Atomic Number MCQs Quiz | Class 9
This quiz covers Class IX Science (Code 086), Unit I: Matter – Its Nature and Behaviour. It specifically targets the concept of Atomic Number, including its definition and relation to protons and electrons. Solve these 10 multiple-choice questions to test your understanding, then submit to view your score and download the solution PDF.
Topic Overview: Atomic Number
In the study of Matter – Its Nature and Behaviour (Class IX Science, Unit I), the concept of Atomic Number is fundamental to understanding the structure of an atom. The atomic number defines the identity of an element and distinguishes one element from another.
1. Definition
The Atomic Number of an element is defined as the total number of protons present in the nucleus of an atom. It is denoted by the symbol Z.
Mathematically: Z = Number of Protons
2. Relation to Electrons
Atoms are electrically neutral. This means the total positive charge (from protons) must equal the total negative charge (from electrons). Therefore, in a neutral atom:
- Number of Protons = Number of Electrons
- Atomic Number (Z) = Number of Protons = Number of Electrons
Note: If an atom loses or gains electrons to become an ion, the number of electrons changes, but the Atomic Number (Z) remains exactly the same because the number of protons in the nucleus does not change during chemical reactions.
3. First 10 Elements Reference Table
Below is a list of the first few elements with their atomic numbers:
| Element | Symbol | Atomic Number (Z) | Protons | Electrons (Neutral) |
|---|---|---|---|---|
| Hydrogen | H | 1 | 1 | 1 |
| Helium | He | 2 | 2 | 2 |
| Lithium | Li | 3 | 3 | 3 |
| Carbon | C | 6 | 6 | 6 |
| Oxygen | O | 8 | 8 | 8 |
4. Key Points to Remember
- The atomic number is always a whole number.
- No two different elements can have the same atomic number.
- Isotopes are atoms of the same element; they have the same atomic number (Z) but different mass numbers.
- The atomic number is usually written as a subscript to the left of the element symbol (e.g., in 6C, 6 is the atomic number).
5. Extra Practice Questions
Try answering these mentally to solidify your knowledge:
- If an atom has 17 protons, what is its atomic number? (Ans: 17, Chlorine)
- Why does the atomic number not depend on neutrons? (Ans: Neutrons have no charge and do not define element identity.)
- If Magnesium (Mg) has Z=12, how many electrons are in the Mg2+ ion? (Ans: 10 electrons)
- Who introduced the concept of atomic number based on X-ray experiments? (Ans: Henry Moseley)
- Can the atomic number be a fraction? (Ans: No, you cannot have half a proton.)
Content created and reviewed by the CBSE Quiz Editorial Team based on the latest NCERT textbooks and CBSE syllabus. Our goal is to help students practice concepts clearly, confidently, and exam-ready through well-structured MCQs and revision content.