Science — CBSE Class 10 board question

Criteria used by Mendeleev: Mendeleev arranged elements in increasing order of their atomic masses and grouped elements with similar chemical properties in the same vertical column (group).

Isotopes — comparison:

• In Mendeleev's Periodic Table, isotopes of an element would have to be placed in different positions (as they have different atomic masses), which was a drawback.
• In the Modern Periodic Table, isotopes are placed in the same position (same group and period) since elements are arranged by atomic number, which is the same for all isotopes of an element.

• The question has two parts — state the criterion AND compare isotope positions. Allocate roughly one line to Mendeleev's criterion and two lines to the comparison.
• Key terms examiners look for: atomic mass (Mendeleev), similar properties, atomic number (Modern), and the resolution of the isotope problem.
• A common mistake is forgetting to explicitly state that isotopes have the same atomic number but different atomic masses — this is the logical basis for the comparison.