by weighing the metal and the ash, and showing that their combined weight was greater than that of the original metal.

Lavoisier then stated that combustion was not an element, but instead was a chemical reaction of a fuel and oxygen.History of Atomic TheoryPicture an atom. What does it look like? Most likely it will resemble something like this: a fairly large nucleus surrounded by orbiting electrons whizzing around the nucleus. This image is a popular icon of the atom, but it only vaguely represents our current model of what the atom looks like.

The Early GreeksFirst, we are going to travel back a little over 2,000 years ago to the times of Aristotle and Democritus. The Greek philosopher Aristotle believed that matter could be divided infinitely without changing its properties. Democritus disagreed. He thought that matter could only be divided until you got to the smallest particle (which he called the atom, coming from the Greek word atomos, meaning indivisible). So, who was right? Aristotle was very convincing and did many experiments using the scientific method, so more people believed him.Ancient Atomic Theory

One of the first atomic theorists was Democritus, a Greek philosopher who lived in the fifth century BC. Democritus knew that if a stone was divided in half, the two halves would have essentially the same properties as the whole.Therefore, he reasoned that if the stone were to be continually cut into smaller and smaller pieces then; at some point, there would be a piece which would be so small as to be indivisible. He called these small pieces of matter "atomos," the Greek word for indivisible. Democritus, theorized that atoms were specific to the material which they composed. In addition, Democritus believed that the atoms differed in size and shape, were in constant motion in a void, collided with each other; and during these collisions, could rebound or stick together. Therefore, changes in matter were a result of dissociations or combinations of the atoms as they moved throughout the void. Although Democritus' theory was remarkable, it was rejected by Aristotle, one of the most influential philosophers of Ancient Greece; and the atomic theory was ignored for nearly 2,000 years.

John Dalton and AtomsIt wasn't until around 2,000 years later, in the early 1800s, when John Dalton came along and disproved Aristotle. Dalton went on to say that matter is made up of tiny particles, called atoms, that cannot be divided into smaller pieces and cannot be destroyed. He also stated that all atoms of the same element will be exactly the same and that atoms of different elements can combine to form compounds. The really awesome thing about Dalton's model of the atom is that he came up with it without ever seeing the atom! He had no concept of protons, neutrons or electrons. His model was created solely on experiments that were macroscopic, or seen with the unaided eye.Dalton's atomic theory contains five basic assumptions:

All matter consists of tiny particles called atoms. Dalton and others imagined the atoms that composed all matter as tiny, solid spheres in various stages of motion.

Atoms are indestructible and unchangeable. Atoms of an element cannot be created, destroyed, divided into smaller pieces, or transformed into atoms of another element. Dalton based this hypothesis on the law of conservation of mass as stated by Antoine Lavoisier and others around 1785.

Elements are characterized by the weight of their atoms. Dalton suggested that all atoms of the same element have identical weights. Therefore, every single atom of an element such as oxygen is identical to every other oxygen atom. However, atoms of different elements, such as oxygen and mercury, are different from each other.

In chemical reactions, atoms combine in small, whole-number ratios. Experiments that Dalton and others performed indicated that chemical reactions proceed according to atom to atom ratios which were precise and well-defined.

When elements react, their atoms may combine in more than one whole-number ratio. Dalton used this assumption to explain why the ratios of two elements in various compounds, such as oxygen and nitrogen in nitrogen oxides, differed by multiples of each other.John Dalton's atomic theory was generally accepted because it explained the laws of conservation of mass, definite proportions, multiple proportions, and other observations. Although exceptions to Dalton's theory are now known, his theory has endured reasonably well, with modifications, throughout the years.

Thomson and the Discovery of ElectronsA diagram of the Rutherford alpha particle experimentRutherford Experiment DiagramNow, let's fast-forward to the late 1800s when J.J. Thomson discovered the electron. Thomson used what was called a cathode ray tube, or an electron gun. You've probably seen a cathode ray tube without even knowing it! They are the bulky electronic part of old television sets. Thomson used the cathode ray tube with a magnet and discovered that the green beam it produced was made up of negatively charged material. He performed many experiments and found that the mass of one of these particles was almost 2,000 times lighter than a hydrogen atom. From this he decided that these particles must have come from somewhere within the atom and that Dalton was incorrect in stating that atoms cannot be divided into smaller pieces. Thomson went one step further and determined that these negatively charged electrons needed something positive to balance them out. So, he determined that they were surrounded by positively-charged material. This became known as the 'plum pudding' model of the atom. The negatively charged plums were surrounded by positively charged pudding.

Rutherford and the NucleusA few years later, Ernest Rutherford , one of Thomson's students, did some tests on Thomson's plum pudding model. The members of his lab fired a beam of positively charged particles called alpha particles at a very thin sheet of gold foil. (Later on you will learn that alpha particles are really just the nuclei of helium atoms.) Because these alpha particles had so much mass, he fully expected that all of the alpha particles would go right through the gold foil. This is because, if Thomson were correct about the plum pudding model of the atom, the alpha particles would just go through the positively charged matter and hit the detecting screen on the other side.Modern Atomic Theory: Models

Bohr model

In 1913, Neils Bohr, a student of Rutherford's, developed a new model of the atom. He proposed that electrons are arranged in concentric circular orbits around the nucleus. This model is patterned on the solar system and is known as the planetary model. The Bohr model can be summarized by the following four principles:

Electrons occupy only certain orbits around the nucleus. Those orbits are stable and are called "stationary" orbits.Each orbit has an energy associated with it. The orbit nearest the nucleus has an energy of E1, the next orbit E2, etc.Energy is absorbed when an electron jumps from a lower orbit to a higher one and energy is emitted when an electron falls from a higher orbit to a lower orbit.The energy and frequency of light emitted or absorbed can be calculated by using the difference between the two orbital energies.bohr

dilemmaelectrons in orbit experience centripetal accelerationaccelerating charge produces electromagnetic waves, electromagnetic waves transfer energyloss of energy would make atoms unstable (electron should spiral into nucleus)discrete spectra from energetic electronsbohr's major new ideaquantization of angular momentum in terms of = h/2electrons occupy stationary states around the nucleusrestricted momentums lead to restricted radii and energy levelstransition between energy levels accompanied by emission, absorption of photon. The atom only gains or loses energy when its electrons are transferred from one stationary state to another.sommerfeld's major new ideaelectrons form standing waves around the nucleusdiscrete nature of harmonics leads to quantization of angular momentumnew dilemmaone dimensionaldoes not work for other elements anything with more than one electronthere are additional spectroscopic phenomena where discrete lines will split that it cannot explain (to be discussed elsewhere)zeeman effecthyperfine splittingQuantum mechanical model

In 1926 Erwin Schrdinger, an Austrian physicist, took the Bohr atom model one step further. Schrdinger used mathematical equations to describe the likelihood of finding an electron in a certain position. This atomic model is known as the quantum mechanical model of the atom. Unlike the Bohr model, the quantum mechanical model does not define the exact path of an electron, but rather, predicts the odds of the location of the electron. This model can be portrayed as a nucleus surrounded by an electron cloud. Where the cloud is most dense, the probability of finding the electron is greatest, and conversely, the electron is less likely to be in a less dense area of the cloud. Thus, this model introduced the concept of sub-energy levels.

Until 1932, the atom was believed to be composed of a positively charged nucleus surrounded by negatively charged electrons. In 1932, James Chadwick bombarded beryllium atoms with alpha particles. An unknown radiation was produced. Chadwick interpreted this radiation as being composed of particles with a neutral electrical charge and the approximate mass of a proton. This particle became known as the neutron. With the discovery of the neutron, an adequate model of the atom became available to chemists.

Since 1932, through continued experimentation, many additional particles have been discovered in the atom. Also, new elements have been created by bombarding existing nuclei with various subatomic particles. The atomic theory has been further enhanced by the concept that protons and neutrons are made of even smaller units called quarks. The quarks themselves are in turn made of vibrating strings of energy. The theory of the composition of the atom continues to be an ongoing and exciting adventure.

CONTENTS

INTRODUCTION

ACKNOWLEDGEMENT

Evolution and history of the atomic modelThe structure of matter has been the focus of study and thinking since the dawn of modern civilization. The word atom comes from the Greek word that sounded the same and it meant indivisible; that is, the smallest unit of matter, mass, or however the Greeks called it.The current meaning of the atom is derived from its evolution in the 19th century, and in the last century it was discovered that there were subatomic particles, and hence began the elaboration of the current structure of the atom, or interrelation of the types of smallest elementary particles that make up the atom.Given the importance of the evolution of the different atomic models developed, let's briefly comment on the history of the atom in chronological order before showing the current atom model put forth byGlobal Mechanics. 450 BC Atomic model of DemocritusThe philosophical development by Democritus postulated the impossibility of an infinite division of matter and the consequential need for there to be a smallest unit of which all things would consist.It is interesting to think that for 2,500 years Democritus was considered to be absolutely right; the truth is that it seemed to be so but now one of the most important theories or ideas inGlobal Mechanicsproposes the exact opposite.In the current model of theTheory of Global Equivalence,all substances make up a unique particle called Globus that consists of an unbreakable three-dimensional reticular network that is spread throughout the universe.

1808 Atomic model of DaltonThe development of the Dalton model was already pointing the way to the modern atom but as one solitary particle, although it was not very clear at first if theatomic model of Daltonwas supposed to be an atom or a molecule. 1897 Atomic model of ThomsonThe following major step in the history of the modern atom was theatomic theory of Thomsonwith the division of the atom between positive and negative charges, like a fruitcake or garlic soup, with electrical forces of attraction.

1911 Atomic model of RutherfordThe Rutherford model separates the nucleus with positive charge from electrons with negative charge. Electrons are located in circular or elliptical orbits around the nucleus. The neutron was theoretically added to theRutherford modelin 1920 and was experimentally developed in 1932.

The Rutherford model is the visual image we all have of the modern atom, but it had two problems: It contradicted theMaxwell lawsof electromagnetism in which charged particles in movement should be constantly emitting photons. Therefore, electrons should lose energy and fall towards the nucleus of the atom. Theatomic theory of Rutherforddid not explain the atomic spectrum. 1913 Atomic model of BohrTheatomic theory of Bohrintroduced substantial improvements to theRutherfordmodel by incorporating energy aspects derived from Planck energy and fromEinstein'sphotoelectric effect.Although a detailed description of theBohr modelis complicated, the following characteristics are relevant in regards to the model thatGlobal Mechanicsis going to introduce: Electrons are situated in stable circular orbits; that is, where orbits do not emit energy and not all are allowed. The orbits or electrons that are allowed in theBohr' atomic modelhave an angular momentum that is an exact multiple of hbar (Planck constant divided by 2) Electrons emit or absorb a photon by changing atomic orbits in which the energy depends on the difference of energy of the orbits and they do not need to go through intermediate stages. In the Bohr atom, the electron orbits follow the rules ofClassic Mechanicsbut the orbit changes do not.Regardless of the enormous success of this model in many aspects, the problem with the Bohr model and all ofQuantum Mechanicsis that they go on adding assumptions throughout history but without explaining the reasons to justify them, just that they work and better explain reality; which, although is not bad, does not help very much with understanding reality if the reasons are based on misleading physics principles.They could have tried a plausible explanation for a change. 1916 Atomic model of SommerfeldWith the development of the Sommerfeld model, sublevels are included in the Bohr atom structure, circular orbits are dismissed and, to a certain extent,Einstein'sTheory of Relativityis incorporated.TheSommerfeld modelalso makes electrons out to be electrical current and it does not explain why orbits must be elliptical. I think they are ellipsoids and that Sommerfeld is right in that the electron is a special kind of electromagnetic wave, which is called awavoninGlobal Mechanics. 1926 Schrdinger model, or current model according to WikipediaTheSchrdingermodel alters the philosophy of orbits surely because of its new contributions to theatomic theory of De Broglieregarding the wavy nature of mass in 1924, and describes electrons with wave functions. This configuration allows us to determine the probability of finding the electron in a specific point in space. Therefore, we get orbitals ofspatial density of probabilityof finding an electron.This model of theSchrdinger atomis much better adapted to observations; however, by giving up the previous image of the shape of orbits we move away from an intuitive explanation regarding the causes for such arbitrary orbits (Ver video)At the same time,Schrdingergoes into the world of probabilities and of mathematical abstraction which, in large doses, could end up being very detrimental or negative. 2008 Evolution of the current model of an atomIn the following section, this online book ofGlobal Mechanicsproposes a new step in the evolution of the modern model of the atom in an attempt to continue to make progress in what we know about a physics reality that is as beautiful and simple as it is complex.

CONCLUSION

BIBLIOGRAPHY