CBSE Class 7 Science Advanced Chemistry Atomic Structure and Transformation of Matter Very Short Answer Questions Set 03

Very Short Answer Questions

Question 1. State whether the following statement is true or false: The central part of the atom called nucleus accounts for the mass of an atom.
Answer:
True. The protons and neutrons in the nucleus carry almost all the mass, because electrons are very light.
In simple words: The nucleus has nearly all the weight of an atom.

Teacher's Note:
1. Electrons weigh 1/1837 of a proton.
2. Mass number = protons + neutrons.

Question 2. State whether the following statement is true or false: Both protons and neutrons possess the same magnitude of charge but opposite signs.
Answer:
False. Protons carry +1 but neutrons carry no charge. It is protons and electrons that have equal and opposite charges.
In simple words: Neutrons are neutral, so they cannot have an opposite charge.

Teacher's Note:
1. Say which particles do have equal and opposite charges.
2. Neutral means zero charge.

Question 3. State whether the following statement is true or false: The maximum number of electrons in the L shell is two.
Answer:
False. The L shell (n = 2) can hold \( 2n^2 = 8 \) electrons. It is the K shell that holds two.
In simple words: L holds 8 and K holds 2.

Teacher's Note:
1. Use 2n² for any shell.
2. K: 2, L: 8, M: 18.

Question 4. State whether the following statement is true or false: An atom is electrically neutral.
Answer:
True. An atom has equal numbers of protons and electrons, so the charges cancel.
In simple words: Equal positive and negative charges cancel.

Teacher's Note:
1. Protons = electrons.
2. Ions are charged, atoms are not.

Question 5. State whether the following statement is true or false: In phosphide ion, the total number of electrons is equal to 12.
Answer:
False. Phosphorus has 15 electrons and gains 3 to form \(\ce{P^3-}\), so the phosphide ion has 15 + 3 = 18 electrons.
In simple words: The ion has 18 electrons, not 12.

Teacher's Note:
1. A negative ion has gained electrons.
2. Phosphide ion has the argon configuration 2, 8, 8.

Question 6. The symbol of cobalt is ________.
Answer:
Co
In simple words: Cobalt is Co; the capital C alone is carbon.

Teacher's Note:
1. Co is not the same as CO, which is carbon monoxide.
2. Two-letter symbols have a small second letter.

Question 7. The mass of an electron is ________ times that of mass of a proton.
Answer:
\( \frac{1}{1837} \)
In simple words: An electron is about 1837 times lighter than a proton.

Teacher's Note:
1. Proton mass is 1837 times electron mass.
2. That is why electron mass is negligible.

Question 8. The maximum number of electrons that can be accommodated in the outermost orbit is equal to ________.
Answer:
8
In simple words: No atom has more than 8 electrons in its outermost shell.

Teacher's Note:
1. Octet means eight.
2. Helium is the duplet exception.

Question 9. The valency of argon is ________.
Answer:
zero Argon has a complete octet, 2, 8, 8, so it does not gain or lose electrons.
In simple words: Argon is stable, so its valency is zero.

Teacher's Note:
1. All noble gases have valency zero.
2. A complete outer shell means no valency.

Question 10. In the formation of sodium chloride, each chlorine gains ________ electron.
Answer:
one Chlorine (2, 8, 7) gains one electron to become \(\ce{Cl-}\) (2, 8, 8).
In simple words: Chlorine takes one electron from sodium.

Teacher's Note:
1. Sodium loses the electron that chlorine gains.
2. Write both ions.

Question 11. The formula of a zincate radical is ________.
Answer:
\(\ce{ZnO2^2-}\)
In simple words: Zincate has one zinc, two oxygens and charge −2.

Teacher's Note:
1. Remember the charge as well.
2. Chromate is CrO4²⁻, zincate is ZnO2²⁻.

Question 12. The combining capacity of the metal which can exhibit more than one valency is called ________.
Answer:
variable valency
In simple words: A metal with two or more possible charges has variable valency.

Teacher's Note:
1. Iron and copper are examples.
2. Give an example in the answer if asked.

Question 13. Blackening of silverware is a ________ change.
Answer:
chemical Silver reacts with sulphur compounds in air to form black silver sulphide, a new substance.
In simple words: A new black substance forms, so the change is chemical.

Teacher's Note:
1. Tarnishing makes a new compound.
2. It is a harmful chemical change.

Question 14. \(\ce{2HOCl -> 2HCl + O2}\) is a ________ decomposition reaction.
Answer:
photolytic HOCl breaks down in the presence of light.
In simple words: Light breaks HOCl down, so it is photolytic.

Teacher's Note:
1. Photo means light.
2. Thermal needs heat; electrolytic needs electricity.

Question 15. Atomic number of a metal having 12 neutrons and mass number 23 is ________.
Answer:
11 Atomic number = mass number − neutrons = 23 − 12 = 11 (sodium).
In simple words: Subtract the neutrons from the mass number.

Teacher's Note:
1. Z = A − n.
2. Sodium has Z = 11.

Teacher's Note:
1. Na⁺ is an ion, Na is an atom.
2. Subscript 2 means a molecule of two atoms.

Teacher's Note:
1. Stannous is the lower valency of tin, +2.
2. Bi- radicals have charge −1.

Teacher's Note:
1. Zn is more active than Fe.
2. AgCl is the precipitate.