9th chemistry chapter 3 solved exercise short questions

Periodic Table and Periodicity of Properties

9th chemistry chapter 3 solved exercise short questions

Q#1 : Why noble gases are not-reactive ?


Because their valence shells are completely filled and do not tend to lose or gain electrons from any other atom.

Therefore, they are not reactive .

Q#2 : Why Cesium (at.no.55) requires little energy to release its one electron present in the outermost shell ?

  • Cesium releases its valence electrons easily due to following reasons:

Atomic size of cesuim is very large .

Greater shielding effect.

Less nuclear charge.

Q#3 : How is periodicity of properties dependent upon number of protons in an atom ?

  • The trend of number of protons in periodic table is :

In periodic table , when we move from left to right , proton number increases in nucleus.

Hence, nuclear charge will also increase .

  • When nuclear charge changes , properties of elements will also change .

Q#4 : Why shielding effect of electron makes cation formation easy ?

With the increase in shielding effect, cation formation becomes easy due to following reasons :

  • increase of atomic number results in increase of shielding effect.
  • it makes easy to take away electron from the outermost shell.

Q#5 : What is difference between Mendeleev’s periodic law and modern periodic law ?

Q#6 : What do you mean by groups and periods in the periodic table ?

Groups :

” The vertical columns in the periodic table are called groups .”

  • They are numbered from left to right as 1 to 18 .

Periods :

” The horizontal rows of elements in the periodic table are called periods .”

  • There are 7 periods numbered from top to bottom .

Q#7 : Why and how are elements arranged in 4th period ?

Fourth period has 18 elements .

First 2 elements are put in s-block ;

Next 10 elements are in d-block transition metals and

Next 6 elements are in p-block.

  • These elements are put in 4th period as all they have four number of shells .

Q#8 : Why the size of atom does not decrease regularly in a period ?

  • Atomic radius decreases in a periodic table due to reason .


  • Because of change in Shielding Effect is not regular .

If shielding effect increases, the size will also increase .

If shielding effect decreases, the size will also decrease .

Q#9 : Give the trend of ionization energy in a period . Also give reasons .

Trend of Ionization Energy :

The ionization energy of elements increases from left to right in a period .

  • Due to following factors :

Increase in nuclear charge.

Shell number remains same .

Decrease in atomic size .

9th Chemistry, Chapter 3, Periodic Table and Periodicity of Properties , Solved exercise short questions , free notes

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