Chemistry ebook

Discovery and Characteristics By Upasana S Gupta 1

Index Discovery of a proton………..3 Characteristics of a proton…………4 Discovery of a neutron…………5 Characteristics of a neutron…………6 Discovery of an electron…………7 Characteristics of an electron………...8 2

Discovery of a proton The discovery of protons dates back to the year 1815 when the English chemist William Prout suggested that all atoms are made up of hydrogen atoms. When canal rays (positively charged ions formed by gases) were discovered by the German physicist Eugen Goldstein, it was observed that the charge-to-mass ratio of the hydrogen ion was the highest among all gases. It was also observed that the hydrogen ion had the smallest size among all ionized gases. The discovery of the proton is credited to Ernest Rutherford, who proved that the nucleus of the hydrogen atom is present in the nuclei of all other atoms in the year 1917. • Ernest Rutherford observed that his scintillation detectors detected hydrogen nuclei when a beam of alpha particles was shot into the air. • After investigating further, Rutherford found that these hydrogen nuclei were produced from the nitrogen atoms present in the atmosphere. • He then proceeded to fire beams of alpha particles into pure nitrogen gas and observed that a greater number of hydrogen nuclei were produced. • He concluded that the hydrogen nuclei originated from the nitrogen atom, proving that the hydrogen nucleus was a part of all other atoms. • This experiment was the first to report a nuclear reaction, given by the equation: N14 + α → O17 + p [Where α is an alpha particle which contains two protons and two neutrons, and ‘p’ is a proton] • The hydrogen nucleus was later named ‘proton’ and recognized as one of the building blocks of the atomic nucleus. 3

Characteristics of proton Two important characteristics of a proton are its mass and charge. Mass of a proton: The proton is actually a hydrogen atom which has lost its electron. Since the mass of an electron is very small, we can say that the mass of a proton is equal to the mass of a hydrogen atom. But the mass of a hydrogen atom is 1 u, therefore, the relative mass of a proton is 1 u. if, however, we compare the mass of a proton with that of an electron, then the mass of a proton is 1840 times that of an electron. The absolute mass of a proton is 1.6 x 10^-24 gram. Charge of a proton: The charge of a proton is equal and opposite to the charge of an electron . so, the absolute charge of a portion is 1.6 x 10^-19 coulomb of positive charge . Now , 1.6 x 10^-19 coulomb has been found to be the smallest positive charge by any particle. So , this is taken as the unit of positive charge. This means that proton carries 1 unit positive charge. In other words, the relative charge of a proton id +1 . Ernest Rutherford Eugen Goldstein 4

Discovery of a neutron The discovery of neutrons can be traced back to the year 1930 when the German nuclear physicists Herbert Becker and Walther Bothe observed that a penetrating form of radiation was produced when the alpha particles emitted by polonium was incident on relatively light elements such as lithium, beryllium, and boron. This penetrating radiation was unaffected by electric fields and was, therefore, assumed to be gamma radiation. In the year 1932, the French scientists Frederic Joliot-Curie and Irene Joliot-Curie observed that this unusually penetrating radiation, when incident on paraffin wax (or other compounds rich in hydrogen), caused the ejection of high energy protons (~5 MeV). The Italian physicist Ettore Majorana suggested the existence of a neutral particle in the nucleus of the atom which was responsible for the way the radiation interacted with protons. The presence of neutral particles in the nuclei of atoms was also suggested by Ernest Rutherford in the year 1920. He suggested that a neutrally charged particle, consisting of a proton and an electron bound to each other, also resided in the nuclei of atoms. He coined the term ‘neutron’ to refer to these neutrally charged particles. The British physicist Sir James Chadwick discovered neutrons in the year 1932. He was awarded the Nobel Prize in Physics in the year 1935 for this discovery. It is important to note that the neutron was first theorized by Ernest Rutherford in the year 1920. • James Chadwick fired alpha radiation at beryllium sheet from a polonium source. This led to the production of an uncharged, penetrating radiation. • This radiation was made incident on paraffin wax, a hydrocarbon having a relatively high hydrogen content. • The protons ejected from the paraffin wax (when struck by the uncharged radiation) were observed with the help of an ionization chamber. • The range of the liberated protons was measured and the interaction between the uncharged radiation and the atoms of several gases was studied by Chadwick. • He concluded that the unusually penetrating radiation consisted of uncharged particles having (approximately) the same mass as a proton. These particles were later termed ‘neutrons. 5

Characteristics of neutron Two important characteristics of neutron is its mass and charge. Mass of a neutron: The mass of a neutron is equal to the mass of a proton. In other words, the relative mass of a neutron is 1 u. the absolute mass of a neutron is 1.6 x 10^-24 gram. Charge of a neutron: Neutron has no charge. It is electrically neutral. We are now in a position to explain why the atomic mass of carbon is 12 u. it is now that a carbon atom contains 6 protons and 6 neutrons, each having a mass of 1 u. Now, Atomic mass of carbon = mass of 6 protons + mass of 6 neutrons. James Chadwick 6

Discovery of an electron The existence of electrons in an atom was shown by J.J. Thomson in 1897. Thomson passed electricity at high voltage through a gas at very low pressure taken in a discharge tube. Streams of minute particles were given out by the cathode (negative electrode). These streams of particles are called cathode rays (because they come out of cathode). The mass and charge of the cathode ray particles does not depend on the nature of gas taken in the discharge tube. Cathode rays consist of small, negatively charged particles called electrons. Since all the gases form cathode rays, it was concluded that all the atoms contain negatively charged particles called electrons. The gas taken in the discharge tube consists of atoms contain electrons. When high electricity voltage is applied, the electrical energy pushes out some of the electrons from the atoms of the gas . these fast-moving electrons form cathode rays. Thus, the formation of cathode rays shows that one of the subatomic particle present in all the atoms is the negatively charged ‘electron’. The electron is negatively charged particle found in the atoms of all the elements. the electrons are located outside the nucleus in an atom. Only hydrogen atom contains one electron, all the other atoms contains more than one electron. An electron is usually represented by the symbol e- (e for electron and minus sign for negative charge). 7

Characteristics of an electron The smallest unit in the composition of matter is the atom. The atom is composed of a nucleus at the center and one or more electrons orbiting around the nucleus. The nucleus consists of protons and neutrons, collectively called nucleons. The protons are positively charged particles with a mass of 1.00728 amu and the neutrons are electrically neutral particles with mass of 1.00867 amu. The electrons are negatively charged particles with a mass of 0.000549 amu. The protons and neutrons are about 1836 times heavier than the electrons. The number of electrons is equal to the number of protons thus resulting in a neutral atom of an element. The electronic arrangements determine the chemical properties of an element whereas the nuclear structure dictates the stability and radioactive transformation of the atom. J.J. Thomson 8

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