Chemistry · Periodic Classification Of Properties · NEET
Atomic number is the number of protons in the nucleus. It also equals the number of electrons in a neutral atom. Since chemical properties come from electrons, the atomic number directly controls how an element behaves. Atomic mass only tells you the total weight of protons plus neutrons, and neutrons do not decide chemistry. So atomic number is the true, fundamental property. For NEET, remember: properties are a periodic function of atomic number, not atomic mass.
Mendeleev worked before protons were discovered, so atomic mass was the best tool he had. His table mostly worked, but a few pairs came out in the wrong order by mass. When Moseley later measured atomic number using X-rays, arranging elements by atomic number fixed those wrong pairs automatically. So we did not throw away Mendeleev's idea; we replaced the sorting property with a better one. This gave us the Modern Periodic Law.
Moseley fired high-speed electrons at different metal targets and studied the X-rays they gave out. He found that the square root of the X-ray frequency increased in a straight, regular way as he moved from one element to the next. This regular step was the atomic number (the nuclear charge). Because this number rose one whole unit at a time with no exceptions, it was clearly a more basic property than atomic mass. This is why the periodic law was rewritten in terms of atomic number.
Some element pairs had masses that ran backwards. Argon (mass about 40) is heavier than Potassium (mass about 39), yet Argon must sit before Potassium for the chemistry to match. The same happens with Tellurium and Iodine, and with Cobalt and Nickel. Mendeleev had to place them by properties, breaking his own mass rule. When you sort by atomic number instead, Ar(18) comes before K(19), Te(52) before I(53), and everything lines up with no cheating.
No. Atomic number is simply a count of protons, so it goes 1, 2, 3, 4 and never skips or reverses. Atomic mass can reverse because a heavier isotope mixture can make a lower-numbered element weigh more than the next one. Since atomic number never breaks the sequence, it gives a clean and permanent order for the periodic table. This is the core reason it is called fundamental.
No. Isotopes of one element have the same number of protons but different numbers of neutrons, so they have the same atomic number but different atomic masses. Because their atomic number is the same, all isotopes of an element sit in the same box in the periodic table. This shows again that atomic number, not mass, decides the position, since mass differs between isotopes but the place does not change.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Atomic number equals the number of protons in the nucleus. In a neutral atom it also equals the number of electrons. Because electrons control chemistry, this is why atomic number decides an element's properties.
Henry Moseley discovered it in 1913 using X-ray experiments. He showed that atomic number, not atomic mass, is the true basis of periodicity, which led to the Modern Periodic Law.
Atomic mass depends on protons plus neutrons and on the mix of isotopes. A heavier isotope mix can make a lower-numbered element weigh more than the next one, so mass can reverse. Atomic number never reverses.
Argon and Potassium. Argon is heavier (about 40) than Potassium (about 39), but Argon has atomic number 18 and Potassium 19. Sorting by atomic number puts them in the correct chemical order.
The Modern Periodic Law states that the physical and chemical properties of elements are a periodic function of their atomic numbers. It replaced Mendeleev's atomic-mass version.
