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CHAPTER: 3 CHEMISTRY CLASS:9Th CHAPTER: 3 CHEMISTRY CLASS:9th Periodic table history History of the periodic table of chemical elements In 1669 German merchant and amateur alchemist Hennig Brand attempted to created a Philosopher’s Stone; an object that supposedly could turn metals into pure gold. He heated residues from boiled urine, and a liquid dropped out and burst into flames. This was the first discovery of phosphorus. In 1680 Robert Boyle also discovered phosphorus, and it became public. In 1809 at least 47 elements were discovered, and scientists began to see patterns in the characteristics. In 1863 English chemist John Newlands divided the than discovered 56 elements into 11 groups, based on characteristics. In 1869 Russian chemist Dimitri Mendeleev started the development of the periodic table, arranging chemical elements by atomic mass. He predicted the discovery of other elements, and left spaces open in his periodic table for them. In 1886 French physicist Antoine Bequerel first discovered radioactivity. Thomson student from New Zealand Ernest Rutherford named three types of radiation; alpha, beta and gamma rays. Marie and Pierre Curie started working on the radiation of uranium and thorium, and subsequently discovered radium and polonium. They discovered that beta particles were negatively charged. In 1894 Sir William Ramsay and Lord Rayleigh discovered the noble gases, which were added to the periodic table as group 0. In 1897 English physicist J. J. Thomson first discovered electrons; small negatively charged particles in an atom. John Townsend and Robert Millikan determined their exact charge and mass. In 1900 Bequerel discovered that electrons and beta particles as identified by the Curies are the same thing. In 1903 Rutherford announced that radioactivity is caused by the breakdown of atoms. In 1911 Rutherford and German physicist Hans Geiger discovered that electrons orbit the nucleus of an atom. In 1913 Bohr discovered that electrons move around a nucleus in discrete energy called orbitals. Radiation is emitted during movement from one orbital to another. In 1914 Rutherford first identified protons in the atomic nucleus. He also transmutated a nitrogen atom into an oxygen atom for the first time. English physicist Henry Moseley provided atomic numbers, based on the number of electrons in an atom, rather than based on atomic mass. 1 BY: M. ZIA DOGAR M.Sc CHEM (GCU) CHAPTER: 3 CHEMISTRY CLASS:9th In 1932 James Chadwick first discovered neutrons, and isotopes were identified. This was the complete basis for the periodic table. In that same year Englishman Cockroft and the Irishman Walton first split an atom by bombarding lithium in a particle accelerator, changing it to two helium nuclei. In 1945 Glenn Seaborg identified lanthanides and actinides (atomic number >92), which are usually placed below the periodic table. In 1951, Seaborg was awarded the Nobel Prize in chemistry for his work. Element 106 has been named seaborgium (Sg) in his honor. Q: WHAT IS PERIODIC TABLE? ANS: “ a table of elements obtained by arranging them in order of increasing atomic number in which elements having similar properties are placed in the same group is called periodic table.” It is an arrangement of element with similar properties placed together in the same group. Thus knowing properties of one element, the properties of other elements placed in the same group can be known. Q: WHAT DO YOU KNOW ABOUT PERIODS IN PERIODIC TABLE? ANS: PERIODS: Periodic table is divided into seven horizontal rows of elements. Each row of elements is called "PERIODS”. All the elements of a period are different from each other. Period number of an element indicates number of orbits in the atom of that elements. There are seven periods in periodic table 1st period has 2 elements 2nd and 3rd periods both have 8 elements in each 4th & 5th periods both have 18 elements in each 6th & 7th periods have 32 elements and elements in these periods are termed as lanthanides and actinides. First three periods are known as short periods while other known as long periods From left to right properties of elements gradually change with in a period but when move to next period the pattern of properties repeats. Q: WHAT ARE GROUPS IN PERIODIC TABLE? ANS: GROUPS: Modern periodic table is divided into eight vertical columns of elements. Each vertical column of elements is called a "GROUP". The groups of the periodic table are further divided into two sub- 2 BY: M. ZIA DOGAR M.Sc CHEM (GCU) CHAPTER: 3 CHEMISTRY CLASS:9th groups or families "A and B". Elements of sub-group "A" are known as Normal elements or Representative elements. While the elements of sub-group "B" are known as Transition elements. Group number of an element is represented by roman numerals such as IA, IIA, VIA etc * total number of electrons in the valence shell represents the group number ** SOME GROUPS IN PERIODIC TABLE HAS BEEN GIVEN THE GROUP NAMES Group 1A elements are called alkali metals Group IIA elements are called alkaline earth metals Group VIIA elements are known as halogens Group VIIIA elements are known as noble gases Q: what is meant by periodicity of the properties? Ans: Periodicity of properties: ‘’The repetition of similar properties after regular intervals in the periodic table is called periodicity of properties” The physical and the chemical properties of the elements change periodically with the atomic number. There are some other properties of atoms which are periodic properties. These properties are atomic radius (atomic size), ionization energy, electron affinity, electronegativity and valency. These atomic properties depend upon the location of the element in the periodic table. These properties show periodic variations. 1 - Shielding Effect: “The reduction of force of attraction between nucleus and valence electrons by the electrons present in the inner sub-shells is called shielding effect” The electrons present in the shells between the nucleus and the valence electrons reduce the forse of attraction between the nucleus and the valence electrons. As a result, an atom experiences less number of charges than that of actual charge, which is called effective nucleus charge (Z-eff). Trends in periodic table: Trend in period: when we move from left to right in period, the shielding effect does not change in period. Reason: this is because in period from L→R , the number of inner shells remain same. Trend in group: The shielding effect increases in moving from top to bottom in a group. Reason: this is because the number of inner shells increases in going from top to bottom in a group. 3 BY: M. ZIA DOGAR M.Sc CHEM (GCU) CHAPTER: 3 CHEMISTRY CLASS:9th Atomic Size: The size of an atom depend upon its electronic configuration “ The size of an atom is the average distance between the nucleus of an atom and outer electronic shell”. Trend in period: As you move across a period, atomic radius decreases.( L→R) Reason: As you go across a period, electrons are added to the same energy level. At the same time, protons are being added to the nucleus. The concentration of more protons in the nucleus creates a "higher effective nuclear charge." In other words, there is a stronger force of attraction pulling the electrons closer to the nucleus resulting in a smaller atomic radius. Trend in group: As you move down a group, atomic radius increases. Reason: The number of energy levels increases as you move down a group as the number of electrons increases. Each subsequent energy level is further from the nucleus than the last. Therefore, the atomic radius increases as the group and energy levels increase. 4 BY: M. ZIA DOGAR M.Sc CHEM (GCU) CHAPTER: 3 CHEMISTRY CLASS:9th Ionization Energy (I.E): Ionization energy is the minimum amount of energy required to remove an electron from valence shell an isolated gaseous atom or ion. First ionization energy: the energy required to remove the first electron from an atom is called the first ionization energy. Unit: it is measured in KJ/mol or in electron –volt (eV). For example: the first ionization energy of sodium atom is +496 kJmol-1 + - Mg Mg + e I1= +735KJ/mol So the second ionization energy, which is larger, is the energy required to remove the next electron. + 2+ - Mg Mg + e I2 = +1443 KJ/mol When a filled-shell needs to be "broken" it will require a even larger energy, often 10 times more. 5 BY: M. ZIA DOGAR M.Sc CHEM (GCU) CHAPTER: 3 CHEMISTRY CLASS:9th Trend in Periodic table: In group: ionization energy decreases in going from top to bottom in a group Reason: 1. Nuclear charge increases and atomic size also increases due to addition of new shells at each element 2. The number of inner electrons increases , this increase the shielding effect on outer electrons. In period: I.E increases as we go from left to right in a period Reason: 1. The nuclear charge increase by one unit at each element. 2. the added electrons enter to the same shell. 2. The shielding effect remains the same. Due to these facts size of atoms decrease and valence electrons held strongly by the electrostatic force of nucleus ELECTRON AFFINITY: The amount of energy released when an electron adds up in the valence shell of an atom to form a uninegative gaseous ion. X + e- Xˉ Example: Whaen an electron is added to the outermost shell of Cl, 349 KJ/mol energy is released. Cl + e- Clˉ Trend in group: the value of electron affinity decreases in going from top to bottom in a group. Reason: decreases due to shielding effect increase added electrons are less tightly binds to the nucleus Trend in a Period: the value of electron affinity increases across a period from left to right Reason: In a period atomic size decreases due to increase in nuclear charge.
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