Which of the following is a naturally occurring radioactive element? MCQ with Answer and Explanation

Explanation:
In 2Na + Cl₂ → 2NaCl, sodium is oxidized (Na → Na⁺ + e⁻) and chlorine is reduced (Cl₂ + 2e⁻ → 2Cl⁻). It involves change in oxidation states. Options A and B are double displacement (no change in oxidation states). Option D is decomposition with no change in oxidation states (Ca: +2 remains +2). Redox reactions involve simultaneous oxidation and reduction.

Explanation:
Bohr's model successfully explains the spectrum of hydrogen and hydrogen-like species (single electron systems like He⁺, Li²⁺). However, it fails to explain the spectra of multi-electron atoms due to electron-electron repulsions and cannot account for the splitting of spectral lines in magnetic (Zeeman) and electric (Stark) fields.

Explanation:
Starch (polysaccharide) is a natural polymer made from glucose units. It is found in plants. Cellulose, proteins, natural rubber are natural polymers. PVC, nylon, Teflon are synthetic polymers. Natural polymers are biodegradable and renewable. Starch is used as food, in paper, and bioplastics. Synthetic polymers are often derived from petrochemicals.