Review the electron configurations of the elements in period 3, from sodium (Na) to argon (Ar). What do you observe?

Electronic configuration represents the total number of electrons that a neutral element contains. We add all the superscripts to know the number of electrons in an atom.  The electrons are filled according to Afbau's rule in order of increasing energies.

The last electron enters the third shell in all elements from Sodium to Argon.

it should have an internal chamber and two thermometers sticking out of the top. the internal chamber will also be closed off from the outside.