Chemistry · Some Basic Concepts Of Chemistry · NEET
Atomic mass is the mass of ONE single atom of an isotope, measured in u. For example, one chlorine-35 atom has a mass of about 35 u. Average atomic mass is the weighted mean of ALL the isotopes of that element, using their natural abundance. Chlorine has two isotopes (35 and 37), so its average atomic mass is 35.5 u. The values in the periodic table are average atomic masses, not single-isotope masses. This matters for NEET because every mole and mass calculation uses the average value.
amu means atomic mass unit and u means unified mass. They are the SAME thing. One amu (or 1 u) is defined as exactly one-twelfth (1/12) of the mass of one carbon-12 atom. In older books it is written amu; NCERT and modern NEET papers use u. So if a question gives masses in amu, treat them exactly like u. In grams, 1 u is about 1.66056 x 10^-24 g.
Scientists needed one fixed reference atom so every other atom's mass could be compared to it. Since 1961 they chose carbon-12 and gave it a mass of exactly 12 u. So one unit of mass (1 u) must be 12 divided by 12, which is one-twelfth of a carbon-12 atom. This is just a chosen standard, like using a metre stick. Because of this choice, a carbon-12 atom is exactly 12 u with no decimal.
A single atom's mass is close to a whole number, but the periodic table shows the AVERAGE of the isotopes. Chlorine is a mix of about 75% chlorine-35 and 25% chlorine-37. The weighted average of 35 and 37 in that ratio is 35.5 u. So 35.5 is not the mass of any real single atom; it is the average of the mixture. Elements with more than one common isotope usually have decimal atomic masses.
Multiply each isotope's mass by its fractional abundance (percent divided by 100), then add all the products. Formula: Average atomic mass = (mass1 x fraction1) + (mass2 x fraction2) + ... For chlorine with 75.77% of mass 34.97 u and 24.23% of mass 36.97 u: (0.7577 x 34.97) + (0.2423 x 36.97) = 26.50 + 8.96 = 35.45 u. Always convert percent to a decimal fraction first.
No. Average atomic mass is for ONE element's atom (like Cl = 35.5 u). Molecular mass is for a whole molecule and is the SUM of the atomic masses in it (like Cl2 = 71 u, H2O = 18 u). Molar mass is the mass of one mole (6.022 x 10^23 particles) and has the same number but units of g/mol. They are linked but not equal, so read the question carefully in NEET.
A compound X contains 32% of A, 20% of B and the remaining percentage of C. The empirical formula of X is (atomic masses: A = 64, B = 40, C = 32 u)
The amount of carbon dioxide evolved upon complete combustion of 116 g of n-butane is (Atomic masses: H = 1, C = 12, O = 16):
Try the real previous-year questions from this chapter — each with the answer and a full solution.
1 amu (or 1 u) equals about 1.66056 x 10^-24 g. It is one-twelfth the mass of a carbon-12 atom. You rarely need the gram value in NEET, but it explains why atoms are so light.
Hydrogen (mass 1) was the first reference, but carbon-12 was agreed upon in 1961 because it gives more accurate and consistent atomic masses using mass spectrometry. Carbon-12 is now assigned an exact mass of 12 u.
It shows average atomic mass. The value already includes all the isotopes and their natural abundance, which is why many values have decimals (like Cl = 35.5, Cu = 63.5).
Yes, if an element is basically one isotope (like fluorine, which is almost 100% F-19), the average is close to a whole number. Elements with two or more common isotopes usually have decimal averages.
Yes. Almost every NEET mole, mass percent, empirical formula, and stoichiometry problem uses the average atomic mass from the periodic table. Always use the given u values.
