8th Science Guide Unit 12 Atomic Structure

8th Std Science Guide Lesson 12 Atomic Structure | English Medium

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8th Science Guide Unit 12 Atomic Structure

I. Choose the correct answers:

1. The same proportion of carbon and oxygen in the carbon dioxide obtained from diferent sources proves the law of __________

1. reciprocal proportion
2. defnite proportion
3. multiple proportion
4. conservation of mass

Ans : Supports burning

2. Cathode rays are made up of

1. neutral particles
2. positively charged particles
3. negatively charged particles
4. None of the above

Ans : carbon dioxide

3. In water, hydrogen and oxygen are combined in the ratio of _______by mass.

1. 1:8
2. 8:1
3. 2:3
4. 1:3

Ans : sodium carbonate

4. Which of the following statements made by Dalton has not undergone any change?

1. Atoms cannot be broken.
2. Atoms combine in small, whole numbers to form compounds.
3. Elements are made up of atoms.
4. All atoms of an elements are alike

Ans : blue litmus to red

5. In all atoms of an elemen

1. the atomic and the mass number are same.
2. the mass number is same and the atomic number is different.
3. the atomic number is same and the mass number is different
4. both atomic and mass numbers may vary.

Ans : Nitrogen

II. Fill in the blanks.

1. ________ is the smallest particle of an element. Ans : oxygen
2. An element is composed of ____________ atoms. Ans : lighter
3. An atom is made up of ____________, ____________ and ____________. Ans : nitrogen
4. A negatively charged ion is called ____________, while positively charged ion is called ____________

Ans : refrigerant

5. ____________ is a negatively charged particle (Electron/Proton). Ans : rusting
6. Proton is defected towards the _________ charged plate (positively, negatively). Ans : rusting

III.Match the following

1. Law of conservation of mass – Sir William Crookes
2. Law of constant proportion – James Chadwick
3. Cathode rays – Joseph Proust
4. Anode rays – Lavoisier
5. Neutrons – Goldstein

Ans : 1 – b, 2 – a, 3 – d, 4 – c

IV. Answer briefly.

1. State the law of conservation of mass.

• The law states that during any chemical change, the total mass of the products is equal to the total mass of the reactants.

2. State the law of constant proportions.

• the law of constant proportions that in a pure chemical compound the elements are always present in defnite proportions by mass.

3. Write the properties of anode rays.

• Anode rays travel in straight lines.
• Anode rays are made up of material particles.
• Anode rays are defected by electric and magnetic felds. Since, they are defected towards the negatively charged plate, they consist of positively charged particles.

• The properties of anode rays depend upon the nature of the gas taken inside in the discharge tube.

4. Define valency of an element with respect to hydrogen.

• valency of an element can is defned as the number of hydrogen atoms which combine with one atom of it.

5. Define the term ions or radicals.

• An atom or a group of atoms when they either lose or gain electrons, get converted into ions or radicals

6. What is a chemical equation?

• A chemical equation is a short hand representation of a chemical reaction with the help of chemical symbols and formulae.

7. Write the names of the following compounds.

1. a) CO

• Carbon monoxide

1. b) N2O

• Nitrous oxide

1. c) NO2

• Nitrogen di oxide

1. d) PCl5

• Phosphorous penta chloride

V. Answer the following.

1. Find the valency of the element which is underlined in the following formula.

1. a) NaCl

• NaCl – 1

1. b) CO2

• CO2 – 4

1. c) Al (PO4)

• Al (PO4) – 3

1. d) Ba (NO3)2

• Ba (NO3)2 – 2

1. e) CaCl2

• CaCl2 – 2

2. Write the chemical formula for the following compounds

1. a) Aluminium sulphate

• Al2 (SO4)3

1. b) Silver nitrate

• BaCl2

1. c) Magnesium oxide

• AgNO3

1. d) Barium chloride

• MgO

3. Write the skeleton equation for the following word equation and then balance them.

1. a) Carbon + Oxygen → Carbon dioxide

• C +  O2 →  CO2

1. b) Phosphorus + Chlorine → Phosphorus pentachloride.

• P + Cl2 →   PCl5

1. c) Sulphur + Oxygen → Sulphur dioxide

• S +  O2 →  SO2

1. d) Magnesium + hydrogen → Magnesium + Hydrogen chloride chloride

• Mg + 2HCl →   MgCl2 + H2

4. Balance the following chemical equation.

1. a) Na + O2 → Na2O

• 4Na + O2 → 2Na2O

1. b) Ca + N2 → Ca3N2

• 3Ca + N2 → Ca3N2

1. c) N2 + H2 →NH3

• N2 + 3H2 → 2NH3

1. d) CaCO3 +HCl → CaCl2 + CO2 +H2O

• CaCO3 + 2HCl → CaCl2 + CO2 +H2O

1. e) Pb(NO3)2 → PbO + NO2 + O2

• 2Pb(NO3)2 → 2PbO + NO2 + O2