CBSE Class 10 Chemistry Atomic Structure MCQs Set 06

Answer: (a) 22

Explanation:
1. Carbon has 6 electrons and each oxygen atom has 8 electrons.
2. Total = 6 + 2 × 8 = 22 electrons.

Teacher's Note:
1. In a neutral molecule the number of electrons equals the sum of the atomic numbers of all atoms.
2. Do not confuse electrons with the molecular mass, which is 44.

Answer: (b) Chadwick

Explanation:
1. James Chadwick discovered the neutron in 1932 by bombarding beryllium with alpha particles.

Teacher's Note:
1. Remember the trio: Thomson (electron), Rutherford (nucleus and proton), Chadwick (neutron).
2. Neutrons carry no charge and have a mass nearly equal to that of a proton.

Answer: (c) 140

Explanation:
1. Number of neutrons = mass number − atomic number.
2. Neutrons = 226 − 86 = 140.

Teacher's Note:
1. Mass number counts protons plus neutrons together.
2. Subtract the atomic number (protons) to get the neutrons.

Answer: (d) 12 neutrons and 11 electrons

Explanation:
1. 11 positive charges means 11 protons, so the atomic number is 11.
2. The mass number is 23, so neutrons = 23 − 11 = 12.
3. The atom is neutral, so electrons = protons = 11.

Teacher's Note:
1. A neutral atom always has as many electrons as protons.
2. The mass of a hydrogen atom is taken as 1 unit, so '23 times heavier' means mass number 23.

Answer: (a) 2 electrons

Explanation:
1. Calcium has atomic number 20, so its electrons are arranged as 2, 8, 8, 2.
2. The outermost (N) shell therefore holds 2 electrons.

Teacher's Note:
1. Fill shells in the order K (2), L (8), M (8 for the first 20 elements), N.
2. Two outer electrons make calcium a divalent metal that forms Ca²⁺.

Answer: (b) 195

Explanation:
1. For a neutral atom, protons = electrons = 78.
2. Mass number = protons + neutrons = 78 + 117 = 195.

Teacher's Note:
1. Mass number = number of protons + number of neutrons.
2. Electrons are ignored because their mass is negligible.

Answer: (c) 1

Explanation:
1. Molar mass of NH₃ = 14 + 3 = 17 g/mol, so 17 g is 1 mole of NH₃.
2. Each molecule has one nitrogen atom, so there is 1 mole of nitrogen atoms.

Teacher's Note:
1. First convert mass to moles using the molar mass.
2. Then multiply by the number of atoms of the element in one formula unit.

Answer: (d) nucleus of a hydrogen atom

Explanation:
1. The common hydrogen atom has one proton and no neutron, so its nucleus is a single proton.
2. The nucleus of helium is an alpha particle, and the positive counterpart of the electron is the positron.

Teacher's Note:
1. A hydrogen ion H⁺ is nothing but a proton.
2. Protons and electrons have equal and opposite charges but very different masses.

Answer: (a) 18

Explanation:
1. The capacity of the nth shell is 2n².
2. For the M shell n = 3, so the capacity is 2 × 9 = 18.

Teacher's Note:
1. Use 2n² for K, L, M and N: 2, 8, 18 and 32.
2. The outermost shell of any atom never holds more than 8 electrons (2 for the first shell).

Answer: (b) oxygen

Explanation:
1. Total electrons = 2 + 6 = 8, so the atomic number is 8.
2. The element with atomic number 8 is oxygen.

Teacher's Note:
1. Add the electrons in all shells to get the atomic number.
2. Carbon is 2, 4; nitrogen is 2, 5 and sulphur is 2, 8, 6.

Answer: (c) Calcium

Explanation:
1. Atomic numbers: helium 2, aluminium 13, silicon 14, calcium 20.
2. Calcium has the highest.

Teacher's Note:
1. Learn the first twenty elements in order with their atomic numbers.
2. Position in the modern periodic table follows atomic number.

Answer: (d) a small, dense, positively charged nucleus

Explanation:
1. Most alpha particles passed straight through, a few were deflected and very few rebounded.
2. This showed that the positive charge and almost all the mass are packed into a tiny nucleus.

Teacher's Note:
1. The atom is mostly empty space.
2. Rutherford's model had electrons circling a central nucleus.