Question

Identify the least reactive element of the following?
A) ${}_8^{16}X$
B) ${}_{10}^{20}X$
C) ${}_{11}^{23}X$
D) ${}_9^{19}X$

Answer 1

Hint: We know all the elements known to be arranged in a specific pattern depending on their physical and chemical properties in the periodic table. According to the number of electrons they are given their unique atomic number.

Complete answer:
> We know that an atom is composed of subatomic particles called electrons, protons and neutrons. This composition of element can be represented by using the normal element symbol (X) with superscript on the left hand side as the atomic mass number (A) and subscript (Z) on the left hand side as the atomic number.
> Atomic number is given by the number of electrons in the neutral atom or by the number of protons in the nucleus of the atom whereas atomic mass number is the sum of the number of protons and neutrons. The protons and neutrons are present in the nucleus of the atom and are collectively called nucleons.
> In the periodic table elements are divided into four blocks and react differently to water,air etc. depending upon their electronic configuration. The group-18 elements are least reactive and are referred to as inert or noble gases as they have a very stable electronic configuration which is why they do not react with other elements to lose or share their electrons. Now let us identify the given elements depending upon their atomic number and atomic mass. > Therefore the first element is oxygen with Z = 8, second element is neon with Z = 10, third element is sodium with Z = 11 and the last element is fluorine with Z = 9. From all the above elements it is only neon that belongs to group 18 and is therefore least reactive. Rest all of the elements form various compounds that we are aware of such as $O_2,F_2,NaCl$ etc.

Thus the correct answer is (B).

Note: There are elements that would happen to have the same atomic mass but different atomic number such elements are isobars and the elements with same atomic number and different atomic mass are isotopes.