Atomic Structure — Class 8 Science (Samacheer Kalvi)
TN State Board (Samacheer Kalvi) Class 8 Science, Chemistry — Chapter 12. Inside the atom — particles, laws and valency.
1. About this chapter
This chapter covers the law of definite proportion, cathode rays, the subatomic particles, atomic number and mass number, valency, ions, and writing chemical formulae.
2. Laws of chemical combination
- Law of definite proportion: a compound always contains the same elements combined in the same proportion by mass, whatever its source. For example, carbon dioxide from any source has the same ratio of carbon and oxygen; in water, hydrogen and oxygen combine in the ratio 1 : 8 by mass.
3. Subatomic particles
| Particle | Charge | Location | Note |
|---|---|---|---|
| Proton | +1 | nucleus | — |
| Neutron | 0 | nucleus | — |
| Electron | −1 | shells | cathode rays are streams of electrons |
- Atomic number (Z) = number of protons. Mass number (A) = protons + neutrons.
- Electrons fill shells (K, L, M…) holding 2, 8, 8… electrons.
4. Valency, ions and formulae
- Valency is the number of hydrogen atoms that combine with (or replace) one atom of an element (e.g., oxygen valency 2 in H₂O).
- Ions: a cation (+) forms by losing electrons; an anion (−) forms by gaining electrons.
- Chemical formula: cross-multiply the valencies, e.g. Na (1) and O (2) → Na₂O.
5. Worked examples
Example 1. What is the ratio of hydrogen to oxygen by mass in water? 1 : 8.
Example 2. Cathode rays are made up of which particles? Negatively charged electrons.
Example 3. Write the formula of magnesium chloride (Mg valency 2, Cl valency 1). Cross-multiply → MgCl₂.
6. Book-back questions (Samacheer Kalvi)
I. Choose the correct answer
- The same proportion of carbon and oxygen in carbon dioxide from any source proves the law of — (a) conservation of mass / (b) definite proportion. Ans: (b) definite proportion.
- Cathode rays are made up of — (a) negatively charged particles / (b) positive particles. Ans: (a) negatively charged particles.
- In water, hydrogen and oxygen are combined in the ratio — (a) 1 : 16 / (b) 1 : 8 by mass. Ans: (b) 1 : 8.
- The atomic number of an atom represents the number of — (a) neutrons / (b) protons. Ans: (b) protons.
II. Fill in the blanks 5. A positively charged ion is called a cation and a negatively charged ion is an anion. 6. An electron is a negatively charged particle. 7. The valency of an element is the number of hydrogen atoms that combine with one atom of it.
7. Common mistakes
- Mistake: Counting electrons for the atomic number. Fix: Atomic number = number of protons.
- Mistake: Saying cathode rays are positive. Fix: Cathode rays are negatively charged (electrons).
- Mistake: Forgetting the 1 : 8 ratio in water. Fix: In water, H : O = 1 : 8 by mass (law of definite proportion).
8. Quick revision
- Chemistry Ch 12 · laws, particles, valency, formulae.
- Law of definite proportion: fixed ratio by mass (water H : O = 1 : 8).
- Cathode rays = electrons (negative); proton (+) and neutron (0) in nucleus.
- Z = protons; A = protons + neutrons; shells 2, 8, 8.
- Valency = combining hydrogen atoms; cation (+) loses e⁻, anion (−) gains; formula by cross-multiplying valencies.
