Which of the following elements had NOT been discovered during Mendeleev’s time?

Correct option is D

The correct answer is (D) Radon and Polonium

Explanation:

• Radon and Polonium were not discovered during Mendeleev’s time. These elements were identified only after the discovery of radioactivity, which occurred in the late 19th century, after Mendeleev published his periodic table in 1869.
• Polonium was discovered by Marie and Pierre Curie in 1898, and Radon was discovered by Friedrich Ernst Dorn in 1900.
• Mendeleev predicted the existence of several unknown elements and left gaps in his periodic table for them, such as Gallium and Germanium, which were later discovered, confirming the accuracy of his predictions.

Information Booster:

• Chlorine and Fluorine:
• Chlorine was discovered in 1774 by Carl Wilhelm Scheele.
• Fluorine was isolated in 1886 by Henri Moissan, but its compounds were known earlier.
• Hydrogen and Nitrogen:
• Hydrogen was discovered in 1766 by Henry Cavendish.
• Nitrogen was discovered in 1772 by Daniel Rutherford.
• Gallium and Germanium:
• Gallium was discovered in 1875 by Paul Émile Lecoq de Boisbaudran.
• Germanium was discovered in 1886 by Clemens Winkler.
• These discoveries matched Mendeleev’s predictions for eka-aluminium and eka-silicon.
• Mendeleev's Contribution:
• Published the first periodic table in 1869.
• Arranged elements based on atomic mass and chemical properties.
• Left gaps for undiscovered elements and predicted their properties with remarkable accuracy.