Test: Structure Of The Atom- 1 - Class 9 MCQ

Rutherford's scattering formula fails for very small scattering angles because due to the strong repulsive force present between the α-particles and the nucleus and also the full nuclear charge of the target atom is partially screened by its electrons.

Electronic configuration of ¹²Mg atom = 2, 8, 2 

Number of protons in Mg atom = 2+ 8 + 2 = 12

Number of neutrons in Mg atom = 24 -12 = 12
[as mass number of Mg atom = 24 and number of neutrons = mass number – number of protons]

Therefore, option (d) is the correct answer.

When alpha particles are sent through a thin metal foil, most of them go straight through the foil because most part of the atom is empty space. In other words, there is no obstruction in the path of alpha particles.

Neils Bohr proposed that electrons revolve around the nucleus only in specific allowed orbits called discrete orbits or energy levels.

- Neutrons:
- Neutrons are subatomic particles found in the nucleus of an atom.
- They have no electric charge, making them neutral.
- Neutrons help stabilize the nucleus by reducing repulsive forces between positively charged protons.

- Protons:
- Positively charged particles located in the nucleus.

- Electrons:
- Negatively charged particles found outside the nucleus.

- Nucleons:
- A term for particles in the nucleus, including both protons and neutrons.

Points (ii) and (iv) are correct. An atom consists of a positively charged, dense and very small nucleus which have all the protons and neutrons. Positive charge is due to protons, as neutrons have no charge. Most of the space is empty because most of the alpha particles pass straight through the gold foil without any deflection.
Electrons have negative charge, it was explained by Thomson. The existance of neutron was discovered by Chadwick.

- The atomic number of an element is defined as the number of protons in the nucleus of an atom.
- Each element has a unique atomic number, which determines its identity and position in the periodic table.
- Protons are positively charged particles found in the nucleus.
- Electrons orbit the nucleus, but their number equals the atomic number only in neutral atoms.
- Neutrons are also in the nucleus but do not affect the atomic number.

- Isotopes Definition: Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons.

- Atomic Number: This is the number of protons in an atom's nucleus, which defines the element.

- Mass Number: This is the total number of protons and neutrons in an atom's nucleus.

- Correct Answer Explanation:
- Choice B describes isotopes accurately: they have different mass numbers due to varying neutrons, while their atomic number (protons) remains the same.

- Isobars are atoms with different elements.
- They have:
- Different atomic numbers (number of protons).
- The same mass number (sum of protons and neutrons).
- Therefore, the correct answer is:
- C: The same mass number but different atomic numbers.
- Isobars differ in chemical properties due to different proton numbers but have similar overall nuclear mass.

- The correct answer is B: Law of Definite Proportions.
- This law, also known as Proust's Law, states that a chemical compound always contains its component elements in a fixed ratio by mass.
- For example, water (H₂O) always contains hydrogen and oxygen in a mass ratio of about 1:8.