Railway Science (Chemistry-Structure of Atom) (Part-II)

Total Questions: 50

31. Fill in the blank with the most appropriate option. [RRB ALP Tier - I (21/08/2018) Evening]

An element attains stability when it achieves _______ state.

Correct Answer: (d) a duplet or octet
Solution:

An octet configuration is that the outermost orbit/ Valence shell of electrons has 8 electrons in it and the duplet has 2 electrons in its outermost orbit/Valence shell. Example - Water (H₂O) in which oxygen gets its octet configuration and Hydrogen gets its duplet configuration.

32. If the electronic configuration of elements A' and B' are 1s² , 2s² 2p⁶, 3s¹ and 1s² , 2s² 2p⁶ , 3s² 3p⁴ respectively, then the formula of the compound formed by the combination of these elements will be: [RRB ALP Tier - I (21/08/2018) Evening]

Correct Answer: (b) A2B
Solution:

Electronic configuration - It illustrates how electrons can be distributed in atomic orbitals. Configuration of electrons involves three terms - Energy level, Orbital type, No. of electrons present in the orbital. 1s¹ - 1 in front of the s denotes the energy level. s indicates the type of orbital. Power 1 represents the no. of electrons in the orbital.

33. If the valency of Aluminum is 3 and that of Oxygen is 2, then the chemical formula of Aluminum oxide is: [RRB ALP Tier - I (29/08/2018) Morning]

Correct Answer: (d) Al₂O₃
Solution:

The valency is the number of the electrons that an atom gains, loses or even shares during a chemical reaction. Metal oxides like aluminium oxide, zinc oxide (ZnO) etc., show both acidic as well as basic behavior. Amphoteric Oxides: The metal oxides which react with both acids as well as bases to produce salts and water. Aluminum (Al) - Atomic number - 13, Group - 13. Oxygen (O), Atomic number - 8, Group - 16.

34. What is the radius of an atom used to measure? [RRB ALP Tier - I (29/08/2018) Morning]

Correct Answer: (d) Atomic size
Solution:

It depends on the number of electron orbits in the atom. Atomic radius (measured in Angstroms (Å)) is the distance between an atom's nucleus and its valence electrons. Atomic radius is used to determine the size of an atom. Atomic number (Z) :
The number of protons in the nucleus of an atom is the atomic number of that element. Atomic mass of an element is equal to the sum of protons and neutrons. Atomicity refers to the number of atoms present in a single molecule of an element, substance or compound (Atomicity of H₂O is 3 :- 2 hydrogen atoms and 1 oxygen atom).

35. Select the option that correctly matches the contents of the first column with the contents of the second column. [RRB ALP Tier - I (31/08/2018) Morning]

Correct Answer: (b) A-III, B-I, C-IV, D-II
Solution:

Dalton's Atomic Theory: All atoms of a specific element are identical in mass, size, and other properties. Compounds are combinations of two or more different types of atoms. All matter is made of atoms, which are indivisible. Law of constant proportions : a chemical compound always contains the same elements combined together in the same proportion by mass. Ions : a chemical species which holds a positive or negative charge of some magnitude. The ions with a negative charge are called anions and the ones with a positive charge are called cations.

36. An element A forms an oxide with formula A₃O₄. What is the valency of element A? [RRB ALP Tier - I (31/08/2018) Morning]

Correct Answer: (b) 4
Solution:

Valency : The combination power of an element or Valency is the number of atoms of a particular element that is combined with one atom of another element to form a molecule.How to determine the valency of an atom: The Maximum number of electrons in a valence shell can be 8.

Hence the number of electrons in a valence shell can vary from 1 to 8. If an atom has 1, 2, 3 numbers of valence electrons. It is easier for them to lose 1,2 or 3 e- respectively from their valence to achieve an octet. (Since now the previous shell would be its valence shell, containing 8 electrons). Valency of atoms losing 1 electrons = 1, valency of atoms losing 2 electrons = 2, valency of atoms losing 3 electrons = 3.

37. The mass number of an isotope of an element is 298. If its nucleus has 189 neutrons, what is its atomic number ? [RRB Group D 17/09/2018 (Afternoon)]

Correct Answer: (c) 109.0
Solution:

Atomic number = Number of protons present in the atom.
Mass number = Proton + Neutron. Hence, 298 = Proton + 189
∴ Proton = 298 - 189 = 109.

38. How many moles of sulphuric acid are present in 25 gms ? [RRB Group D 17/09/2018 (Evening)]

Correct Answer: (a) 0.255
Solution:

Given: Mass of sulfuric acid = 25 grams.
Molecular mass of sulphuric acid (H₂SO₄)
is = 2 (1) + (32) + 4 (16) = 98 u.
Number of moles = 𝑀𝑎𝑠𝑠(𝑔)/𝑀𝑜𝑙𝑎𝑟 𝑚𝑎𝑠𝑠 (𝑔/𝑚𝑜𝑙) ⇒ 25/98 = 0.255.

39. What is the number of electrons that can be placed in the L orbital ? [RRB Group D 19/09/2018 (Evening)]

Correct Answer: (c) 8
Solution:

The maximum number of electrons present in a shell is given by the formula 2n² , where ‘n’ is the orbit number or energy level index, 1,2,3,… .
Hence, the maximum number of electrons in different shells are as follows: First orbit or K-shell = 2 × 1² = 2, second orbit or L-shell = 2 × 2²  = 8, third orbit or M-shell = 2 × 3²  = 18, fourth orbit or N-shell = 2 × 4² = 32, and so on.

40. The atomic number of Potassium is 19 and its mass number is 39. The number of neutrons in its atom is ____________ . [RRB Group D 20/09/2018 (Afternoon)]

Correct Answer: (c) 20
Solution:

As we know, Number of Neutron = Mass number - Atomic Number
(∵ Number of Proton = Atomic Number) According to the question,
Number of neutrons = 39 - 19 = 20.