Short Questions & Answers : Chemistry Class 9 - KP Boards - Unit: 1 - FUNDAMENTALS OF CHEMISTRY

Answer briefly the following questions.

i.          How many elements are present in each of the following?

(a). HF and Hf       (b). Co and CO      (c). Si and SiO₂      (d). PoCl₂ and POCl₃

Answer:

        (a) HF have two elements (H & F) while Hf has one element (Hf).

        (b) Co has one element (Co) while CO has two elements (C and O).

       (c) Si has one element (Si) while SiO₂ has two elements (Si and O).

      (d) PoCl2 has two elements (Po and Cl) while POCl₃ has three elements (P, O and Cl).

ii.         Cm is the chemical symbol of curium, named after the famous scientist Madam curie. Why wasn’t the symbol C, Cu or Cr used instead?

Answer:

        It’s because the symbols C, Cu or Cr were already been taken and used for other elements in the periodic table, so for curium, it was decided to use the symbol Cm. The two letters C and m are taken from the initial and final letter of the element’s name.

iii.        What is the atomic number of an element? How does it differ from mass number?

Answer:

        Atomic number represented by Z can be defined as, “The number of protons in the nucleus of an atom of that element or the number of electrons revolving around the nucleus”. While mass number represented by A is, “the sum of the number of protons and neutrons present in the nucleus of an atom of that element.” Mass number is also known as nucleon number.

iv.        Students often mix up the following elements. Give the name of each element.(a). Mg and Mn           (b). K and P                 (c). Na and S                (d). Cu and Co

Answer:

    (a). Mg = Magnesium and Mn = Manganese.

    (b). K = Potassium and P = Phosphorus.

    (c). Na = Sodium and S = Sulfur.

    (d). Cu = Copper and Co = Cobalt.

v.         (a). Classify the following molecules as mono-atomic, diatomic, tri-atomic and polyatomic molecules.

            H₂O, N₂, S₈, He, HCl, CO₂, Ar, H₂SO₄, C₆H₁₂O₆

            (b). Classify the following as cation, anions, molecular ion, free radical and molecules.

            CH₄⁺, O^-2, CH₃^., CO⁺, CO₂, Cl^-, Mg⁺², CO₃^-2, O₂, Na⁺, C₂H₅O^-1, H₂O, Cl₂

Answer:                    (a)	(b)
(a). Mono-atomic molecules: He, Ar	(a). Cations: Mg+2, Na+
(b). Diatomic molecules: N2, HCl	(b). Anions: O-2, Cl-, CO3-2, C2H5O-1
(c). Tri-atomic molecules: H2O, CO2	(c). Molecular ions: CH4+, CO+, CO3-2, C2H5O-1
(d). Polyatomic molecules: S8, H2SO4, C6H12O6	(d). Free radicals: CH3.
	(e). Molecules: CO2, O2, H2O, Cl2