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9.32×10⁻³

Explanation:

Scientific notation is the way to express the large value in short form.

The number in scientific notation have two parts.

. The digits (decimal point will place after first digit)

× 10 ( the power which put the decimal point where it should be)

How to add the number in scientific notation with same exponent.

7.21 ×10⁻³ + 2.11 ×10⁻³

exponent of both values are same thus we will take it common.

[7.21 + 2.11 ]×10⁻³

9.32×10⁻³

increased stability in the completion of fluorine's outer shell.