The International System of Units (SI)

The International System of Units (SI, from the French "Système International d'Unités") is the modern form of the metric system and the world's most widely used system of measurement. Scientific notation is essential when working with SI units, especially when dealing with very large or very small measurements.

To truly appreciate the range of scales in our universe - from the smallest subatomic particles to the largest cosmic structures - explore the interactive Scale of the Universe. This visualization perfectly demonstrates why scientific notation is essential in science, showing measurements that span over 60 orders of magnitude!

Writing Numbers: Formatting Conventions

Thousands Separation

Despite its common use, it's not recommended to use commas or periods to separate thousands in scientific contexts. Instead, the SI recommends using spaces to group digits in threes:

• Recommended: 1 234 567 - International standard, used in scientific contexts
• Not recommended: 1,234,567 (US/UK) or 1.234.567 (Continental Europe)

Decimal Notation

⚠️ Important: Never use apostrophes or quotes (') for thousands separation. This is not recommended in any standard.

For decimal numbers, different regions use different symbols:

• Decimal point (.): 3.14 - Used in USA, UK, and international scientific contexts
• Decimal comma (,): 3,14 - Used in Europe and many other countries worldwide

Note: For very long decimal numbers, use spaces every three digits after the decimal point as well, for example: 0.123 456 789

Scientific Notation

In science, it is common to work with extremely large or small numbers. Scientific notation facilitates handling big or small numbers, makes calculations easier, and can indicate the error of the measurement.

A number in scientific notation has the form: \(a.bc... \times 10^n\), where \(n\) is an integer (positive or negative).

How to Convert to Scientific Notation

1. Find the first non-zero digit in the number on the left.
2. Place the decimal point after this digit and continue with the rest of the number.
3. Zeros at the end of the number are not significant and can be omitted.
4. Count how many places you moved the decimal point to apply it to the exponent.
5. If you moved the decimal point to the left, the exponent is positive (to make it bigger); if you moved it to the right, the exponent is negative (to make it smaller).

Examples:

• 45 300 = 4.53 × 10⁴ (decimal point moved 4 places to the left).
• 0.006 78 = 6.78 × 10⁻³ (decimal point moved 3 places to the right).

How to Convert from Scientific Notation

1. Identify the coefficient (\(a\)) and the exponent (\(n\)).
2. Move the decimal point in the coefficient \(|n|\) places.
3. If \(n\) is positive, move the point to the right; if negative, move it to the left.
4. Add zeros as necessary.

Examples:

• 3.82 × 10⁵ = 382 000 (decimal point moved 5 places to the right).
• 7.5 × 10⁻² = 0.075 (decimal point moved 2 places to the left).

SI Base Units

The SI system is built on seven fundamental base units:

Why SI Units Matter in Science

• Universal Standard: Enables global scientific communication and collaboration
• Precision: Based on fundamental physical constants and phenomena
• Consistency: All derived units can be expressed in terms of base units
• Scientific Notation: Perfectly compatible with powers of 10 system

SI Base Units Table

SI Prefixes and Scientific Notation

SI uses prefixes to express multiples and submultiples of units. These prefixes correspond directly to powers of 10, making scientific notation essential:

Solved Examples

Example 1: Express in scientific notation the distance to the Moon: 384 000 km.

Step 1: Identify the first significant digit The first significant digit is 3, so we need to write 3.84... Step 2: Count decimal places From the original decimal position (384 000.0) to after the first digit (3.84000) there are 5 places to the left. Step 3: Write in scientific notation

\[384 \space 000 \text{ km} = 3.84 \times 10^5 \text{ km}\]

Result: 3.84 × 10⁵ km (average distance from Earth to Moon)

Example 2: The size of a small particle is approximately 0.000167 kg. Express it in scientific notation.

Step 1: Locate the first significant digit The first significant digit is 1, followed by 67. Step 2: Count decimal places From 0.000167 to 1.67... we need to move the decimal 4 places to the right. Step 3: Write in scientific notation

\[0.000167 \text{ kg} = 1.67 \times 10^{-4} \text{ kg}\]

Result: 1.67 × 10⁻⁴ kg (small particle mass)

Example 3: Calculate how many carbon atoms are in 12 g of carbon-12, knowing that Avogadro's number is 6.022 × 10²³ mol⁻¹ and that 12 g of C-12 equals 1 mol.

Step 1: Identify the data

• Carbon mass: 12 g = 1 mol of C-12
• Avogadro's number: 6.022 × 10²³ atoms/mol

Step 2: Apply the relationship

\[ \text{Number of atoms} = 1 \text{ mol} \times 6.022 \times 10^{23} \frac{ \text{atoms}}{ \text{mol}}\]

Step 3: Perform the calculation

\[ \text{Number of atoms} = 6.022 \times 10^{23} \text{ atoms}\]

Result: In 12 g of carbon-12 there are exactly 6.022 × 10²³ atoms (by definition of the mole).

Example 4: Compare the size of an atom (radius ≈ 10⁻¹⁰ m) with that of an atomic nucleus (radius ≈ 10⁻¹⁵ m). How many times larger is the atom?

Step 1: Establish the relationship

\[ \text{Factor} = \frac{ \text{Atom radius}}{ \text{Nucleus radius}} = \frac{10^{-10} \text{ m}}{10^{-15} \text{ m}}\]

Step 2: Apply exponent rules

\[ \text{Factor} = 10^{-10} \div 10^{-15} = 10^{(-10)-(-15)} = 10^{5} = 100,000\]

Step 3: Interpret the result The atom is 100,000 times larger than its nucleus. Analogy: If the nucleus were the size of a soccer ball (20 cm), the complete atom would have a radius of 20 km.

Example 5: Light travels at 3.00 × 10⁸ m/s. How long does light take to reach from the Sun to Earth? (Sun-Earth distance: 1.496 × 10⁸ km)

Step 1: Convert units

\[d = 1.496 \times 10^8 \text{ km} = 1.496 \times 10^8 \times 10^3 \text{ m} = 1.496 \times 10^{11} \text{ m}\]

Step 2: Apply the time formula

\[t = \frac{d}{v} = \frac{1.496 \times 10^{11} \text{ m}}{3.00 \times 10^8 \text{ m/s}}\]

Step 3: Perform the division

\[t = \frac{1.496}{3.00} \times 10^{11-8} = 0.499 \times 10^3 = 499 \text{ s}\]

Step 4: Convert to minutes

\[t = 499 \text{ s} \times \frac{1 \text{ min}}{60 \text{ s}} = 8.32 \text{ min}\]

Result: Sunlight takes approximately 8.3 minutes to reach Earth.

Example 6: Calculate the mass in grams of a single water molecule (H₂O), knowing that its molar mass is 18.0 g/mol.

Step 1: Establish the relationship 1 mol of H₂O contains 6.022 × 10²³ molecules and has a mass of 18.0 g. Step 2: Calculate the mass of one molecule

\[ \text{Mass of 1 molecule} = \frac{18.0 \text{ g}}{6.022 \times 10^{23} \text{ molecules}}\]

Step 3: Perform the division

\[ \text{Mass} = \frac{18.0}{6.022} \times 10^{-23} = 2.99 \times 10^{-23} \text{ g}\]

Step 4: Express in other units

\[2.99 \times 10^{-23} \text{ g} = 2.99 \times 10^{-23} \times 10^{-3} \text{ kg} = 2.99 \times 10^{-26} \text{ kg}\]

Result: A water molecule has a mass of 2.99 × 10⁻²³ g or 2.99 × 10⁻²⁶ kg.