Account for the following. limit your answer to two sentences: 'Atomic weight of most of the elements are fractional'.

Explanation

**Step-by-Step Solution:**

1. **Definition of Atomic Mass**: Atomic mass is defined as the average mass of an element's isotopes, weighted by their natural abundance.

2. **Presence of Isotopes**: Most elements exist as a mixture of isotopes, which are atoms of the same element that have different masses due to varying numbers of neutrons.

3. **Calculation of Atomic Weight**: The atomic weight is calculated by taking the weighted average of the atomic masses of the isotopes. For example, for iron, if the isotopes have masses of 54, 56, and 57 with respective abundances of 5%, 90%, and 5%, the atomic weight is calculated as:
\[ \text{Atomic weight of iron} = \frac{(5 \times 54) + (90 \times 56) + (5 \times 57)}{100} = 55.9 \]

4. **Conclusion**: The fractional atomic weights of most elements arise from the averaging of the masses of their isotopes, reflecting their natural abundance.

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