How many electrons would be expected in the outermost electron shell of an atom with atomic number 12?

First, what does the atomic number 12 tell us about the atom?  Well, it tells us the number of protons(+) that atom has. If the atom is not bonded to any other atoms the number of electrons(-) is usually equal to the number of protons. Therefore, we can assume this atom has 12 total electrons. 

Next, it is important to understand electron shells.
The first electron shell around an atom always has 2 valence electrons with the exception of Hydrogen because it only has one electron to begin with. It doesn't have enough electrons to fill it up with 2e-. 
Every electron shell after the first shell fills up with electrons until it runs out of electrons or until it reaches 8 electrons. Once a shell fills up with 8 electrons a new shell is started. This pattern continues until all the atom's electrons are used up.

I drew a picture of this atom and its electron shells so that you can see what I mean. The answer is: The outermost shell will have 2 electrons on it. The electrons on the outermost shell are called valence electrons. 

If you understand the material, often times you can use a shortcut for this problem. (atomic #)12 - (shell 1)2- (Shell 2)8= 2e-    You keep subtracting 8 until your answer is less than or equal to 8.

Hope that made sense! Comment with any questions if things are still unclear or if you think I made a mistake. :)

assuming that your periodic table has eighteen columns, you should highlight   the four elements in the third column from the left of the table.

explanation

neutral atoms of elements after calcium contain d electrons. the energy of d orbitals of a particular main shell lies between the energy of the s and p orbitals of the next main shell. for example, electrons in the have energy higher than those in the orbital while lower than those of the orbital.

by the aufbau principle, electrons fill orbitals one at a time, starting from those of the lowest energy. the orbital is therefore filled right after orbitals and before orbitals.  

potassium has electron configuration and calcium . the s orbital of a main shell holds a maximum of two electrons. the next element would thus have the electron configuration and satisfy the requirements.

a neutral atom of the element contains three electrons on top of the most recent noble gas element. it should thus be placed in the third column of the periodic table. both scandium and yttrium would satisfy the conditions. an aufbau diagram indicates that the 4f orbital is filled before the 5d. however, it appears that exceptions exists for both lanthanum and actinium. neutral atoms of the two d-block elements also demonstrate the and should be highlighted as well.