How many valence electrons are in an atom of magnesium? (atomic number 12)?

From the periodic table, Magnesium is actually the element 12 and this belongs to Group 2. We know that an element in Group 2 has 2 valence electrons.

We can also determine this from the electron configuration of Mg which is 1s² 2s²2p⁶ 3s² or [Ne]3s². Since the outermost electron is 3s² therefore magnesium has 2 valence electrons.

c. heat could easily travel through the molecules in the air and escape/enter the flask. removing the air slows down the heat transfer.  

the electron configuration that you gave me has 16 electrons ergo it has 16 protons ergo it is the element sulfur.