CBSE Class 7 Science Advanced Chemistry Atomic Structure and Transformation of Matter Short Answer Questions Set 02

Short Answer Questions

Question 1. Derive the formulae of the following compounds. (a) Ammonium nitrate (b) Ferrous sulphate
Answer:
(a) Ammonium \(\ce{NH4+}\) and nitrate \(\ce{NO3-}\): both have valency 1. Formula: \(\ce{NH4NO3}\).
(b) Ferrous \(\ce{Fe^2+}\) and sulphate \(\ce{SO4^2-}\): both have valency 2, which cancel. Formula: \(\ce{FeSO4}\).
In simple words: Equal valencies cancel, so one of each radical is enough.

Teacher's Note:
1. Write the charges first.
2. Ferrous is Fe²⁺.

Question 2. How do you distinguish metals and nonmetals with respect to electronic arrangement?
Answer:
Metals have 1 to 3 electrons in the valence shell, which they tend to lose to form cations (example: sodium 2, 8, 1).
Nonmetals have 4 to 7 electrons in the valence shell, which they tend to gain to form anions (example: chlorine 2, 8, 7).
In simple words: Few outer electrons means metal; many outer electrons means nonmetal.

Teacher's Note:
1. Count the valence electrons.
2. Name one metal and one nonmetal.

Question 3. Write the electronic configurations of the following elements: (a) magnesium (b) silicon (c) sulphur (d) argon
Answer:
(a) Magnesium (Z = 12): 2, 8, 2
(b) Silicon (Z = 14): 2, 8, 4
(c) Sulphur (Z = 16): 2, 8, 6
(d) Argon (Z = 18): 2, 8, 8
In simple words: Fill 2 in K, then 8 in L, then the rest in M.

Teacher's Note:
1. Use K = 2, L = 8, M = 8 up to Z = 20.
2. Check that the sum equals Z.

Question 4. Explain the formation of sodium chloride.
Answer:
Sodium (Z = 11) is 2, 8, 1. It loses one electron to form \(\ce{Na+}\) (2, 8), like neon.
Chlorine (Z = 17) is 2, 8, 7. It gains one electron to form \(\ce{Cl-}\) (2, 8, 8), like argon.
The oppositely charged ions attract each other strongly and bind together to form sodium chloride, NaCl.
In simple words: Sodium hands one electron to chlorine, and the two oppositely charged ions stick together.

Teacher's Note:
1. Show both ions and their configurations.
2. Mention the force of attraction.

Question 5. Write the symbols of the following elements. (a) cadmium (b) bismuth (c) silicon (d) argon
Answer:
(a) Cadmium: Cd
(b) Bismuth: Bi
(c) Silicon: Si
(d) Argon: Ar
In simple words: Each symbol is the first letter plus one small letter.

Teacher's Note:
1. Use a small second letter.
2. Si is not S, which is sulphur.

Question 6. Explain the observable change taking place during reaction of iron with the aqueous solutions of \(\ce{H2SO4}\) and \(\ce{CuSO4}\) respectively.
Answer:
With \(\ce{H2SO4}\): iron liberates hydrogen gas, which is seen as bubbles, and the solution turns green because of \(\ce{FeSO4}\). \(\ce{Fe + H2SO4 -> FeSO4 + H2 ^}\)
With \(\ce{CuSO4}\): iron is more reactive than copper, so it displaces copper. The blue solution turns green and brown copper is deposited. \(\ce{Fe + CuSO4 -> FeSO4 + Cu}\)
In simple words: Iron pushes hydrogen out of the acid and copper out of its salt, and the solution turns green.

Teacher's Note:
1. Mention colour changes.
2. Write both equations.

Question 7. While explaining the modern concept of an atom, Jack asked the teacher "why are electrons, assumed to be in continuous motion around the nucleus instead of being stationary?" What could be the explanation given by the teacher?
Answer:
Electrons are negatively charged and the nucleus is positively charged, so the nucleus pulls the electrons towards itself. To balance this attraction, the electrons must keep moving at high speed around the nucleus. If they were stationary, they would be pulled into the nucleus.
In simple words: Motion balances the pull of the nucleus, just as a whirling stone stays in orbit.

Teacher's Note:
1. Mention both the attraction and the motion.
2. Opposite charges attract.

Question 8. A stable neutral atom "X" contains two completely filled orbits. Find out its mass number if it consists of 10 neutrons.
Answer:
Two completely filled shells: K = 2 and L = 8, so the configuration is 2, 8.
Electrons = protons = 10, so Z = 10 (neon).
Mass number = protons + neutrons = 10 + 10 = 20.
In simple words: Two full shells hold 10 electrons, so there are 10 protons; add 10 neutrons.

Teacher's Note:
1. Count the electrons first.
2. A = Z + n.

Question 9. Mass of total positive charge present in an atom is 7348 times to that of the mass of a negatively charged particle. Find the atomic number of an element. Find out its valency.
Answer:
The positive charge is carried by protons. The mass of a proton is 1837 times the mass of an electron.
Number of protons = 7348 ÷ 1837 = 4, so the atomic number is 4 (beryllium).
Configuration 2, 2. It loses 2 electrons to form \(\ce{Be^2+}\), so the valency is 2.
In simple words: Divide by 1837 to get the number of protons, then see how many outer electrons it can lose.

Teacher's Note:
1. Check: 4 × 1837 = 7348.
2. Two valence electrons give valency 2.

Question 10. While coming back from school in the school bus, Bill asked his friend Gill to predict the atoms X and Y which form a stable dinegative ion and a uninegative ion, respectively. They have the same number of electrons in the L shell, which is the shell inner to the valence shell. Predict the answer given by Gill.
Answer:
L is the shell inside the valence shell, so M is the valence shell and L has 8 electrons.
A stable dinegative ion \(\ce{X^2-}\) has 2, 8, 8, so the atom X is 2, 8, 6. Its atomic number is 16, so X is sulphur.
A stable uninegative ion \(\ce{Y-}\) is 2, 8, 8, so the atom Y is 2, 8, 7. Its atomic number is 17, so Y is chlorine.
In simple words: The two ions both have 2, 8, 8, so X is S (2, 8, 6) and Y is Cl (2, 8, 7).

Teacher's Note:
1. Take away the gained electrons from the ion.
2. Both ions have the argon arrangement.

Question 11. A negative ion of an atom of element X has 18 electrons and 16 protons. Calculate the number of electrons and protons in its corresponding atom. Also calculate the number of neutrons, if the mass number is 2 units more than double the number of protons.
Answer:
The ion has 16 protons and 18 electrons, so it has gained 2 electrons (\(\ce{X^2-}\)).
The neutral atom has 16 protons and 16 electrons.
Mass number = 2 × 16 + 2 = 34.
Neutrons = 34 − 16 = 18.
In simple words: Remove the extra electrons to get the atom, then use A − Z for the neutrons.

Teacher's Note:
1. Protons never change when an ion forms.
2. The element is sulphur.

Question 12. If the formula of metal sulphite of a metal M is \(\ce{MSO3}\), give the formulae of its (a) phosphate (b) hydrogen phosphate (c) chloride
Answer:
Sulphite is \(\ce{SO3^2-}\) and one is needed, so M has valency 2 (\(\ce{M^2+}\)).
(a) Phosphate \(\ce{PO4^3-}\): \(\ce{M3(PO4)2}\)
(b) Hydrogen phosphate \(\ce{HPO4^2-}\): \(\ce{MHPO4}\)
(c) Chloride \(\ce{Cl-}\): \(\ce{MCl2}\)
In simple words: Find the valency of M first, then criss-cross with each radical.

Teacher's Note:
1. Brackets are needed around PO4 only in (a).
2. Valencies 2 and 2 cancel in (b).

Question 13. What is the valency of iron in the products formed (a) when iron is exposed to humid air for a long time? (b) when a red hot iron is made to react with steam?
Answer:
(a) Iron rusts to form hydrated ferric oxide, \(\ce{Fe2O3.xH2O}\). The valency of iron is 3.
(b) \(\ce{3Fe + 4H2O -> Fe3O4 + 4H2}\). \(\ce{Fe3O4}\) is a mixed oxide (FeO and \(\ce{Fe2O3}\)), so iron shows valencies 2 and 3.
In simple words: Rust has iron with valency 3; in Fe3O4 iron shows both 2 and 3.

Teacher's Note:
1. Fe₃O₄ is ferroso-ferric oxide.
2. Write the equation for part (b).

Question 14. Give the necessary chemical equations for the following and balance them. (a) reaction of a red hot iron with steam to give \(\ce{Fe3O4}\) (b) burning magnesium ribbon in an atmosphere of nitrogen (c) displacement of copper from the solution of its sulphate by zinc
Answer:
(a) \(\ce{3Fe + 4H2O -> Fe3O4 + 4H2}\)
(b) \(\ce{3Mg + N2 -> Mg3N2}\)
(c) \(\ce{CuSO4 + Zn -> ZnSO4 + Cu}\)
In simple words: Write the formulae, then add numbers in front until each atom count matches.

Teacher's Note:
1. Check the atoms on both sides.
2. Magnesium nitride is Mg₃N₂.