Features of the Atom PDF

Features of the Atom Electrical Nature of Matter Quite number if experiments have demonstrated that two kinds of electrical charges are possible. Bodies having the same charge repel one another; while opposite charge attract one another. (first law of electrostatic charges) The second l...

Features of the Atom Electrical Nature of Matter Quite number if experiments have demonstrated that two kinds of electrical charges are possible. Bodies having the same charge repel one another; while opposite charge attract one another. (first law of electrostatic charges) The second law of electrostatic charges, also known as Coulomb’s law, states that the force between two point charges is directly proportional to the product of their magnitude and inversely proportional to the square of the separation distance between them. Coulomb’s Law It was formulated by a French physicist Charles Coulomb. The coulomb (C), an electrical unit of charge, was named in his honor.  Other electrical discoveries Luigi Galvani, an Italian scientist, was successful in passing a current by making use of the energy derived from a chemical change. He pioneered the study and use of electricity. Count Alessandro Volta, Italian physicist invented the wet battery. This sparked the widespread investigation into the phenomenon of electrolysis. Electrolysis is passing of a direct electric current through an electrolyte producing chemical reactions at the electrodes and decomposition of materials. Electrolyte is a substance that produces an electrically conducting solution when dissolved in a polar solvent such as water.  Other electrical discoveries Sir Humphry Davy and Michael Faraday who studied the electrical decomposition of numerous compounds. Faraday proposed the quantitative law known as Faraday’s law to express magnitudes of electrolytic effects. He suggested the existence of a fundamental unit of electricity to account fro his results involving the electrolysis of solutions of aqueous acids and salts. George Stoney, Irish physicist who proposed the name electrine for the unit charge.  The Electron William Crookes, a British chemist showed that there was some kind of matter coming from the cathode(negative electrode) using vacuum discharge. These particles were called cathode rays because that originated from the cathode.  The Electron Numerous experiments made by Julius Plucker, Johann Hittorf, William Crookes and others showed that cathode rays exhibit the following properties: a. Cathode rays are emitted from the cathode when an electric current is passed through an evacuated tube. b. The rays travel in a straight line, if small objects are placed in the discharge tube, the rays form a shadow. c. The cathode rays, upon striking a glass or other material, will cause it to fluoresce. d. Cathode rays bear a negative charge. This is shown by the fact that they are attracted toward positively charged objects held outside the tube.  The Electron Sir Joseph John Thomson established the particle nature of cathode rays after a series investigation. He managed to determine the velocity and the ratio of electric charge e to mass m, or e/m. The value is about -1.759 x coulombs per gram(C/g) He further concluded that cathode rays are negatively charged fundamental particles found in all atoms Cathode rays are now called electrons. He revolutionized the knowledge of atomic structure by his discovery of the electron.  The Electron Eugene Goldstein, a German physicist, known for work on electrical phenomena in gases, discovered new particles which he called canal rays. The name arose from the fact that the rays are attracted to the cathode ( - ) and pass through the holes of “canals” in the cathode. These particles are positively charged and are massive. He hypothesize that these particles could be what were left out of the atoms or molecules after electrons had been pulled out. Canal rays are now known to be proton.  Thomson’s Plum Pudding Model In 1897, J.J.Thomson, a British physicist discovered the negatively charged particle called electron. -He was studying the effect of passing an electrical current through a gas when the gas gave off rays made of negatively charged particles. Since the gas was known to be made of uncharged atoms, he thought that these negatively charged particles came from within the atom. Natural Radioactivity 1895 German physicist, Wilhelm Roentgen brought about the discovery of radioactivity. This discovery also overthrew the notion of an indestructible atom. He discovered X-rays. 1890s Antoine Becquerel and Marie Curie observed that radioactivity causes some atoms to break down spontaneously. Several experiments verified that the rays were emitted by the uranium material. These rays were called Becquerel rays. Natural Radioactivity Pierre and Marie Curie were cooking on pitchblende, a uranium ore. They succeeded in isolating polonium and radium. These elements, which were sources of radiation were termed radioactive substances. Marie Curie was awarded a second Nobel Prize for the discovery of radium and polonium. Natural Radioactivity Ernest Rutherford, a British physicist, completely unraveled the nature of Becquerel rays. He discovered with his coworkers that the rays are of three types: alpha rays(+ charged particles), beta rays(- charged particles) and gamma rays(uncharged particles) 1910-1911 Ernest Rutherford observed that atoms are mostly empty space.  Nuclear Model -In 1908, Ernest Rutherford, an English physicist discovered the positively charged particles or proton. -in his experiment, he fired a stream of alpha particles, which are tiny positively charged particles at a thin sheet of gold foil. He discovered that most of the positively charged “bullets” This shows that the atom is mostly an easily passed through the gold empty space, contrary to Thomson’s foil without changing direction at idea of a plum pudding model filled with all. positively charged materials. He also concluded that electrons are located outside the nucleus Rutherford’s Nuclear Model of an Atom He was able to calculate the relative size of the nucleus and “electron space” based on the number of alpha particles deflected and their angle of deflections. His calculations showed that the nucleus was about m in diameter, while the diameter of the entire atom was about m The Nucleus Rutherford’s experiment opened the door for the investigation of the nucleus. He found that the number of positive charges in the nucleus increases from atom to atom by a single unit when the elements are arranged consecutively. This was the important findings because it means that the nuclear charge is the basis of the atom’s individuality. Rutherford also established that the positive charge is due to the presence of a fundamental particles called proton. For this discovery, he was awarded the Nobel Prize for Chemistry in 1908. Atomic number Written as Z It is the number of protons in the nucleus. Since an atom is electrically neutral, the number of protons must be equal to the number of electrons. Proton = electron Z= Neutron The discovery of the third fundamental particle in 1932 is credited to British physicist James Chadwick. 1932 James Chadwick discovered neutron. It has a mass roughly equal to that of proton but has no electric charge.  These discoveries indicate that the proton, neutron and electron are the building blocks of atoms. Since the mass of an atom is concentrated in the nucleus, the mass number (A) is equal to the sum of the massed of the protons and neutrons. Mass number(A)= protons + neutrons (For neutral atoms) Number of proton (Z) = number of electrons Number of neutrons(n) = mass number (A) – atomic number (Z) n=A–Z  In writing the chemical symbols of elements, a superscript and subscript are often seen on the left side of the symbol. Superscript refers to the mass number (A), while Subscript refers to the atomic number (Z). 12 is the mass number and 6 is the atomic number  Isotopes – atoms of the same element that have different atomic masses. Atoms that have the same atomic number but different atomic masses. Z= 1 ; A= 1 Z=1 ; A = 2 21 Z= 1 ; A= 3 An example, hydrogen has protium, deuterium and tritium as its isotopes.