Atomic Structure

Step 1: Understanding the Concept:
This question asks about the composition of the atomic nucleus, which is the central, dense region of an atom.
Step 2: Detailed Explanation:
The nucleus of an atom is composed of two types of subatomic particles:
- Protons, which have a positive electrical charge.
- Neutrons, which are electrically neutral (have no charge).
These two particles, protons and neutrons, are collectively known as nucleons. They are held together in the nucleus by the strong nuclear force.
Let's analyze the other options:
- (A) proton: The nucleus contains protons, but for all elements except the most common isotope of hydrogen (protium), it also contains neutrons. So this is incomplete.
- (B) proton and electron: Electrons are negatively charged particles that orbit the nucleus; they are not part of the nucleus itself.
- (C) -particle: An alpha particle is the nucleus of a helium atom, consisting of two protons and two neutrons. While it is a type of nucleus, not all atomic nuclei are α-particles.
Step 3: Final Answer:
The nucleus of an atom is made up of protons and neutrons. Therefore, option (D) is the correct answer.

Step 1: Understanding isotopes.
Isotopes are variants of the same element that have the same number of protons (atomic number) but differ in the number of neutrons. This difference in neutrons results in a different atomic mass but does not affect the chemical properties of the element.
Step 2: Explanation of options.
- (A) Number of protons: Isotopes of the same element have the same number of protons (this defines the element).
- (B) Number of neutrons: The key difference between isotopes of the same element is the number of neutrons.
- (C) Number of electrons: In isotopes, the number of electrons remains the same because they are neutral atoms.
- (D) Atomic number: The atomic number is the same for all isotopes of the same element, as it is determined by the number of protons.
Step 3: Conclusion.
The main difference between isotopes is the number of neutrons.