Electron Configuration (Orbitals, Valance Electrons, and Core Notation)

The electron configuration of an atom is a form of notation which shows how the electrons are distributed among the various atomic orbital and energy levels.  

before we start, you have to know these principles so you can fill the electrons correctly.
 

I.  Principle Quantum Number (n) and Sublevels

The number of sublevels that an energy level can contain is equal to the principle quantum number of that level.  So, for example, the second energy level would have two sublevels, and the third energy level would have three sublevels.  The first sublevel is called an s sublevel.  The second sublevel is called a p sublevel.  The third sublevel is called a d sublevel and the fourth sublevel is called an f sublevel.  Although energy levels that are higher than 4 would contain additional sublevels, these sublevels have not been named because no known atom in its ground state would have electrons that occupy them.

II.  Sublevels and Orbitals

An orbital is a space that can be occupied by up to two electrons.  Each type of sublevel holds a different number or orbitals, and therefore, a different number of electrons.  s sublevels have one orbital, which can hold up to two electrons.  p sublevels have three orbitals, each of which can hold 2 electrons, for a total of 6.  d sublevels have 5 orbitals, for a possible total of 10 electrons.  f sublevels, with 7 orbitals, can hold up to 14 electrons.  The information about the sublevels is summarized in the table below:

Table 3-6a - Orbital and Electron Capacity for the Four Named Sublevels
Sublevel	# of orbitals	Maximum number of electrons
s	1	2
p	3	6
d	5	10
f	7	14

III.  Total Number of Electrons per Energy Level

An easy way to calculate the total number of electrons that can be held by a given energy level is to use the formula 2n².   For example, the fourth energy level (n=4) can hold 2(4)² = 32 electrons.  This makes sense because the fourth energy level would have four sublevels, one of each of the named types.  The s sublevel hold 2 electrons, the p sublevel holds 6 electrons , the d sublevel holds 10 electrons and the f sublevel holds 14 electrons.  2 + 6 + 10 + 14 = 32, so the formula 2n² works!  We can summarize this information in the table below:

Table 3-6b Orbitals and Electron Capacity of the First Four Principle Energy Levels
Principle energy level (n)	Type of sublevel	Number of orbitals per type	Number of orbitals per level(n2)	Maximum number of electrons (2n2)
1	s	1	1	2
2	s	1	4	8
	p	3
3	s	1	9	18
	p	3
	d	5
4	s	1	16	32
	p	3
	d	5
	f	7

  

V.  Order of Filling Sublevels with Electrons

The next thing that you need to recall is the fact that the energy sublevels are filled in a specific order that is shown by the arrow diagram seen below:

Remember to start at the beginning of each arrow, and then follow it all of the way to the end, filling in the sublevels that it passes through.  In other words, the order for filling in the sublevels becomes; 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 6s, 4f, 5d, 6p, 7s, 5f, 6d,7p.

So, how to find the numbers of valance electrons?

Its easy!  you just need to add all the electrons in s and p sublevel

Then, how do you write it in core notation.

First, you have to find the noble gas at the front role

Then, you add electrons until you get the number of electrons you want

eg. C = [He]2s²2p²