Periodic Table and Periodicity of Properties

9th chemistry chapter 3 solved exercise short questions

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

Reason:

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 .

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 .