Chemistry · Periodic Classification Of Properties · NEET
The Modern Periodic Law states: "The physical and chemical properties of the elements are periodic functions of their atomic numbers." This is the NCERT wording. Learn it word for word. NEET sometimes gives a slightly wrong version as a trap, so the key words are properties, periodic function, and atomic numbers (not atomic weights).
Mendeleev's law used atomic weight: "properties are a periodic function of their atomic weights." The Modern Periodic Law changed only one word - it uses atomic number instead of atomic weight. So the shape of the idea is the same (properties repeat), but the basis (the thing you arrange by) is different. Just changing weight to number fixed several problems in Mendeleev's table.
In 1913 Henry Moseley shot X-rays at elements. He plotted the square root of the X-ray frequency against atomic number and got a straight line. The same plot against atomic mass was NOT a straight line. This proved that atomic number (Z), the number of protons, is the more fundamental property of an element. So Mendeleev's law was modified to use atomic number, giving the Modern Periodic Law.
"Periodic function" means the property does not just rise or fall forever - it comes back to a similar value again and again at regular gaps. Example: alkali metals Li, Na, K, Rb all behave alike. As Z increases you keep meeting a new alkali metal at fixed intervals. So the property is a repeating (periodic) pattern of Z, not a one-way change. This repetition is what the word periodic captures.
The Modern Periodic Law itself is a modification of Mendeleev's law, made after Henry Moseley's 1913 X-ray work showed atomic number is fundamental. So NCERT credits Moseley's experiment as the reason the law was changed. Mendeleev gave the original periodic law; Moseley's finding led to the modern (atomic-number) version. NEET usually links "atomic number as basis" with Moseley.
Yes. In Mendeleev's table a few pairs like Ar-K, Co-Ni and Te-I looked out of order because they were placed by atomic weight. When you order by atomic number instead, these pairs sit correctly (K > Ar in Z, I > Te in Z). So the Modern Periodic Law automatically removes the anomalous-pair problem. That is one of its main advantages for NEET.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Atomic number. The whole point of the modern law is that atomic number (Z), the proton count, is the fundamental property. Atomic mass belongs to Mendeleev's older law.
Because atomic number equals the number of protons, which fixes the element's identity and its electronic configuration. Atomic mass can vary due to isotopes and can even fall out of order (like Ar and K), so it is less reliable as a basis.
Yes. It is a direct one-mark fact. NEET tests the exact wording, the Mendeleev-vs-Modern difference, and links it to Moseley's X-ray experiment. Memorising the statement and the reason for the change is easy free marks.
Helium is placed with the noble gases (Group 18) because of its properties, even though its configuration is 1s2. The law arranges by atomic number, but final placement also respects repeating chemical behaviour, which is why He sits in the p-block noble-gas column.
