Chemistry · Some Basic Concepts Of Chemistry · NEET
Because most elements are found in nature as a mixture of isotopes. Isotopes are atoms of the same element with different numbers of neutrons, so they have different masses. The atomic mass on the periodic table is the weighted average of all these isotopes. An average of different numbers is almost never a clean whole number, so it comes out with a decimal, like 35.5 for chlorine.
Chlorine has two main isotopes: Cl-35 (about 75.77%) and Cl-37 (about 24.23%). To get the average, you multiply each mass by its abundance and add them: (35 x 0.7577) + (37 x 0.2423) = 35.5 u. No single chlorine atom weighs 35.5 u. That number is just the average of the crowd, which is why it has a decimal.
For one single atom, the mass number (protons + neutrons) is a whole number. But the atomic mass on the periodic table is NOT the mass of one atom. It is the average mass of all the isotopes of that element as they occur in nature. Because different isotopes have different mass numbers, their average is not a whole number.
Carbon-12 is used as the standard and is defined as exactly 12 u. But natural carbon also contains a small amount of Carbon-13 (about 1.1%), which is heavier. When you take the weighted average of C-12 and C-13, the value rises slightly above 12 to 12.011 u. NCERT calculates this same value in Unit 1.
Almost all do, but a few elements exist as only one stable isotope (like fluorine or sodium), so their atomic masses are very close to whole numbers. Even then, tiny mass differences and binding-energy effects mean they are not perfectly whole. Most elements have decimals because they are mixtures of two or more isotopes.
It is the mass you would measure if you took a large, real sample of the element and averaged the mass of every atom in it, giving more weight to the isotopes that appear more often. NCERT states that the atomic masses printed in the periodic table are actually these average atomic masses, not the mass of one specific atom.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
No. Mass number is the count of protons plus neutrons in one atom and is always a whole number. Atomic mass is the weighted average of all isotopes of the element and usually has a decimal.
It is measured in unified atomic mass units, written as u (older books call it amu). One u is exactly 1/12 the mass of one Carbon-12 atom.
Multiply the mass of each isotope by its fractional abundance, then add all the results. For example, carbon: (0.98892 x 12) + (0.01108 x 13.00335) = 12.011 u, exactly as NCERT shows.
Elements with only one stable isotope, such as fluorine (19), sodium (23), and phosphorus (31), have atomic masses very close to whole numbers because there is no isotope mixture to average.
NEET asks you to calculate average atomic mass from isotope data and to know why periodic-table values have decimals. Understanding isotopes prevents the common trap of treating atomic mass as a simple whole number.
