Chemistry · Some Basic Concepts Of Chemistry · NEET
Chlorine has two natural isotopes: Cl-35 (mass 34.969 u, abundance 75.77%) and Cl-37 (mass 36.966 u, abundance 24.23%). Convert percentages to fractions and multiply: (34.969 x 0.7577) + (36.966 x 0.2423) = 26.496 + 8.957 = 35.45 u. In most NEET problems the isotope masses are rounded to 35 and 37, giving (35 x 0.7577) + (37 x 0.2423) = 35.48 u, which rounds to 35.5 u. That is the number on the periodic table.
A single chlorine atom is either 35 or 37 - it can never be 35.5. The value 35.5 is an AVERAGE. Because about 3 out of every 4 chlorine atoms are Cl-35 and 1 out of 4 is Cl-37, the weighted average lands between them, closer to 35. So 35.5 does not describe one atom; it describes the whole natural mixture. This is why almost no element has a perfectly whole-number atomic mass.
Average atomic mass = (mass1 x fraction1) + (mass2 x fraction2) + ... for every isotope. 'Fraction' means abundance divided by 100. Two quick checks: (1) the fractions must add up to 1 (or the percents to 100), and (2) the answer must lie BETWEEN the smallest and largest isotope mass. If your answer is outside that range, you made an error.
Let the fraction of the lighter isotope be x, so the heavier one is (1 - x). Set up: (mass1)(x) + (mass2)(1 - x) = average atomic mass, then solve for x. Example for chlorine with average 35.5: 35x + 37(1 - x) = 35.5, so 35x + 37 - 37x = 35.5, giving -2x = -1.5, so x = 0.75. That means 75% Cl-35 and 25% Cl-37. This 'reverse' type is a common NEET twist.
Argon has three isotopes: Ar-40 (99.6%), Ar-36 (0.34%), and Ar-38 (0.06%). Because Ar-40 makes up almost all argon, the average is pulled very close to 40. Calculation: (40 x 0.996) + (36 x 0.0034) + (38 x 0.0006) = 39.84 + 0.1224 + 0.0228 = about 39.99 u, so roughly 40 u. Lesson: when one isotope dominates, the average sits almost on top of that isotope's mass.
Mass number is a whole number for ONE isotope - it counts protons plus neutrons (for example, Cl-35 has mass number 35). Average atomic mass is usually a decimal and describes the natural MIXTURE of all isotopes of that element. Mass number has no units; average atomic mass is written in u (unified mass unit). In short: mass number = one atom, average atomic mass = the whole element as found in nature.
You use the isotope MASS in u (from the given data, like 34.969 or just 35) and the abundance as a fraction (percent divided by 100). Never mix grams in here. The word 'mass' in this formula means atomic mass in u, and the word 'abundance' means how common that isotope is, written as a decimal fraction that adds to 1.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Numerically yes for a single element. The average atomic mass of chlorine is 35.5 u, and its molar mass is 35.5 g/mol. The unit changes from u (per atom) to g/mol (per mole), but the number is the same. This link is why NEET mole problems use the periodic-table value directly.
To keep the arithmetic fast. The exact masses (34.969 and 36.966) give 35.45 u, while rounding to 35 and 37 gives 35.48 u. Both round to 35.5 u, so NEET accepts the simpler numbers unless the exact masses are given in the question.
Rarely. It happens only if an element has one stable isotope (like fluorine-19, so fluorine is about 19 u) or if isotope masses and abundances happen to average to a whole number. Most elements are mixtures, so their average atomic masses are decimals.
Convert the ratio to fractions first. If Cl-35 : Cl-37 is 3 : 1, the total parts are 4, so fractions are 3/4 and 1/4. Then apply the formula: (35 x 3/4) + (37 x 1/4) = 26.25 + 9.25 = 35.5 u.
Average atomic mass feeds directly into molar mass, mole calculations, and stoichiometry. If you use the wrong atomic mass, every following step - moles, mass, and even empirical formula - becomes wrong. So getting 35.5 for chlorine correct protects a whole chain of NEET calculations.
