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Class 10 Science Chapter 5 Periodic Classification of Elements
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Periodic Classification of Elements
Chapter – 5
TEXTUAL QUESTIONS AND ANSWERS
Page – 81
Q.1. Did Dobereiner’s triads also exist in the columns of Newlands Octaves ? Compare and find out.
Ans :- Dobereiner’s triads are also found in Newland’s octave . For example , Li , Na , k .
Q.2. What the limitations of Dobereiner’s classification ?
Ans :- The limitation of Dobereiner’s classification was that it failed to arrange all the taken known elements in the form of triads of elements having similar chemical properties . Dobereiner could identify only three triads from the elements known at that time . So , his classification of elements was not much successful .
Q.3. What were the limitations of Newland’s law of octaves ?
Ans :- Newlands ‘ law of octaves for the classification of elements has the following limitations :
( i ) Newlands ‘ law of octaves was applicable to the classification of elements up to calcium only . After calcium every eight elements did not posses the properties similar to that of the first element . Thus Newlands ‘ law of octaves worked well with lighter elements only.
( ii ) Newands ‘ assumed that only 56 elements existed in nature bet and no more elements would be discovered in the future . But later on , several new elements were discovered whose proportics did not fit into Newlands , law of octaves .
( ii ) In order to fit elements into his table , Newlands put even two elements together in one slot and that too in the column of unlike elements having very different properties .
( iv ) Iron element which resembles cobalt and nickel elements in properties was placed far away from these elements .
Page – 85
Q.1. Use Mendeleev’s periodic table to predict the formula for the oxides of the following elements :
K , C , AI , Si , Ba
Ans :- ( i ) The element K is in group 1 of Mendeleev’s periodic table is which the general formula of the oxides of elements is R₂ O . So , the formula of oxide of K will be K₂ O .
( ii ) The element C is in group IV of Mendeleev’s periodic table in which the general formula of the oxides of elements is RO₂So , the formula of oxide of C will be CO₂.
( iii ) The element Al is in group III of Mendeleev’s periodic table in which the general formula of the oxides of elements is R₂O₃ . So the formula of oxide of Al will be Al₂O₃.
( iv ) The element Si is in group IV of Mendelev’s periodic table in which the general formula for the oxides of elements is RO₂ So , the formula of the oxide of Si will be SiO₂.
( v ) The element Ba is in group II of Mendeleev’s periodic table in which the general formula for the oxides of elements is RO . So , the formula of oxide of Ba will be BaO .
Q.2. Besides gallium , which other elements have since been discovered that were left by Mendeleev in this periodic table ? ( any two )
Ans :- Scandium and Germanium .
Q.3. What were the criteria used by Mendeleev in creating his periodic table ?
Ans :- When mendeleev started his work 63 elements were known . He examined the relationship between the atomic masses of the elements and their physical and chemical properties .
Among chemical properties , Mendeleev concentrated on the compounds formed by elements with oxygen and hydrogen . He selected hydrogen and oxygen as they are very reactive and formed compounds with most elements . The formula of the hydrides and oxides formed by an element were treated as one of the basic properties of an element for its classification . He then took 63 cards and on each card he wrote down the properties of one element .
He sorted out the elements with similar properties and pinned the cards together on a wall . He observed that most of the elements got a place in a periodic table and were arranged in the order of their increasing atomic masses . It was also observed that there occurs a periodic recurrence of elements with similar physical and chemical properties . On this basis Mendeleev formulated a periodic law . Which states that ” The properties of elements are the periodic function of their atomic masses .
Q.4. Why do you think the noble gases are placed in a separate group ?
Ans :- Noble gases like helium , neon and argon are chemically inert and are present in atmosphere in extremely low concentrations . Thus owing to their similar inert behaviour and similar electronic configuration . They are justified to be placed in a separate group .
Page – 90
Q.1. How could the Modern Periodic Table remove various anomalies of Mendeleev’s Periodic Table ?
Ans :- Since all the isotopes of an element have the same atomic number . They can be put at one place in the same group of the periodic table . For example , both the isotopes of chlorine . Cl – 35 and Cl -37 , have the same atomic number of 17. so both of them can be put at one place in the same group of the periodic table .
The atomic number of cobalt is 27 and that of nickel is 28 . Now , according to modern periodic law , the elements are arranged in the order of increasing atomic numbers . So , cobalt with lower atomic number ( 27 ) should come first and nickel with higher atomic number (28 ) should come later , even if their atomic masses are in the wrong order.
Q.2. Name two elements you would expect to show chemical reactions similar to magnesium . What is the basis of your choice ?
Ans :- Beryllium and Calcium . Both Beryllium and calcium have some electrons in outermost shell with calcium , i.e. in outermost shell they have two electrons each .
( a ) Three elements that have a single electron in their outermost shells .
( b ) Two elements that have two electrons in their outermost shells .
( c ) Three elements with filled outermost shells .
Ans :- ( a ) Lithium , sodium , potassium.
( b ) Magnesium , calcium .
( c ) Helium , Neon , Argon .
Q.4. ( a ) Lithium , sodium , potassium are all metals that react with water to liberate hydrogen gas . Is there any similarity in the atoms of these elements ?
( b ) Helium is an unreactive gas and neon in a gas of extremely low reactivity . What if anything , do their atoms have in common ?
Ans :- ( a ) The atoms of lithium , sodium , potassium all have only one electron in their outer most shells .
( b ) The atoms of helium and neon have their outer most shell completely filled .
Q.5. In the Modern Periodic Table , which are the metals among the first ten elements ?
Ans :- Li , Be and B are metals .
Q.6. By considering their position in he periodic table , which one of the following elements would you expect to have maximum metallic characteristic ?
Ga , Ge , As , Se , Be
Ans :- Be .
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