What is 2n^2 rule ( of science)

2n² rule:

This rule is used to determine number of electrons in particular shell.

n=1 means K shell

n=2 means L shell

n=3 means M shell

n=4means N shell

The first K shell can hold upto 2 electrons, L shell can hold up to 8, third M shell can hold up to 18and the fourth N shell can hold upto 32 electrons. This rule of arrangement of electrons according to the shell is known 2n2 rule where n means number of shell.

For example: There is one proton in the nucleus of hydrogen atom, which moves in K shell path. It has no neutron.

6.4 cm (max) - 25 cm (min)

"record the time to complete a chemical reaction" would most likely be part of a lab procedure.