Question

What is the SI base unit for length?

Answer 1

The SI base units are the standard units of measurement defined by the International System of Units (SI) for the seven base quantities. These base quantities are known as the International System of Quantities. Using these base quantities, other SI units are derived.

Complete answer:
The SI base unit of length is metre. It is represented by the symbol $m$. As per the final act of the $2019$ redefinition of the SI base units, the definition of length is:
“The metre, symbol $m$, is the SI unit of length. It is defined by taking the fixed numerical value of the speed of light in vacuum $c$ to be $299792458$ when expressed in the unit $m{s^{ - 1}}$, where the second is defined in terms of $\Delta {V_{Cs}}$.”
Earlier, metre was defined as:
“$1/10000000$ of the distance from the Earth's equator to the North Pole measured on the median arc through Paris.”
The dimensional symbol of metre is $L$.
In chemistry, Angstroms are usually used. $1$ Angstroms $= 1 \times {10^{ - 10}}m$. It is used because the bond length between atoms is usually around $1 - 2$ Angstroms long. Angstroms are represented by the symbol $A$. Also, to describe the size of electrons or protons, picometre is used. $1$ picometre $= 1 \times {10^{ - 12}}m$. The symbol of the picometre is $pm$.

Note:
When reducing the units to their dimensional formula, write the complete form of metric system ${M^a}{L^b}{T^c}$, even if only one or two SI units is used. Write the other basic unit with a power of $0$. For example, the dimensional formula of speed is $L{T^{ - 1}}$, but it should be written as ${M^0}{L^1}{T^{ - 1}}$ or ${M^0}L{T^{ - 1}}$.