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VIII - EM -SCIENCE - ATOMIC STRUCTURE - EVALUATION - QUESTION - ANSWERS
ATOMIC STRUCTURE
TEXTBOOK EXERCISES
1. Choose the best answer.
1. The same proportion of carbon and oxygen in the carbon dioxide obtained from different sources proves the law of __________
a) reciprocal proportion
b) definite proportion
c) multiple proportion
d) conservation of mass
Ans: b) definite proportion
2. Cathode rays are made up of
a) neutral particles
b) positively charged particles
c) negatively charged particles
d) None of the above
Ans: c) negatively charged particles
3. In water, hydrogen and oxygen are combined in the ratio of _______by mass.
a) 1:8
b) 8:1
c) 2:3
d) 1:3
Ans: a) 1:8
4. Which of the following statements made by Dalton has not undergone any change?
a) Atoms cannot be broken.
b) Atoms combine in small, whole numbers to form compounds.
c) Elements are made up of atoms.
d) All atoms of an elements are alike
Ans: c) Elements are made up of atoms.
5. In all atoms of an element
a) the atomic and the mass number are same.
b) the mass number is same and the atomic number is different.
c) the atomic number is same and the mass number is different
d) both atomic and mass numbers may vary.
Ans: c) the atomic number is same and the mass number is different
II. Fill in the blanks.
1. ____________ is the smallest particle of an element.
Ans: Atom
2. An element is composed of ____________ atoms.
Ans: only one type of
3. An atom is made up of ____________, ____________ and ____________.
Ans: electrons, protons and neutrons
4. A negatively charged ion is called ____________, while positively charged ion is called ____________.
Ans: anion, cation
5. ____________ is a negatively charged particle (Electron/Proton).
Ans: Electron
6. Proton is deflected towards the _________ charged plate (positively, negatively).
Ans: negatively
III. Match the following.
Ans:
IV. Answer briefly.
1. State the law of conservation of mass.
It 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.
It states that in a pure chemical compound the elements are always present in definite 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 deflected by electric and magnetic fields. Since, they are deflected 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.
Since a hydrogen atom loses one electron in its outermost orbit, its valency is taken as one and it is selected as the standard.
Thus, valency of an element can also be defined 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.
Every chemical equation has two components:
Reactants and products.
Reactants are the substances that take part in a chemical reaction and
the products are the substances that are formed in a chemical reaction.
7. Write the names of the following compounds.
a) CO b) N2O c) NO2 d) PCl5
Ans:
V. Answer the following.
1. Find the valency of the element which is underlined in the following formula.
a) NaCl b) CO2 c) Al (PO4) d) Ba (NO3)2 e) CaCl2
Ans:
2. Write the chemical formula for the following compounds
a) Aluminium sulphate b) Silver nitrate c) Magnesium oxide d) Barium chloride
Ans:
3. Write the skeleton equation for the following word equation and then balance them.
a) Carbon + Oxygen → Carbon dioxide
b) Phosphorus + Chlorine → Phosphorus pentachloride.
c) Sulphur + Oxygen → Sulphur dioxide
d) Magnesium + hydrogen chloride → Magnesium chloride + Hydrogen
Ans:
a) C + O2 → CO2
b) P + Cl2 → PCl5
2P + 5Cl2 → 2PCL5
c) S + O2 → SO2
d) Mg + HCl → MgCl + H2
Mg + 2HCl → MgCl2 + H2
4. Balance the following chemical equation.
a) Na + O2 → Na2O
b) Ca + N2 → Ca3N2
c) N2 + H2 →NH3
d) CaCO3 +HCl → CaCl2 + CO2 +H2O
e) Pb(NO3)2 → PbO + NO2 + O2
Ans:
a) 4Na + O2 → 2Na2O
b) 3Ca + N2 → Ca3N2
c) N2 + 3H2 → 2NH3
d) CaCO3 + 2HCl → CaCl2 + CO2 + H2O
e) 2Pb(NO3)2 → 2PbO + 4NO2 + O2
VI. Higher Order Thinking Questions.
1. Why does a light paddle wheel placed in the path of cathode rays begin to rotate, when cathode rays fall on it?
A light paddle wheel placed in the path of cathode rays begins to rotate, because the cathode rays are made up of material particles, whicha have mass and kinetic energy.
2. How can we prove that the electrons carry negative charge?
When an electric field is applied on the cathode rays(which are made up of electrons), they are deflected towards the positive plate of the electric field. This proves that they carry negative charge.
3. Ruthresh, Hari, Kanishka and Thahera collected different samples of water from a well, a pond, a river and underground water. All these samples were sent to a testing laboratory. The test result showed the ratio of hydrogen to oxygen as 1:8. a) What conclusion would you draw from the above experiment?
b) Which law of chemical combination does it obey?
Water is a chemical compound, which always contains two elements namely Hydrogen and Oxygen with the ratio of 1:8.
It obeys the law of Constant proportion, which states that a pure chemical compound the elements are always present in definite proportions by mass
