History of Atomic Structure

The ancient Greek thinker DemocritusThe Democritus Atomic Theory revolves around the atoms that are present in the atmosphere. The atoms are present in all the forms of existence, like solid or liquid. The theory states that these atoms are all individually created and cannot be separated, no matter what scientific procedures are applied. The Democritus Atomic Theory is pretty much clear and the ideas are portrayed in manner that can be easily understood. The theory claims that there is no specific size that can be attributed to the various atoms and that they can differ depending on the various factors influencing them. It can be arrived at that the atoms cannot be split no matter what their size. Democritus proposed 6 points to his theory of atoms.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 lifeModern atomic theory was born with Dalton when he published his theories in 1803. His theory consists of five important points, which are considered to be mostly true today: Elements are composed of tiny particles called atoms.All atoms of a given element are identical.The atoms of a given element are different from those of any other element; the atoms of different elements can be distinguished from one another by their respective relative weights.Atoms of one element can combine with atoms of other elements to form chemical compounds; a given compound always has the same relative numbers of types of atoms.Atoms cannot be created, divided into smaller particles, nor destroyed in the chemical process; a chemical reaction simply changes the way atoms are grouped together.In the year 1889 the British physicist J.J. Thomson discovered the electron. Thomson conducted a number of experiments using cathode rays. Cathode rays are constructed by sealing two electrodes in a glass tube and removing the air from it. When the electrodes are attached to high voltage, a beam of radiation is emitted from the negative electrode. These beams are called cathode rays. Thomson concluded that these rays are made of negatively charged particles; today we call them electrons. Thompson found that he could produce cathode rays using electrodes of various materialsHe then concluded that electrons were found in all atoms and is over a thousand times smaller than protons. A "plum pudding" model. In this model the bits of "plum" were the electrons which were floating around in a "pudding" of positive charge to match that of the electrons and make an electrically neutral atom. Ernest Rutherford is known for his famous gold foil experiment in 1911. Alpha particles, which are heavy and positively charged, were fired at a very thin layer of gold. The result of the experiment allowed Rutherford to conclude that the plum pudding model is wrong.Atoms have a nucleus, very small and dense, containing the positive charge and most of the atom's mass.The atom consists of mostly empty space.The electrons are attracted to the nucleus, but remain far outside it. Bohr created an equation that could predict the spectral lines of the hydrogen atom. He then realized that electrons must orbit the nucleus in "shells", each with a certain energy level. An atom will absorb and release photons that have a specific amount of energy. The energy is the result of an electron jumping to a different shell. The Bohr model depicts the atom as a nucleus with electrons orbiting around it at specific distances.Dalton's Atomic Theory1) All matter is made of atoms. Atoms are indivisible and indestructible.2) All atoms of a given element are identical in mass and properties3) Compounds are formed by a combination of two or more different kinds of atoms.4) A chemical reaction is a rearrangement of atoms.Modern atomic theory is, of course, a little more involved than Dalton's theory but the essence of Dalton's theory remains valid. Today we know that atoms can be destroyed via nuclear reactions but not by chemical reactions. Also, there are different kinds of atoms (differing by their masses) within an element that is known as "isotopes", but isotopes of an element have the same chemical properties.Many heretofore unexplained chemical phenomena were quickly explained by Dalton with his theory. Dalton's theory quickly became the theoretical foundation in chemistryGreek  EraDemocritus"by convention bitter, by convention sweet, but in reality atoms and void"1803John DaltonProposed an "atomic theory" with spherical solid atoms based upon measurable properties of mass.1869Dmitri MendeleevArranged elements into 7 groups with similar properties.  He discovered that the properties of elements”were periodic functions of their atomic weights".  This became known as the Periodic Law.1898RutherfordStudied radiations emitted from uranium and thorium and named them alpha and beta.1911 Ernest RutherfordUsing alpha particles as atomic bullets, probed the atoms in a piece of thin (0.00006 cm) gold foil. He established that the nucleus was: very dense, very small and positively charged. He also assumed that the electrons were located outside the nucleus1919 AstonDiscovered the existence of isotopes through the use of a mass spectrograph. 1922Niels BohrDeveloped an explanation of atomic structure that underlies regularities of the periodic table of elements. His atomic model had atoms built up of successive orbital shells of electrons.RUTHERFORD’S ATOMIC MODEL Rutherford's atomic model shows the existence of nucleus in the atom, nature of charge on the nucleus   and the magnitude of charge on the nucleus.In his experiments, Rutherford bombarded alpha particles on very thin metallic foils such as gold foil. In     order to record experimental observations; he made use of circular screen coated with zinc sulfide.MAIN POINTS OF RUTHERFORD’S THEORY  Major portion of the atom is empty.  The whole mass of the atom is concentrated in the center of atom called nucleus.  The positively charged particles are present in the nucleus of atom.  The charge on the nucleus of an atom is equal to (+z.e) where Z= charge number, e = charge of  proton.  The electrons revolve around the nucleus in different circular orbits.  Size of nucleus is very small as compare to the size of atom.DEFECT OF RUTHERFORD’S THEORY  There were two fundamental defects in Rutherford's atomic model:  According to classical electromagnetic theory, being a charge particle electron when accelerated must   emit energy. We know that the motion of electron around the nucleus is an accelerated motion; therefore,   it must radiate energy. But in actual practice this does not happen. Suppose if it happens then due to   continuous loss of energy orbit of electron must decrease continuously. Consequently electron will fall into   the nucleus. But this is against the actual situation and this shows that atom is unstable.  If the electrons emit energy continuously, they should form continuous spectrum .But actually line spectrum is obtained