CBSE – 10th Standard Science

Class 10 Question Bank Chapter 5 – Periodic Classification of Elements

Qus 1. How many elements are known till date?

Ans. 118 elements are known to us till date.

Qus 2. Who was Johann Wolfgang Döbereiner?

Ans. Johann Wolfgang Döbereiner was a pharmacist who discovered similar triads of elements which led to the development of the Periodic Table of elements

Qus 3. Why does Dmitri Ivanovich Mendeléev is said to be the most important contributor to the early development of a Periodic Table?

Ans. Dmitri Ivanovich Mendeléev, a Russian chemist was the one who arranged elements on the basis of their fundamental property, the atomic mass on the periodic table.

Qus 4. Which two elements are very reactive and can form compounds with most elements?

Ans. hydrogen and oxygen  are very reactive and can be formed compounds with most elements.

Qus 5. When Mendeléev’s Periodic Table was published?

Ans. Mendeléev’s Periodic Table was published in a German journal in 1872.

Qus 6. Who showed that the atomic number of an element is a more fundamental property than its atomic mass?

Ans. In 1913, Henry Moseley showed that the atomic number of an element is a more fundamental property than its atomic mass

Qus 7. How many elements are there in the first, second, third and fourth periods?

Ans. The number of elements in these periods is based on how electrons are filled into various shells.

Qus 8. Explain the term ‘atomic size’?

Ans. The term atomic size refers to the radius of an atom.

Qus 9. Why atomic radius decreases in moving from left to right along a period?

Ans. Atomic radius decreases because when it is increased in nuclear charge which tends to pull the electrons closer to the nucleus and reduces the size of the atom.

Qus 10. Why is silicon classified as a semi-metal or metalloid?

Ans. Silicon is classified as a semi-metal or metalloid because it exhibits some properties of both metals and non-metals.

Qus 11. Name the borderline elements in the periodic table?

Ans. The borderline elements in the periodic table are boron, silicon, germanium, arsenic, antimony, tellurium and polonium.

Qus 12. What are Periods?

Ans. Elements in the Modern Periodic Table are arranged in 18 vertical columns called groups and 7 horizontal rows called periods

Q.13. Name two elements you would expect to show chemical reactions    similar to magnesium. What is the basis for your choice?

Ans.13. All the elements like beryllium calcium strontium which belong to   group 2,they show similar electronic configuration

Q.14. Name

(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.14.   (a) lithium,sodium ,potassium

(b)beryllium, magnesium

(c) helium,neon,argon


Q.1. What were the limitations of Döbereiner’s classification?

Q.2. What were the limitations of Newlands’ Law of Octaves?

Q.3. Besides gallium, which other elements have since been discovered that were left by Mendeléev in his Periodic Table? (any two)

Q.4. What were the criteria used by Mendeléev in creating his Periodic Table?

Q.5. Why do you think the noble gases are placed in a separate group?