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Seven SI Base Units and the Fundamental Quantities

Chemistry · Some Basic Concepts Of Chemistry · NEET

The SI system has exactly 7 base units for 7 fundamental (basic) quantities. They are: metre (m) for length, kilogram (kg) for mass, second (s) for time, ampere (A) for electric current, kelvin (K) for temperature, mole (mol) for amount of substance, and candela (cd) for luminous intensity. All other units (like speed or density) are "derived" by combining these seven. Memory hook: "My Kind Sister Takes Money, Cakes Cheerfully" -> Metre, Kilogram, Second, Time... use "Mrs LTC MTA": think of 7 friends, one for each base quantity.
The 7 SI Base Units (Fundamental Quantities)Lengthmetre (m)Masskilogram (kg)Timesecond (s)Electric currentampere (A)Temperaturekelvin (K)Amount of substancemole (mol)Luminous intensitycandela (cd)All other units are DERIVED from these 7:speed = m/s volume = m^3 density = kg/m^3 pressure = pascal
The seven SI base units, one for each fundamental quantity. Blue boxes are the physics-style quantities; orange boxes include the chemistry-important mole and kelvin. Every other unit (speed, volume, density, pressure) is derived by combining these seven.

Your doubts, answered

What are the 7 SI base units and their symbols?

There are exactly seven. Length -> metre (m). Mass -> kilogram (kg). Time -> second (s). Electric current -> ampere (A). Temperature -> kelvin (K). Amount of substance -> mole (mol). Luminous intensity -> candela (cd). Learn them as pairs of (quantity -> unit -> symbol). Note the symbols named after people (ampere A, kelvin K) start with a capital letter, but their full names are written in small letters.

Is the mole a base unit or a derived unit?

The mole (mol) is a BASE unit. It measures the fundamental quantity called 'amount of substance'. Many students wrongly think mole is derived because it links to grams and Avogadro number, but SI officially lists it as one of the seven base units. This is a common NEET trap.

Why is the SI unit of mass the kilogram and not the gram?

By international agreement (CGPM), the base unit of mass was defined as the kilogram (kg), not the gram. The gram is just a smaller fraction, where 1 kg = 1000 g. In chemistry labs we use grams because chemical amounts are small, but for the SI base unit answer you must write kilogram. NEET often asks 'SI unit of mass?' and the correct answer is kilogram, not gram.

What is the difference between fundamental (base) units and derived units?

Base units cannot be broken into simpler units - they are the seven building blocks (m, kg, s, A, K, mol, cd). Derived units are made by combining base units. For example speed = metre/second (m s^-1), volume = m^3, and density = kg/m^3. So if a unit is a single one of the seven, it is a base unit; if it is a combination, it is derived.

What is the SI base unit of temperature?

The SI base unit of temperature is the kelvin (K), NOT degree Celsius. Kelvin is related to Celsius by K = deg C + 273.15. This matters for NEET because gas-law and thermodynamics formulas need temperature in kelvin, and kelvin cannot be negative while Celsius can.

Is candela (luminous intensity) really tested in NEET?

Candela (cd) rarely appears in a direct chemistry calculation, but you must still know it is one of the seven base units and that it measures luminous intensity. NEET can ask 'which of these is NOT an SI base unit?' and list a wrong option, so knowing candela belongs to the seven protects you from that trap.

What quantity does the ampere measure and what is its symbol?

The ampere measures electric current. Its symbol is a capital A (because it is named after the scientist Ampere). It is one of the seven base units. Do not confuse the symbol A with amu (atomic mass unit) - they are unrelated.

⚠️ The NEET trap
✗ Choosing gram (g) as the SI unit of mass, or thinking the mole is a derived unit.
✓ The SI base unit of mass is the kilogram (kg); the mole (mol) IS one of the seven base units.
🧠 Seven base units = m, kg, s, A, K, mol, cd. Mass is KILOgram, and MOLE is a base unit, not derived. If a question asks 'which is NOT a base unit', a derived one like the litre, newton, pascal or m/s is the odd one out.

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Frequently asked

How many base units are there in the SI system?

Exactly seven. They correspond to seven fundamental quantities: length, mass, time, electric current, temperature, amount of substance, and luminous intensity.

What is the SI unit of amount of substance?

The mole (mol). It is a base unit and is used to count particles like atoms, molecules and ions using the Avogadro number.

Which body decides SI units?

The General Conference on Weights and Measures (CGPM). The SI system was established by the 11th CGPM. In India, the National Physical Laboratory (NPL), New Delhi maintains the national standards.

Are litre, newton and pascal base units?

No. They are not among the seven base units. Litre is a non-SI volume unit; newton (force) and pascal (pressure) are derived units built from base units. Only m, kg, s, A, K, mol and cd are base units.

Why do we need SI units at all?

So scientists everywhere measure in the same way. A common, uniform system removes confusion when comparing results, and every derived unit can be built cleanly from the seven base units.

Related concepts:
SI Prefixes for Multiples and SubmultiplesDerived SI Units (Speed, Volume, Density, Pressure)Difference Between Mass and WeightCelsius to Kelvin Temperature ConversionUnits of Volume: Litre, dm3, cm3
Practise Chemistry for NEET:
Chemistry MCQs (free, chapter-wise)Some Basic Concepts Of Chemistry PYQs