PERIODIC CLASSIFICATION OF ELEMENTS

Classification of elements: The arrangement of elements in such a manner that elements with similar properties are grouped together while elements with dissimilar properties are separated. The classifications are as follows

Dobereiner’s Triads Drawback – 1st Real attempt – By Johann Wolfgang Dobereiner in 1817. Tried to arrange known elements into groups of three. { TRIADS } Basis of classification – When written in increasing order of atomic masses, the mass of middle element was toughly the average of the masses of the other elements. Drawback – He could identify only 3 such triads, namely, (a) Lithium – Sodium – Potassium , (b) Calcium – Strontium – Barium, (c ) Chlorine –Bromine –Iodine.

Newland’s Octaves Basis of Classification – John Newlands tried classifying elements in increasing order of atomic masses in 1866. Basis of Classification – He realised that every 8th element had properties similar to the 1st element. He compared this to the octaves found in music. “ Law of Octaves “ Newland’s law of octaves: He arranged the elements in the order of increasing atomic masses starting with Hydrogen (least atomic mass) and ended with Thorium having atomic mass 56. Acc. To him, the properties of every 8th element are similar to the 1st element. It was compared to music notations Sa, Ri, Ga, Ma, Pa, Da, Ni, Sa, and thus the name Newland’s law of octaves (note of music).

Drawbacks – 1. Applicable only till Calcium. ( worked well with lighter elements ) 2. He didn’t consider the chance of discovering any new elements and made his table only for the 56 elements known then. 3. In order to make his law valid, he out 2 elements in one slot ( eg: Co & Ni ) But this also couldn’t make the law universal.

Mendeleev’s Periodic Table Mendeleev’s Periodic Law – The properties of elements are periodic function of their atomic masses. Features - 1. Examined relationship of atomic mass & physical and chemical properties of elements. 2. chemical properties – considered the compounds formed by elements with hydrogen & oxygen, as these 2 were very reactive, forming compounds with most of the known elements. 3. He arranged the 63 known elements into 6 horizontal periods and into 8 vertical groups. 4. Periods – based on increase in atomic mass, Groups – based on similarity in properties.

4. The 4th,5th & 6th periods have 2 series of elements each while 7 groups have A & B sub-groups based on difference of properties of elements in same group. The 8th group has 9 elements belonging to 3 different periods. 5. He left some gaps in his table, predicting the existence of some undiscovered elements. He even named three such elements by comparing the properties of other elements in the group. They were – Eka-Boron( Scandium ), Eka-aluminium ( Gallium ), Eka-silicon ( Germanium ).

Drawbacks – The position of hydrogen was not fixed. Some lighter elements were placed after heavier elements in account of property variation, but this was a contradiction to his periodic law. ( Eg; Co before Ni, Te before I , etc ) Isotopes of elements were not considered for classification. As they possessed different atomic masses, they had to be assigned separate slots, but this was not done by Mendeleev. Increase in atomic masses was not in a regular manner which raised the possibility of existence of elements between 2 of the known elements ( especially between heavier elements, where the mass gap was huge ).

Modern Periodic Table Modern periodic table was the brainchild of Henry Moseley. He showed by experimentation that the atomic number of elements was a more significant property than atomic mass. Modern Periodic Law – The properties of elements are a periodic function of their atomic number. The table consists of 7 horizontal periods and 18 vertical groups. The elements of a period consists of the same no.of shells, while the elements of a group have the same no.of valence electrons.

Drawbacks – The position of hydrogen is still not fixed and hence it had to be given a separate slot. 2. Lanthanoids ( Ce to Lu ) & Actinoids ( Th to Lw ) were not given place in the main table. Instead, they have been listed as 2 separate periods down the table.

Trends In The Periodic Table ( 1 ) VALENCY VARIATIONS – Valency of an element is determined by the number of valence electrons in the atom. The valence electrons are also the factors that determine the chemical properties of an element. Along a period, the valency increases from 1 to 4, and then it decreases to zero as we reach rare gases. Within a group, the valency of elements remains the same.

( 2 ) ATOMIC SIZE VARIATIONS – Atomic size refers to the distance from the nucleus of an atom to the outermost shell of the isolated atom. As we move from left to right along a period, the atomic size decreases. This is due to the increase in effective nuclear charge on the valence shell, that leads to shrinking of the shell as move along a period. As we move down a group, the atomic size increases. This is because as we move down, the number of shells increases, which overcomes the increase in nuclear charge.

( 3 ) METALLIC & NON-METALLIC PROPERTIES – Metals are characterised by the tendency to lose electrons (electropositivity) while non-metals have the tendency to gain electrons (electronegativity). As we move from left to right in a period, electropositivity decreases, ie, metallic character decreases and non-metallic character increases. As we move down the group, metallic character increases and non-metallic decreases. Semi-metals ( Metalloids ) – These are a group of elements that have properties intermediate that of metals & non-metals. Example : B, Si, Ge, As, Sb, Te & Po.

Pen-Paper Test What is the position of Hydrogen in the Modern periodic table? 1 An element M is in the 3rd group of the periodic table. Write the formula of its oxide. 1 What is the valency of Magnesium with atomic no. 12 and Chlorine with atomic no. 17? 1 What is the difference between no. of shells in Magnesium and Sulphur? 1 On the basis of electronic configuration, how will you select: (i) the terminating member in a period? (ii) the chemically similar elements? 1 6. Give reason as to why the atomic radii of elements increase in a group while moving from top to bottom? 2

Elements in a group of the periodic table have similar chemical properties. Why? 2 Explain why atomic no. is more important than atomic weight in determining chemical properties? 2 In a group reactivity of metal increases while those of non metals decreases when we going a group. Explain. Give an example for (a) an alkali metal. (b) a noble gas. (c) atom having smallest size. (d) atom having largest size. 2

11. An element has 6 electrons in its L shell. (a) Identify the element. (b) Which type of ion will it form? (c) What will be its valency? 3 12. Following are the atomic no.s of some elements which of them belong to (a) same period. (b) same group. Explain with steps. 3

13. Write the electronic configuration of atoms of (a) Potassium(K) (d) Fluorine (F) (b) Argon (Ar) (e) Chlorine (Cl) (c) Lithium (Li) These have atomic no.s as 19, 18, 3, 9 and 17 respectively. Use the electronic configuration to explain. (i) Why is Potassium more reactive than Lithium? (ii) Why is Fluorine more reactive than Chlorine? (iii) Which is smaller in size-Cl or Ar? (iv) Which is smaller in size-Li of F? (v) Which is more electronegative-F or Cl? 5 14. The atomic no. of an element is 17. (i) What is its valency? (ii) Whether it is a metal or non metal? (iii) Whether it is smaller or bigger in size than an element of atomic no. 18? (iv) What type of bonds it will form with the elements of group 18? (v) How would its oxide behave with litmus solution? 5

F.A-IV QUIZ Name a liquid metal. Give the symbol of liquid non metal. Name a lustrous non metal. Name the element which was known as Eka aluminium at the time of Mendeleev. Mendeleev’s periodic table is based on ____________. Modern periodic law is on the basis of ____________. Give two examples of metalloids. MCQ Atomic mass of the middle element is approximately the average of the other two elements in _____________. (a) Law of Octaves (b) Mendeleev’s periodic table (c) Dobereiner’s triads. (d) None of these

Isotopes of an element have similar properties due to (a) same atomic no. (b) same atomic mass (c) same valency (d) (a) & (c) In a group , all the elements have same valency due to (a) same no.of shells. (b) same no.of electrons in the outermost energy level (c) same atomic mass (d) none of these In a period, atomic size of elements decreases due to (a) increase in atomic no. and same no.of energy levels (b) decrease in atomic no. and same no.of energy levels (c) same atomic no. and same no.of energy levels Where would you locate an element with electronic configuration 2, 8, 7 in the modern periodic table? (a) group 7 and period 2 (b) group 17 and period 2 (c) group 7 and period 3 (d) group 17 and period 3

Projects PowerPoint Presentation on (a) The classification of elements (b) Property variation in the modern periodic table ii. Mendeleev periodic table and modern periodic table-Charts