Test: Structure of the Atom- Assertion & Reason Type Questions - Class 9 MCQ

- The assertion states that isotopes are electrically neutral, which is true. This is because they have an equal number of protons (positively charged) and electrons (negatively charged), resulting in no overall charge.
- The reason correctly explains that isotopes of an element have the same number of protons and electrons.
- Therefore, both the assertion and reason are true, and the reason provides the correct explanation for the assertion.
Correct answer: a.

Option A: Both assertion (A) and reason (R) are true and reason (R) is the correct explanation of assertion (A).

Explanation:

• Assertion (A): Thomson’s atomic model is known as the "plum pudding" model. This is a true statement.
• Reason (R): The atom is visualized as a pudding of positive charge with electrons (plum) embedded in it. This is also true and provides the correct explanation for why it is called the "plum pudding" model.

In Thomson's model, the atom is described as a positively charged "pudding" with negatively charged electrons distributed within it, resembling plums in a pudding.

Isobars are not identical in chemical properties because they have same mass number and different atomic numbers.

Direction of electric field is from region of high potential to low potential & electron or any -ve charged particle will move against the field or lower potential to higher potential. 

Assertion (A):

"A few positively charged α-particles are deflected in Rutherford’s experiment" is true.

In Rutherford's gold foil experiment, most α-particles passed straight through the gold foil, but a few were deflected at small angles, and very few were deflected at large angles.

Reason (R):

"Most of the space in the atom is empty" is also true.

This explains why most α-particles passed straight through the foil without any deflection.

Relation between A and R:

While the reason (R) is true, it does not directly explain the deflection of a few α-particles. The deflection occurs because of the repulsion between the positively charged α-particles and the positively charged nucleus, which occupies a very small central space in the atom.

Isotopes have the same number of electrons. Hence, their valencies do not differ. Isotopes have the same atomic number but different atomic mass numbers: The total number of protons present in the atomic nucleus is known as its atomic number whereas the mass number is defined as the total number of protons and neutrons present in the atomic nucleus.

Assertion (A):
The statement "The atoms of different elements having the same mass number but different atomic numbers are known as isobars" is true.

Isobars are atoms of different elements with the same mass number (sum of protons and neutrons) but different atomic numbers (number of protons).

Reason (R):
The statement "The sum of protons and neutrons in the isobars is always different" is false.

In isobars, the mass number (sum of protons and neutrons) is the same, while the atomic numbers differ.

Thus, the assertion is true, but the reason is false, making option (c) the correct choice.

The correct option is A

Both assertion and reason are correct and the reason is the correct explanation of assertion.

Explanation for assertion:

Group 18 elements are called inert gases, as they do not react easily.

Explanation for reason statement:

1. Noble gas elements have their duplet or octet complete, which means that they have completely filled valence shells.
2. Due to this reason, they have very low electronegativity and very high Ionization Potential, hence they are then fairly unreactive and called inert gases.

Hence, A) Both assertion and reason are true, reason is the correct explanation of assertion.