Chemistry · Some Basic Concepts Of Chemistry · NEET
Move the decimal point until exactly ONE non-zero digit stays to its left. Count how many places you moved. That count is the power (n). If you moved the decimal to the LEFT (big number), n is positive. If you moved it to the RIGHT (small number), n is negative. Example: 232.508 → shift decimal 2 places left → 2.32508 × 10^2. Example: 0.00016 → shift 4 places right → 1.6 × 10^-4.
Scientific notation has ONE fixed correct form so everyone writes the same number the same way. N must be from 1.000... up to 9.999..., meaning exactly one non-zero digit sits before the decimal point. Writing 23.2 × 10^1 or 0.232 × 10^3 is the same value but is NOT proper scientific notation. Always fix it so only one digit is before the dot.
Simple rule: numbers BIGGER than 1 get a POSITIVE power (you moved the decimal left). Numbers SMALLER than 1 (like 0.000...) get a NEGATIVE power (you moved the decimal right). So 6022 = 6.022 × 10^3 (positive), and 0.0052 = 5.2 × 10^-3 (negative). The power just tells you how many places and which direction to shift back.
You must make the POWERS equal first. To add 2.0 × 10^2 and 3.0 × 10^3, rewrite the smaller-power one: 2.0 × 10^2 = 0.20 × 10^3. Now add the digit terms: (0.20 + 3.0) × 10^3 = 3.2 × 10^3. Rule: match powers, then add/subtract only the N parts, keep the common power. This is different from multiply/divide, where you do NOT need equal powers.
For multiply: multiply the N parts and ADD the powers. (2 × 10^3)(3 × 10^4) = 6 × 10^7. For divide: divide the N parts and SUBTRACT the powers. (6 × 10^5) ÷ (2 × 10^2) = 3 × 10^3. Unlike addition, you do NOT match powers first. If the answer's N goes outside 1–10, shift the decimal and fix the power.
Only the digits in N (the digit term) count. The 10^n part never counts as significant figures. So 4.01 × 10^2 has 3 significant figures, and 8.256 × 10^-3 has 4 significant figures. This is why scientific notation is loved in NEET: it removes the confusion about trailing/leading zeros. Writing 4500 as 4.5 × 10^3 clearly shows 2 significant figures.
The numbers 17.0145 and 21.0235 were rounded to three figures after the decimal point. The resulting numbers, respectively, are:
Try the real previous-year questions from this chapter — each with the answer and a full solution.
A way to write any number as N × 10^n, where N is between 1 and 10 and n is a whole-number power of 10. It makes very big or very small numbers easy to write and read.
Chemistry uses numbers from 10^-31 (electron mass) to 10^+23 (Avogadro's number). Writing all those zeros is slow and error-prone. Scientific notation makes calculations fast and clearly shows significant figures, which NEET tests directly.
No. Only the digits in N count. For example, 8.256 × 10^-3 has 4 significant figures — the 10^-3 is ignored for counting.
For multiply/divide you do NOT match powers — you multiply/divide N and add/subtract the powers. For add/subtract you MUST make the powers equal first, then add or subtract the N parts.
Numbers greater than 1 give a positive power. Numbers less than 1 (starting with 0.00...) give a negative power. The power equals how many places you shifted the decimal.
