Atomic Weight of Chlorine:
The atomic weight of chlorine is 35.5. This means that the average mass of a chlorine atom is 35.5 atomic mass units (amu). However, chlorine has two isotopes with different atomic weights.

Isotopes of Chlorine:
Isotopes are atoms of the same element that have different numbers of neutrons in their nuclei. Chlorine has two stable isotopes: chlorine-35 and chlorine-37.

Percentage of Heavier Isotope:
To determine the percentage of the heavier isotope in a sample of chlorine, we need to consider the relative abundance of each isotope.

Let's assume that the sample contains x% of chlorine-35 and (100-x)% of chlorine-37.

Calculating Average Atomic Weight:
The average atomic weight of an element is the weighted average of the atomic masses of its isotopes, taking into account their relative abundances.

Using the given atomic weights of chlorine-35 (35 amu) and chlorine-37 (37 amu), we can set up the following equation:

(35 amu * x%) + (37 amu * (100-x)%) = 35.5 amu

Simplifying the equation, we have:

(35x/100) + (37(100-x)/100) = 35.5

Solving for x, we get:

35x + 37(100-x) = 3550

35x + 3700 - 37x = 3550

-2x = -150

x = (-150)/(-2)

x = 75

Therefore, the percentage of the heavier isotope (chlorine-37) in the sample is 100 - x = 100 - 75 = 25%.

Explanation:
- The atomic weight of an element is the average mass of its atoms, taking into account the isotopic composition.
- Chlorine has two stable isotopes: chlorine-35 and chlorine-37.
- The average atomic weight of chlorine is 35.5 amu.
- To calculate the percentage of the heavier isotope (chlorine-37) in a sample, we set up an equation based on the relative abundances of the isotopes.
- Solving the equation, we find that the percentage of chlorine-37 is 25% in the sample.