Tamilnadu Board Class 10 Science Chapter 8
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Weak Pull HCl Strong Pull + PERIODIC– CLASSIFICATION 8 H Cl OF ELEMENTS Elctronon TrTfansferredd Learning Objectives After a thorough perusal of this unit, the students will be able to: recognize the basis of the modern periodic law and its development. list the features of groups and periods of the modern periodic table. explain the trend in periodic properties along the periods and groups. distinguish between ores and minerals . list out the types of separation of impurities from the ores. recall the various places of occurrences of minerals in the state of Tamil Nadu. put forth the properties of metals. identify the stages involved in metallurgical processes. think scientifically on alloys and their types. develop an idea on amalgam. understand the reason for corrosion and the methods of its prevention. periods in such a way that each group contained INTRODUCTION a certain number of elements (like an array of The eighteenth and nineteenth centuries fruits and vegetables showing orderliness) witnessed a rapid development in chemistry with similar properties and periods showing a in all spheres of scientific activities. By 1860, regular gradation. So, scientists made several scientists had already discovered 60 elements attempts to arrange elements in a logical way. and determined their atomic masses. They You have studied about all these early attempts noticed that some elements had similar of arrangement of elements in standard IX. In properties and hence arranged them into continuation of the knowledge gained in the groups. During this period, several new topic periodic classification of elementsin elements were discovered. These elements standard IX with earlier concepts and their were found to have different properties. It was subsequent deliberations, you get set to go realized that instead of studying the properties ahead with the higher order of thinking to of all these elements individually, it would be enhance your knowledge on the properties of more convenient to divide them into groups and elements. 106 10th_Science_Unit-8.indd 106 21-02-2019 18:38:56 Figure 8.1 shows the modern periodic table of 8.1 MODERN PERIODIC LAW 118 elements discovered so far. Mendeleev’s periodic table had some As you have studied the features of the discrepancies, which were difficult to overcome. modern periodic table in standard IX, here For example, the atomic mass of argon (39.95 let us confine to the study of the features of amu) is greater than that of potassium (39.10 periods and groups. amu), but argon comes before potassium in the periodic table. If elements were arranged 8.2.1 Features of Periods solely according to increasing atomic mass, argon would appear in the position occupied The horizontal rows are called periods. by potassium in our modern periodic table (see There are seven periods in the periodic in Figure 8.1). No chemist would place argon, a table. gas with no tendency to react, in the same group First period (Atomic number 1 and 2): as lithium and sodium,which are two highly This is the shortest period. It contains reactive metals. This kind of discrepancies only two elements (Hydrogen and suggested that some fundamental property Helium). other than atomic mass must be the basis of Second period (Atomic number 3 to 10): periodicity. The fundamental property turned This is a short period. It contains eight out to be the number of protons in an atom’s elements (Lithium to Neon). nucleus, something that could not have been known by Mendeleev and his contemporaries. Third period (Atomic number 11 to 18): This is also a short period. It contains Henry Moseley, a British scientist in 1912, eight elements (Sodium to Argon). discovered a new property of elements called atomic number, which provided a better basis Fourth period (Atomic number 19 to 36): for the periodic arrangement of the elements. This is a long period. It contains eighteen It is a well-known fact that atomic number of elements (Potassium to Krypton). This an element is equal to the number of protons includes 8 normal elements and 10 or the number of electrons present in the transition elements. neutral atom of an element. The periodic law Fifth period (Atomic number 37 to 54): was, therefore, modified to frame a modern This is also a long period. It contains periodic law, which states that 18 elements (Rubidium to Xenon). This “The physical and chemical properties includes 8 normal elements and 10 of the elements are the periodic functions of transition elements. their atomic numbers”. Sixth period (Atomic number 55 to 86): This is the longest period. It contains 32 elements (Caesium to Radon). This 8.2 MODERN PERIODIC TABLE includes 8 normal elements, 10 transition With reference to the modern periodic law, elements and 14 inner transition elements the elements were arranged in the increasing (Lanthanides). order of their atomic numbers to form the Seventh period (Atomic number 87 modern periodic table. The modern periodic to 118): Like the sixth period, this table is a tabular arrangement of elements in period also accommodates 32 elements. rows and columns, highlighting the regular Recently 4 elements have been included repetition of properties of the elements. by IUPAC. 107 Periodic Classificaiton of Elements 10th_Science_Unit-8.indd 107 21-02-2019 18:38:56 Modern Periodic Table Periodic Modern Figure 8.1 Figure 10th Standard Science 108 10th_Science_Unit-8.indd 108 21-02-2019 18:38:59 8.2.2 Features of Groups 8.3 PERIODIC TRENDS IN The vertical columns in the periodic table PROPERTIES starting from top to bottom are called The electronic groups. There are18 groups in the periodic configurations of elements table. help us to explain the Based on the common characteristics periodic recurrence of of elements in each group, they can be physical and chemical grouped as various families. properties. Anything which repeats itself after a regular Group Number Family interval is called periodic and this behaviour 1 Alkali Metals is called periodicity. Some of the atomic 2 Alkaline earth metals properties of the elements are periodic. 3 to 12 Transition metals Properties such as atomic radius, ionic 13 Boron Family radius, ionisation energy, electronegativity, electron affinity, show a regular periodicity 14 Carbon Family and hence they are called periodic properties. 15 Nitrogen Family The main significance of the modern periodic 16 Oxygen Family (or) table is that it gives a clear understanding of the Chalcogen family general properties and trends within a group or 17 Halogens a period to predict with considerable accuracy, the properties of any element, even though that 18 Noble gases element may be unfamiliar to us. Let us discuss the periodic trend of some of the properties. The Lanthanides and Actinides, which form part of Group 3 are called inner 8.3.1 Atomic Radius transition elements. Except 'group 0', all the elements present Atomic radius of an atom is defined as in each group have the same number of the distance between the centre of its nucleus electrons in their valence shell and thus and the outermost shell containing the valence have the same valency. For example, all electron. Direct measurement of the radius of the elements of group 1 have one electron an isolated atom is not possible. Except for in their valence shells (1s1). So, the noble gases, usually the atomic radius is valency of all the alkali metals is ‘1’. referred to as covalent radius or metallic radius depending on As the elements present in a group the nature of the have identical valence shell electronic bonding between the configurations, they possess similar concerned atoms. chemical properties. Atomic radius in metal (a) The physical properties of the elements atoms is known as in a group such as melting point, boiling metallic radius. It is point and density vary gradually. defined as half the (b) The atoms of the 'group 0' elements have distance between the Figure 8.2 stable electronic configuration in their nuclei of adjacent (a) Metallic Radius valence shells and hence they are unreactive. metal atoms (Figure 8.2 (b) Covalent Radius 109 Periodic Classificaiton of Elements 10th_Science_Unit-8.indd 109 21-02-2019 18:38:59 (a)). In non-metallic elements, their atomic Li Li+ FF– radius is known as Covalent radius. It is defi ned as half the distance between the nuclei of two covalently bonded atoms of the 156 90 69 119 same element in a molecule (Figure 8.2 (b)). Na Na+ Cl Cl– For example, let us consider H2 molecule. Th e distance between the two hydrogen nuclei of the molecule is 0.74 Å. So its covalent radius is 0.74/2 = 0.37 Å. 186 11691 167 Atomic radius of Hydrogen When you look at the Radii in Picometers variation of the atomic Figure 8.5 HH Relative ionic radii of cation radii in the periodic table, and anion there are two distinct Figure 8.3 trends. Along the period, Atomic radius the ion. You know that ions are formed when from left to right, the an atom lose or gain electrons. When a neutral atomic radius of the elements decreases whereas atom loses an electron, it becomes a positively along the groups, from the top to bottom, the charged ion called cation, whereas the gain of atomic radius increases. Th e increase, down a an electron by a neutral atom forms a negatively group, is due to the increase in the valence shell charged ion called anion. Th e size of the ions number down the group. As the shell number is important to determine their behaviours increases, the distance between the valence shell in solutions and the structure of ionic solids.