Question

All members of the group $1A$ have similar reactivity because of:
A.They have the same number of protons.
B.They have the same number of electrons.
C.They have similar outer electronic configurations.
D.They have valence electrons with the same quantum numbers.
E.They have the same number of neutrons.

Answer 1

Group $1A$ elements are known as alkali metals. There are $6$ elements present in Group $1A$ of the Modern periodic table. They are also known as ‘s-block’ elements. They are known as ‘s-block’ elements because their last electrons lie in the s-orbitals.

Complete step by step answer:
-Group $1A$ consists of alkali metals also known as ‘s-block’ elements.
-They are known as ‘s-block’ elements because of their last electrons that lie in the s-orbitals.
-The s-block elements are present at the extreme left of the modern periodic table.
-It consists of 6 elements namely: Lithium $\left( {Li} \right)$ , Sodium $\left( {Na} \right)$ , Potassium $\left( K \right)$ , Rubidium $\left( {Rb} \right)$ , Cesium $\left( {Cs} \right)$ and Francium $\left( {Fr} \right)$ .
-They are very soft, thus can be easily cut with a knife.
-Of all these elements present in Group $1A$ , sodium metal is present abundantly.
-Whereas Francium metal is very rarely found because of its radioactive nature.
-All of these elements are very highly reactive and shiny.
-Because of its high reactivity it is kept under solutions such as oil, kerosene.
-The reactivity of an element mostly depends on the number of electrons that are present in its outermost shell.
-So all the elements in Group $1A$ have the same number of electrons in their outermost shell.
-All of them have $1$ electrons in their outermost shell.
-That is why all the members of Group $1A$ have similar reactivity because of the same number of electrons in their outermost shell.
So the correct answer is option b) They have the same number of electrons.

Note:
In Group $1A$ Hydrogen (H) is not included because though it is highly reactive and has the same electronic configurations it is not an alkali metal. And also it is not a metal because it behaves differently at different temperatures.