Reasons for the fractional atomic masses of elements:

There are several reasons why atomic masses of elements are usually fractional:

1. Mixtures of isotopes:
- Isotopes are atoms of the same element that have different numbers of neutrons but the same number of protons.
- Many elements have multiple isotopes, and each isotope has a different mass.
- The atomic mass of an element is calculated by taking the weighted average of the masses of its isotopes, where the weighting is based on the relative abundance of each isotope in nature.
- Since the abundances of isotopes are often not whole numbers, the atomic mass calculated as the weighted average will be fractional.

2. Impurities of other atoms:
- Atomic masses are determined by analyzing the purest form of an element available.
- However, it is often difficult to obtain elements in a completely pure form, and they may contain impurities of other atoms.
- These impurities can contribute to the overall atomic mass of the element, leading to fractional values.

3. Mixtures of isobars:
- Isobars are atoms or ions that have the same mass number but different atomic numbers (i.e., they have the same sum of protons and neutrons, but different numbers of protons).
- In some cases, elements may exist as mixtures of isobars, which can contribute to fractional atomic masses.

4. Accuracy of weighing atomic masses:
- Weighing atomic masses is a complex process that involves using mass spectrometry and other techniques.
- While modern techniques are highly accurate, there is always a margin of error in the measurements.
- The measured masses may not always be whole numbers, resulting in fractional atomic masses.

Conclusion:
In conclusion, the fractional atomic masses of elements primarily occur due to the presence of isotopes, impurities, mixtures of isobars, and the accuracy limitations of weighing techniques. The weighted average of the masses of isotopes, the presence of impurities, and mixtures of isobars can all contribute to fractional values. It is important to consider these factors when calculating and interpreting atomic masses of elements.