Structure of Atom — Important Questions

Electrons are embedded in a uniform sphere of positive charge

Step 1: J.J. Thomson proposed his atomic model in 1898. Step 2: He described the atom as a positively charged sphere (radius ~10⁻¹⁰ m) in which positive charge is uniformly distributed. Step 3: Electrons are embedded in this sphere like plums in a pudding, which is why it is called the 'plum pudding model'. Step 4: This model explained overall electrical neutrality of the atom. Step 5: However, it was disproved by Rutherford's alpha-particle scattering experiment. The other options describe Rutherford's model, a misconception, and the quantum mechanical model respectively.

A very few alpha particles (~1 in 20,000) bounced back at nearly 180°

Step 1: Rutherford directed a beam of alpha particles at a thin gold foil. Step 2: Most alpha particles passed straight through, showing the atom is mostly empty space. Step 3: A small fraction was deflected by small angles due to repulsion from positive charges. Step 4: A very few (~1 in 20,000) bounced back at nearly 180° – this was the key surprise. Step 5: This 'head-on' collision was only possible if a very small, dense, positively charged region (the nucleus) existed at the centre of the atom. If positive charge were spread out (as in Thomson's model), no such large-angle deflection woul

45

Step 1: The notation ⁸⁰₃₅Br tells us: mass number A = 80, atomic number Z = 35. Step 2: Atomic number Z = number of protons = 35. Step 3: Using the formula: Number of neutrons = A − Z. Step 4: Number of neutrons = 80 − 35 = 45. Step 5: The option '35' is the number of protons (= electrons for a neutral atom), '80' is the mass number, and '115' is the incorrect sum instead of difference.

Isobars

Step 1: The prefix 'iso' means same, 'bar' relates to mass (like barometer measures pressure/weight). Step 2: Isobars: same mass number (A), different atomic number (Z). Example: ¹⁴₆C and ¹⁴₇N both have A = 14. Step 3: Isotopes: same atomic number (Z), different mass number (A). Example: ¹H, ²H (deuterium), ³H (tritium) all have Z = 1. Step 4: Isotones: same number of neutrons but different atomic numbers. Step 5: Allotropes are different physical forms of the same element (like diamond and graphite for carbon) – not related to nuclear composition.