In Mendeleev’s Periodic Table the position of hydrogen could not be explained, while in Modern Periodic table hydrogen is placed above alkali metals. Mendeleev left some gaps in his periodic table. The properties of elements are periodic functions of their atomic mass. (ii) The position of elements in Modem Periodic Table: (iii) Table showing Electronic Configuration of First 20 Elements: (i) Modem Periodic Law: Properties of elements are a periodic function of i their atomic number. In Period: On moving from left to right in a period, the valency first increases from 1 to 4 and then decreases to zero (0). The position of isotopes could not be explained because isotopes have the same chemical properties but different atomic masses. The position of isotopes should be separated according to Mendeleev’s periodic rule but they were kept within the same group. (iii) Limitations: Dobereiner could identify only three triads. F > Cl > Br > I. Characteristics of triads of J.W. (ii) The atomic size or atomic radius increases: as we move down in a group and it decreases as we move from left to right in a period. (c) Hydrogen is placed at the topmost of periodic table because it resembles both alkali metals and halogens some of the properties so the position in the periodic table is unclear. For example, hydrogen reacts with halogens, oxygen, and sulphur to form compounds whose formulae are similar to those of alkali metals. Also, with increase in atomic number by one unit on moving from left to right, the valence shell electrons increases by one unit. Atomic size increases down a group due to the increase in the number of shells. There is an increase in distance between the outermost shell electrons and the nucleus of the atom. The atomic mass of middle element of the triad being nearly equal to the average of the atomic masses of the other two elements. Due to the large positive charge on the nucleus, the electrons are pulled in more closely to the nucleus and the size of the atom decreases. Li has 1+ valency, then Be2+, B3+, C4+ covalency, N3- valency, then O2- and F(-) valency. Law of octaves was applicable only upto calcium (only for lighter elements). Hydrogen’s position was not justified in the very old Mendeleev’s periodic table. (ii) Wrong order of atomic masses of some elements could not be explained. (d) Silicon and Germanium. Example: Valency of first group elements = 1 Valency of second group elements = 2. Limitations of Mendeleev’s classification: Modem Periodic Law: This law was proposed by Henry Moseley, a scientist in 1913. Inert gases were not known at the time of Mendeleev. property of elements, ‘atomic number’ and this was I adopted as the basis of Modem Periodic Table. Example: Eka-boron, Eka-aluminium and Eka-silicon. The placement of elements in the periodic table is based on their electronic configuration. On moving from left to right in a period, the number of valence electrons increases from 1 to 8 in the elements present. Valency in a particular period from left to right first increases as positive valency and then decreases as negative valency. Johann Wolfgang Dobereiner, a German chemist, classified the known elements in groups of three elements on the basis of similarities in their properties. added in spite of the increase in nuclear charge. Classification of Isotopes : Elements having same atomic number but different Mass Number are called Isotopes. 2) Electropositive character: Like alkali metals ,hydrogen also loses its only electron to form hydrogen ion i.e. (2013 D, 2014 D) Answer: H + The Modern Periodic Table consists of 18 vertical columns termed as ‘groups’. POSITION IN THE PERIODIC TABLE Hydrogen is the first element in the periodic table of the elements and is represented by the symbol H. Hydrogen, with only one proton, is the simplest element. In Mendeléev’s periodic table only 63 elements were arranged that were examined on the basis of the relationship between the atomic masses of elements and their physical and chemical properties. Increase in the discovery of different elements made it difficult to organise all that was known about the elements. In some versions of the Periodic Table, uniquely among the elements, the hydrogen symbol appears twice. Trends in the Modern Periodic Table: We hope the given CBSE Class 10 Science Notes Chapter 5 Periodic Classification of Elements Pdf free download will help you. Na – Sodium The table consists of eight vertical column called ‘groups’ and horizontal rows called ‘periods’ Limitations of mendeleev’s classification : (i) He could not assign a correct position of hydrogen in his periodic table, as the properties of hydrogen resembles both with alkali metals as well as with halogens. the combining capacity of an element is known as its valency). Modem Periodic Table: Henry Moseley, gave a new ! Example ; Li < Na < K < Rb < Cs (1st group) In non-metals: Chemical reactivity of non-metals decreases down the group because tendency to gain electrons decreases. He did so because hydrogen and alkali metals have similar properties. It is known as ‘Newlands’ Law of Octaves’. There are 18 groups and 7 (seven) periods in the Modern Periodic Table. Example: Position of hydrogen with alkali metals and halogens (17th group). It contained the elements from hydrogen to thorium. Hydrogen has the atomic number 1 and so it should occupy the first position in the first group of the modern periodic table.But due to its chemical similarities with the elements of group 17, its position is still being debated.so,the position of the hydrogen is not fixed in the modern periodic table.. hope this helps u. thumbs up plsss.. The modem periodic table consists of 18 groups and 7 periods. Metals like Na and Mg occupy the left-hand side whereas the non-metals like sulphur and chlorine occupy the right-hand side of the Periodic Table. Later on the elements were discovered, for example, gallium replaced Eka-aluminium and it showed properties similar to that of aluminium. Class 10 Chemistry Periodic Classification of Elements Limitations of Mendeleevs classification Limitations of Mendeléev’s classification No fixed position could be assigned to hydrogen … Properties of elements in each triad were similar. CBSE 10 Science Periodic Classification of Clements. Limitation: It fails to arrange all the known elements in the form of triads, even having similar properties. Periodicity of Properties: The repetition of properties of elements after certain regular intervals is known as Periodicity of Properties. Trends in Modern Periodic Table: Valency, Atomic size, metallic and non-metallic characters, and Electronegativity. Non-remnant stars are mainly composed of hydrogen in the plasma state. Note that while hydrogen is normally shown at the top of the Group 1 elements in the periodic table, the term "alkaline metal" refers to the Group 1 elements from lithium downwards and not hydrogen. With a standard atomic weight of 1.008, hydrogen is the lightest element in the periodic table.Hydrogen is the most abundant chemical substance in the universe, constituting roughly 75% of all baryonic mass. Periodic Classification of Elements Class 10 Important Questions Very Short Answer Type. 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