Mendeleev's Periodic Law vs Modern Periodic Law: The Key Difference
Chemistry · Periodic Classification Of Properties · NEET
Mendeleev's Periodic Law says element properties repeat based on their atomic mass (weight). The Modern Periodic Law says properties repeat based on their atomic number (Z), the number of protons. So the ONLY basic difference is the sorting key: mass then, number now. Memory hook: "Mendeleev = Mass, Modern = Number (M-M, N-N)."
Both laws say properties repeat periodically. The only change is the sorting key: Mendeleev used atomic mass (which mis-orders pairs like Te and I), while the Modern law uses atomic number Z, which orders every element correctly.
Your doubts, answered
What is the exact difference between Mendeleev's and the Modern Periodic Law?
The difference is one word: the sorting property. Mendeleev's law states 'the properties of elements are a periodic function of their ATOMIC MASSES.' The Modern law states 'the properties of elements are a periodic function of their ATOMIC NUMBERS.' Everything else (periods, groups, repeating pattern) stays the same. For NEET, if asked for the 'basic difference in approach', the answer is atomic mass (Mendeleev) vs atomic number (modern).
Why did scientists switch from atomic mass to atomic number?
Because atomic mass gave a few wrong orderings. If you strictly follow increasing mass, some elements land in the wrong group. Atomic number (number of protons) fixed this and never fails, so it is more fundamental. This switch happened after Henry Moseley's 1913 experiment, not because Mendeleev was 'wrong' overall.
Who discovered that atomic number should be the basis, not mass?
Henry Moseley (1913), an English physicist. He fired X-rays at elements and plotted the square root of the X-ray frequency (√ν) against atomic number Z. This gave a perfect straight line, while plotting against atomic mass did NOT. This proved atomic number is the true fundamental property of an element. Mendeleev made the original table; Moseley corrected the basis to atomic number.
What are the anomalous pairs that Mendeleev had to reverse?
There are pairs where the heavier element comes FIRST by properties, breaking the mass order. The classic NCERT example is Iodine (I, mass 127) placed AFTER Tellurium (Te, mass 128) even though Te is heavier. Other famous pairs are Argon (Ar) before Potassium (K), and Cobalt (Co) before Nickel (Ni). Mendeleev placed them by properties, ignoring mass. The modern law (atomic number) explains all of them naturally: Te (Z=52) < I (Z=53), so the order is correct by number.
Did Mendeleev use atomic number at all?
No. Atomic number was not even known in Mendeleev's time. He built his 1869 table using only atomic weights (masses) and the similarity of properties, especially the formulae of oxides and hydrides. The concept of protons and atomic number came much later, which is why students must NOT say Mendeleev used atomic number.
What is the biggest achievement of Mendeleev's law even though it used mass?
He left GAPS for undiscovered elements and predicted their properties. He called them 'eka-boron', 'eka-aluminium', and 'eka-silicon'. When Scandium, Gallium, and Germanium were later found and matched his predictions closely, it made his table famous. So even with the 'wrong' basis (mass), his law was powerful because of periodicity and prediction.
⚠️ The NEET trap ✗ Modern Periodic Law was given by Mendeleev and is based on atomic mass. ✓ Mendeleev's law is based on atomic MASS. The MODERN Periodic Law (based on atomic NUMBER) came from Moseley's work; the fundamental property is atomic number, not mass. 🧠 Two names, two keys: Mendeleev = Mass, Moseley/Modern = Number. Never mix the person with the property.
Real NEET questions
NEET 2022
The IUPAC name of an element with atomic number 119 is:
A · ununennium ✓
B · unnilennium
C · unununnium
D · ununoctium
Solution: This question depends on the MODERN periodic table basis (atomic number, not mass). For Z > 100, IUPAC uses digit-by-digit roots + 'ium'. For Z=119: 1=un, 1=un, 9=enn -> un-un-enn-ium = ununennium (symbol Uue). Distractors: 109=unnilennium, 111=unununnium, 118=ununoctium. Remember: naming is by atomic NUMBER, the modern law's key property.
NEET 2022
Identify the incorrect statement from the following:
A · The largest and the smallest species among Mg, Mg2+, Al and Al3+ are Al and Mg2+ respectively. ✓
B · The IUPAC name of the element with atomic number 107 is Unnilseptium.
C · The similarity in behaviour of Li with Mg is referred to as the 'diagonal relationship'.
D · The oxidation state and covalency of Al in [AlCl(H2O)5]2+ are 3 and 6, respectively.
Solution: Statement A is INCORRECT (so it is the answer). Among Mg, Mg2+, Al, Al3+, the largest is neutral Mg (weaker nuclear pull than Al) and the smallest is Al3+ (highest charge, fewest electrons) - not Al and Mg2+ as claimed. The other statements are correct, including B: naming by atomic NUMBER gives Z=107 = Un(1)-nil(0)-sept(7)-ium = Unnilseptium. This shows why the modern basis (atomic number) matters.
Solved Periodic Classification Of Properties NEET PYQs
Try the real previous-year questions from this chapter — each with the answer and a full solution.
The properties of the elements are a periodic function of their atomic masses (weights).
State the Modern Periodic Law.
The properties of the elements are a periodic function of their atomic numbers.
What is the basic difference in approach between the two laws?
Mendeleev classified elements by increasing atomic mass; the modern law classifies them by increasing atomic number (Z). Atomic number is the more fundamental property.
Who is responsible for the modern periodic law?
Henry Moseley (1913), whose X-ray experiments proved atomic number, not atomic mass, is the fundamental property of an element.
Give one anomaly the modern law fixed.
Iodine (mass 127) comes after Tellurium (mass 128). By mass this is wrong order, but by atomic number Te (52) < I (53) is correct. The modern law removes the anomaly.
