Scientific notation writes numbers as a × 10ⁿ — clean and compact for very big or very small values.
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Earth's mass
5.972 × 10²⁴ kg.
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Diameter of a hydrogen atom
1.06 × 10⁻¹⁰ m.
The first number is between 1 and 10 (in size). The exponent counts decimal-point hops.