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We observe that the elements Mg and Sr, to gain electrons as we go from left to right aa, will have properties similar to both Mg and S, have physical and chemical properties resembling, atomic numbers 12, 19, 21 and 38. Which of these, Example 7. In the Modern Periodic Table, calcium, but the electronegative character increases, the electropositive character of elements de, the large positive charge on the nucleus, elect, Reason: When we move from left to right in o, decreases and non-metallic charactér increase, two valence electrons just like Ca. Therefore, (A) Group 1 elements when, arranged in incre, their elec, (atomic number 20) is surrounded by elements with, calcium?, [NCERT], N-shell, Ans. To find the elements having similar physical and, M-shell, L-shell, 2, K-shell, 8, Element, Atomic Number, 2, 2, Calcium, 20, 8, 1, 2, 8., Magnesium, 12, 2, 8, 3., Potassium, 19, 8., Scandium, 21, 18, 8., Strontium, 38, metallic, Variation along a period: The, Atomic size, The size of an atom refers to the radius of atom and, move from left to right in a period., is the distance between the centre of the nucleus and, the outermost shell of an isolated atom. It is expressed, in picometer., 1 picometer = 10-12 m, Variation along a period: On moving from left to right, in a period, the size of atoms decreases., pulled more strongly towards the nucleus, perio, Reason: When we move from left to right in a period,, the number of electrons and protons increases. Due to, the large positive charge on the nucleus, electrons are, pulled more strongly towards the nucleus., Example: Consider the elements of 3id, variation of metallic character is shown he, P, S, CL, Ar, Si,, Na, Mg, Al,, Metals, Metalloid Non-metals, Variation along a group: The size of an atom (radius), increases as we go down in a group, Metallic property decreases, Reason: When going from top to bottom in a group,, a new shell is added to the atoms which increases, Variation along a group: The metallic che, increases and non-metallic character decreases, move from top to bottom in a group., the distance between the valence electrons and the, nucleus. So, the effective nuclear charge experienced, Reason: When we go down a group, the electrope, character of elements increases as the tender, an atom to lose electrons increases as the ef, nuclear charge experienced by the valence ele, decrease. The electronegative character decr, down a group as the tendency to gain elec, decreases., by the valence electrons decreases., Metallic and Non-metallic Character, Metals are the elements that have 1, 2 or 3 electrons, in their valence shell and lose electrons easily to form, positive ions or cations. They are present on the left, side and centre of the periodic table. They are called, electropositive elements as they have a tendency of, losing electrons., Example 8. Case Based:, Let us study how the tendency to lose electroe, change in a group and how this tendency will d, in a period., Non-Metals are the elements that have 4, 5, 6 or 7, electrons in their valence shell and gain electrons to, form negative ions or anions. They are present or, the right side of the periodic table. They are called, electronegative elements as they have a tendency of, gaining electrons., In the periodic table, studų the change in the tem, Metalloids: There are some elements known as, period and as we go down a group., metalloids that exhibit properties of both metals and, [NCERT Activity 5.10,*, non-metals., In the Modern Periodic Table, a zig-zag line separates, metals from non-metals. The border line elements -, boron, silicon, germanium, arsenic, antimony, tellurium, and polonium- are intermediate in properties and are, called metalloids or semi-metals,, order of their reactivity is:, (a) Na < Li < K < Rb < Cs, (b) Cs < Rb < K< Na < Li, (c) Li < Na < K< Rb < Cs, (d) K< Na < Li < Rb < Cs