CBSE X Periodic Classification of Elements Solved assignment

Periodic Classification of Elements Assignment Solved

Q1. Explain Dobereiner’s Law of triads along with an example .

Ans. Dobereiner tried to arrange elements with similar properties into groups of 3 elements each. He called these as triads. He showed that when the 3 elements into a triad were arranged in the order of increasing atomic masses, the atomic mass of the middle element was roughly the average of the masses of the other two elements. e.g.           Li                   Na                          K

atomic  mass                                     6.9                 23                          39

                                                         39+6.9/2  =    45.9/2   =    22.9   =  23

Q2. What were the limitations of Dobereiner’s Law of triads?

Ans. Dobereiner could identify only 3 triads from the element known at that time. Hence, this system is not useful.

Q3. State newland’s law of Octaves.

Ans. Newland arranged the elements in the increasing order of the atomic masses and found that every eight element had properties similar to that of the first like the notes of music.

Q4. What were the limitations of newland’s Law of Octaves?

Ans. 1. It was only applicable only up to Calcium as after Calcium every eighth element did  not posses , properties similar to that of first.

2. At a few places unlike elements were put together like cobalt and nickel along with F, Cl and Br.

Q5. Which 2 criteria did Mendeleev used to classify elements in his periodic table ?

Ans. Atomic mass and similarity in chemical property.

Q6. State Mendeleev’s periodic law.

Ans. Properties of elements are a periodic function of their atomic masses.

Q7. Why did Mendeleev leave gap in his periodic table?

Ans. Mendeleev place elements with similar properties one below the other leaving gap  for yet undiscovered elements.

Q8. Name the elements which have since been discovered that were left by Mendeleev  in his Periodic Table?

Ans. Eka Boron – Scandium             Eka   Al - Gallium         Eka   Si - Germanium

Q9. What are the limitations of Mendeleev's periodic table?

Ans. 
1. Position of Isotopes was not clear in his table.       

2. Wrong Order of atomic mass of certain elements  – Elements with higher atomic  mass were placed before elements having lower atomic mass.    e.g. Cobalt was placed before nickel.

3. Position of hydrogen was not clear in his periodic table.

Q10. What is Modern periodic Law ?

Ans. The Properties of elements are a periodic function of their atomic number.

Q11. How were the drawbacks of Mendeleev's periodic table resolved in the modern  periodic table?

Ans. As the modern periodic table was based on the atomic numbers so all the isotopes could be  placed together in one slot.  Secondly, the wrong order of atomic masses was also sorted as Cobalt's atomic number  was lower than Nickel even though it had a higher atomic mass.

Q12. How are the position of elements related to their atomic number in the Modern periodic table?

Ans. The number of valence electrons present in an atom of an element is equivalent to  the group number, while the total number of shells give the period number in which the element to present.

e.g 1. Atomic no. of Mg=12 ;  Electronic configuration=2,8,2     therefore, Gp. no.=2 or IIA   Period no.=3

e.g 2. Atomic no. of Cl=17;  Electronic Configuration=2,8,7       therefore, Gp. no.=17^th  or VIIA

Q13. Explain the gradation in properties of elements in the modern periodic table.

Ans. a. Valency: 

 Valency of elements remain the same  while going down a group 

 Moving across a   period, the valency of  elements increases   from 1 to 4 and then decreases till 0.

b. Atomic size or radius:

  It increases on going down a group

  It decreases in   Moving from left to right along a period.

c. Metallic and non-metallic  properties: :   Moving down a group of metals, metallic nature increases due to increasing size. However, in a group of non-metals the non-metallic character decreases on going    down

Going across a period, the metallic character decreases while Non-metallic character increases

Q14. Give examples of somea. Metalloids – Boron , Silicon , Germanium

b. Alkali metals – Sodium , lithium , Potassium

c. Halogens – Fluorine , Chlorine , Bromine

d. Alkaline Earth metals – Magnesium , Calcium

e. Nobel gases or Inert elements – Helium , Neon , Krypton