Chemistry · Some Basic Concepts Of Chemistry · NEET
Average atomic mass = (mass of isotope 1 x its fractional abundance) + (mass of isotope 2 x its fractional abundance) + ... for all isotopes. 'Fractional abundance' means the percent written as a decimal. Example: 75% becomes 0.75. NCERT computes carbon this way: (0.98892)(12) + (0.01108)(13.00335) + (tiny)(14.00317) = 12.011 u.
You can use either, but stay consistent. If you use decimals (0.75, 0.25) the abundances add to 1, and you just add the products. If you use percentages (75, 25) the products must be divided by 100 at the end. The safest habit for NEET is to convert to decimals first, because they must total 1.00.
Chlorine is a mixture of two isotopes: Cl-35 (about 75%) and Cl-37 (about 25%). No single chlorine atom weighs 35.5 u. The value 35.5 is only the weighted average of the two masses. Because natural chlorine has more of the lighter isotope, the average lands closer to 35 than to 37.
Mass number is a whole number for ONE isotope (protons + neutrons), like 35 for one Cl-35 atom. Average atomic mass is the weighted average of ALL isotopes of that element found in nature, so it is usually a decimal (like 35.5). The periodic table shows average atomic mass, not mass number.
Let the abundance of isotope 1 be x (as a decimal). Then isotope 2 is (1 - x). Set up: (mass1)(x) + (mass2)(1 - x) = average mass. Solve for x. Example for Cl: 35x + 37(1 - x) = 35.5 gives x = 0.75, so 75% is Cl-35 and 25% is Cl-37.
A plain average (35 + 37)/2 = 36 would only be correct if both isotopes were equally common. In nature they are not. Cl-35 is three times more common than Cl-37, so it should pull the average down. Multiplying by abundance gives each isotope the correct 'weight,' which is why it is called a weighted average.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
It is the average mass of one atom of an element, taking into account all its natural isotopes and how common each one is. It is a weighted average, so common isotopes count more than rare ones.
It is measured in unified atomic mass units, written as 'u' (older books use 'amu'). One u is defined as 1/12 the mass of one carbon-12 atom.
Because most elements are mixtures of isotopes with different masses. The weighted average of these masses is almost always a decimal, such as 35.5 for chlorine or 12.011 for carbon.
They are numerically equal for a single element. Average atomic mass is in 'u' per atom; molar mass is the same number in grams per mole. So chlorine is 35.5 u per atom and 35.5 g per mole.
Yes. All isotope percentages of one element must total 100% (or 1.00 as a decimal). This is a quick way to check that you have all the isotopes and that your data is correct.
