CBSE Class 7 Science Advanced Chemistry Atomic Structure and Transformation of Matter Very Short Answer Questions Set 01

Very Short Answer Questions

Question 1. The symbol of calcium is ______.
Answer:
Ca
In simple words: Calcium starts with C like carbon, so a second letter is added: Ca.

Teacher's Note:
1. Carbon is C, so calcium needs two letters.
2. The first letter is capital and the second is small.

Question 2. Magnesium attains an octet configuration by ______ electrons.
Answer:
Magnesium is 2, 8, 2. By losing two electrons it becomes \(\ce{Mg^2+}\) with the stable octet 2, 8.
In simple words: Magnesium finds it easier to give away its 2 outer electrons than to take in 6.

Teacher's Note:
1. Metals reach an octet by losing electrons.
2. Losing the 2 outer electrons exposes the full L shell.

Question 3. The electronic arrangement of an atom with two electrons in the first shell is called ______ if the first shell is the valence shell.
Answer:
A first shell holding two electrons is complete. This is called a duplet configuration (as in helium).
In simple words: Two electrons in the only shell is a duplet, not an octet.

Teacher's Note:
1. The K shell can hold only 2 electrons.
2. Helium is the example of a duplet.

Question 4. Two molecules of sulphur trioxide are indicated by ______.
Answer:
A coefficient written before the formula gives the number of molecules, so two molecules of sulphur trioxide are written as \(\ce{2SO3}\).
In simple words: The big number in front counts molecules; the small number below counts atoms.

Teacher's Note:
1. Sulphur trioxide is SO3.
2. Never change the subscript to show more molecules.

Question 5. ______ is the suffix used while naming the salt of a metal with lower valency.
Answer:
The suffix -ous is used for the lower valency (for example ferrous for \(\ce{Fe^2+}\)) and -ic for the higher valency.
In simple words: Lower valency gets -ous and higher valency gets -ic.

Teacher's Note:
1. Think of ferrous (2) and ferric (3).
2. Cuprous (1) and cupric (2) follow the same rule.

Question 6. Name of the radical \(\ce{PO4^3-}\) is ______.
Answer:
The radical \(\ce{PO4^3-}\) is phosphate.
In simple words: It is the phosphate ion, with charge −3.

Teacher's Note:
1. Do not confuse it with phosphide, \(\ce{P^3-}\).
2. -ate radicals have more oxygen than -ite radicals.

Question 7. Formulae of sodium sulphite and sodium sulphate are respectively ______ and ______.
Answer:
Sodium sulphite is \(\ce{Na2SO3}\) and sodium sulphate is \(\ce{Na2SO4}\).
In simple words: Two \(\ce{Na+}\) ions balance one sulphite or one sulphate ion (charge −2).

Teacher's Note:
1. Use the criss-cross method.
2. Sulphite has 3 oxygens and sulphate has 4.

Question 8. Generally ______ changes are reversible because there is no change in the ______ of the substance.
Answer:
Generally physical changes are reversible because there is no change in the molecular composition of the substance.
In simple words: Physical changes only alter appearance, so the original can be got back.

Teacher's Note:
1. Reversible means the original form can be recovered.
2. Chemical changes are usually permanent.

Question 9. Decomposition of solid ammonium chloride is associated with ______ change.
Answer:
Heating solid \(\ce{NH4Cl}\) first changes it directly into vapour (physical, sublimation), and the vapour then splits into \(\ce{NH3}\) and \(\ce{HCl}\) (chemical). So it is associated with both physical and chemical changes.
In simple words: Both a change of state and a change of composition take place.

Teacher's Note:
1. Name both the sublimation and the decomposition.
2. A single change cannot describe it.

Question 10. The number of atoms of a molecule or compound is balanced on either side of a chemical equation by keeping appropriate ______.
Answer:
The atoms are balanced by keeping appropriate coefficients in front of the formulae. The subscripts are never changed.
In simple words: Put numbers in front of formulae, not inside them.

Teacher's Note:
1. Changing a subscript changes the substance itself.
2. Coefficients only change how many molecules there are.

Teacher's Note:
1. A symbol alone means an atom; a formula with a subscript means a molecule.
2. Na without a charge is an atom, not an ion.

Teacher's Note:
1. Learn the -ic ion as the higher charge: stannic is +4.
2. Nitride is the ion of nitrogen, N³⁻.

Teacher's Note:
1. One product from two reactants is combination.
2. Two reactants swapping partners is double displacement.

Question 14. What is the difference between symbol and formula?
Answer:
A symbol is the short form for one atom of an element, for example Na or Cl.
A formula represents a molecule of an element or a compound and combines symbols with numbers of atoms, for example \(\ce{H2O}\) or \(\ce{O2}\).
In simple words: A symbol is for one atom; a formula is for one molecule.

Teacher's Note:
1. Write one example of each.
2. A formula is built from symbols.

Question 15. Compare the mass of proton, neutron and electron?
Answer:
Proton: 1.67 × 10⁻²⁷ kg, taken as 1 amu.
Neutron: slightly more than a proton, about 1.67 × 10⁻²⁷ kg, also taken as 1 amu.
Electron: 9.1 × 10⁻³¹ kg, which is \( \frac{1}{1837} \) of the mass of a proton.
So the proton and neutron have nearly equal mass, and the electron is far lighter.
In simple words: Protons and neutrons weigh about the same; an electron is about 1837 times lighter.

Teacher's Note:
1. Say that electron mass is negligible.
2. Neutron and proton are both 1 amu.

Question 16. How does the energy of orbits in an atom vary?
Answer:
The energy of the orbits increases as the distance from the nucleus increases. So K has the least energy and the outer shells have more.
In simple words: The farther a shell is from the nucleus, the higher its energy.

Teacher's Note:
1. K < L < M < N in energy.
2. Electrons fill the lower energy shell first.

Question 17. The atomic number and mass number of an element are 13 and 27, respectively. Find the number of neutrons.
Answer:
Neutrons = A − Z = 27 − 13 = 14.
In simple words: Subtract the atomic number from the mass number.

Teacher's Note:
1. Use the formula n = A − Z.
2. This element is aluminium.

Question 18. Define valency.
Answer:
Valency is the number of electrons gained or lost by an atom of an element to attain a stable octet (or duplet) configuration.
In simple words: Valency tells how many electrons an atom gives or takes to become stable.

Teacher's Note:
1. Mention gain or loss of electrons.
2. Octet or duplet is the target.

Question 19. Name two metals that can exhibit variable valency. Also give the names of radicals.
Answer:
Iron: \(\ce{Fe^2+}\) (ferrous) and \(\ce{Fe^3+}\) (ferric).
Copper: \(\ce{Cu+}\) (cuprous) and \(\ce{Cu^2+}\) (cupric).
In simple words: Iron and copper can form ions with two different charges.

Teacher's Note:
1. Other examples are mercury and tin.
2. -ous is the lower valency and -ic the higher.