Chemistry · Some Basic Concepts Of Chemistry · NEET
An SI prefix is a small word added before a unit to make it bigger or smaller by a power of 10. We use it so we do not have to write long numbers. Instead of writing 0.000000001 m, we write 1 nm (nanometre). Instead of 1000 g, we write 1 kg. It saves space and reduces mistakes. NEET tests whether you know which prefix means which power of 10.
Going from big to small: kilo = 10^3, hecto = 10^2, deca = 10^1, then the base unit = 10^0, then deci = 10^-1, centi = 10^-2, milli = 10^-3. Each step is one power of 10. Multiples (bigger than base) use kilo, hecto, deca. Submultiples (smaller than base) use deci, centi, milli.
They are all submultiples (smaller than 1), each 1000 times smaller than the one before. milli = 10^-3, micro = 10^-6, nano = 10^-9, pico = 10^-12. So micro is 1000 times smaller than milli, nano is 1000 times smaller than micro, and pico is 1000 times smaller than nano. The symbol for micro is the Greek letter mu.
There are 1,000,000,000 (one billion) nanometres in one metre, because nano = 10^-9. So 1 m = 10^9 nm. This matters in NEET because atomic sizes and light wavelengths are often given in nanometres. For example, visible light is about 400 to 700 nm.
Yes, mega is bigger than kilo. kilo = 10^3 (thousand), mega = 10^6 (million), giga = 10^9 (billion), tera = 10^12. So each one is 1000 times bigger than the last. Among common NEET prefixes, tera is the largest and giga, mega, kilo follow in decreasing size.
centi means 10^-2, so 1 cm = 10^-2 m = 0.01 m. To go from cm to m, divide by 100 (or multiply by 10^-2). Example: 250 cm = 250 x 10^-2 m = 2.5 m. To go the other way (m to cm), multiply by 100.
NCERT says the SI base unit of mass is the kilogram (kg), not the gram. But in chemistry labs we use small amounts, so gram is more handy, where 1 kg = 1000 g. This is a special case: mass is the only base unit that already carries a prefix (kilo) in its name.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
micro uses the Greek letter mu, nano uses a small n, and pico uses a small p. So micrometre = mm-like symbol with mu, nanometre = nm, picometre = pm.
They look similar but are opposite. deca = 10^1 (ten times bigger). deci = 10^-1 (ten times smaller, one tenth). Be careful: deca is a multiple, deci is a submultiple.
Focus on kilo (10^3), deci (10^-1), centi (10^-2), milli (10^-3), micro (10^-6), nano (10^-9) and pico (10^-12). These appear in units like dm3, cm3, nm and pm used in atomic structure and measurement questions.
pico = 10^-12, so 1 pm = 10^-12 m. Atomic radii are often given in picometres, for example a hydrogen atom radius is about 53 pm.
Yes. The base unit of mass, the kilogram, already contains the prefix kilo. This is the only SI base unit whose name includes a prefix, which is a common exam trick.
