9th Class Chemistry Chapter 3 Periodic Table & Periodicity of Properties Short Questions Answer
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Noble gases are not reactive because noble gases have eight electrons in their valence shells. So, they do not react with other elements. If elements do not have eight electrons in their valence shells they become reactive and by gaining or losing of electrons make them stable. But noble gases are not reactive.
Cesium requires less energy to release its outermost electron because this electron is far away from nucleus. Hence, its ionization energy is very low. Because of large distance between nucleus and outermost shell it loses its electron easily.
In periodic table, from left to right proton number increases in nucleus. Hence, nuclear charge will also change.
The decrease in the attractive force exerted by the nucleus on the valence shell electrons due to the presence of the electrons lying between the nucleus and valence-shell, is called shielding effect. When an element lose its one electron then its shielding effect will also increases. That’s why, it loses its valence shell easily and became a cation.
Mendeleev’s periodic law:
Properties of the elements are periodic function of their atomic masses.
Atomic masses is the basic property of his law.
Modern periodic law :
Properties of the elements are periodic function of their atomic number.
Atomic number is the basic property of modern periodic law.
The horizontal rows of elements in a periodic table are called periods. The vertical columns in a periods table are called group.
Eight elements are present in 4th period. Their names are
From left to right atomic radius decreases in a periodic table. The change is not regular because shielding effect changes. If shielding effect decreases the size will also decreases.
The ionization energy of elements increases as we go from left to right in a period.