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For Class 8, 9 & 10

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Concept-first questions with clear model answers across all four subjects, aligned to NCERT. Start early, build the habit, and walk into your board exams, NEET and JEE already ahead.

Class 824Class 924Class 1036
SubjectAllPhysics 19Chemistry 19Biology 19Mathematics 27
ChapterAllAtoms and MoleculesIs Matter Around Us PureMatter in Our SurroundingsStructure of the Atom

Showing 2 questions in Chemistry for Class 9. Tap a card to reveal the answer.

ChemistryStructure of the Atomeasy

Define the atomic number and the mass number of an atom.

Reveal answer ↓

What it is

Atomic number = number of protons (the element's identity); mass number = protons + neutrons.

Answer

The atomic number (Z) is the number of protons in the nucleus of an atom, which also equals the number of electrons in a neutral atom; it decides the identity of the element. The mass number (A) is the total number of protons and neutrons in the nucleus. So the number of neutrons = A - Z.

A = Z + number of neutrons

  • •Atomic number Z = number of protons (= electrons if neutral)
  • •Mass number A = protons + neutrons
  • •Neutrons = A - Z
  • •Z identifies the element

Why learn this

These two numbers decide every element in the periodic table and its isotopes.

💡 Memory trick

Z = protons (Zee identity); A = All heavy bits (protons + neutrons). Neutrons = A - Z.

ChemistryStructure of the Atommedium

What are isotopes? Give one example.

Reveal answer ↓

What it is

Isotopes are atoms of the same element with the same protons but different numbers of neutrons.

Answer

Isotopes are atoms of the same element that have the same atomic number (the same number of protons) but different mass numbers (different numbers of neutrons). Because they have the same number of protons and electrons, they show the same chemical properties. For example, carbon-12 and carbon-14 are isotopes of carbon.

Same Z, different A

  • •Same element, same protons (same atomic number)
  • •Different neutrons (different mass number)
  • •Same chemical properties
  • •Example: carbon-12 and carbon-14

Why learn this

Isotopes are used in cancer treatment, carbon dating and nuclear energy.

💡 Memory trick

Same element (same protons), different mass (different neutrons). ISO = same place in the table.