Electrons in an Atom

Niels Bohr proposed that atoms exist in energy levels surrounding the nucleus.

Electrons exist in two states:
1. Ground state- when electrons are in their lowest possible energy level
2-Excited State-when one or more electrons of an atom are in energy levels above their lowest possible level.

An energy level is the amount of energy an electron can possess.
The quantum of energy is the difference in energy between two levels.

Recall that electrons exist in shells,which are sets of orbitals having the same 'n' value
and an orbital is  the space an electron takes up in a particular energy level.
A subshell is a set of orbitals of the same type.

There are 4 different types of orbitals
S, D, P and F.
Each subshell has

1 S Orbital
3 P orbitals
5 D orbitals
7 F orbitals

This means that the most electrons you can put in

S subshell- 2
P subshell- 6
D subshell-10
F subshell-14

This is governed by Pauli's Exclusion Principle which states that each orbital contains 2 electrons max.

This is a diagram which will help when filling orbitals.

To write the electronic configuration of neutral atoms:

1) The Aufbau Principle states that you must start with the lowest energy level.
2) Figure out how many electrons you have.
3)According to the diagram, that is the order you follow, so 1s,2s,2p etc..
4. Remember how many electrons in each subshell
5) Keep filling until you run out of electrons.

Example 1:

Oxygen- has 8 electrons.

So 1s^2 2s^2 2p^4

You write the name of the subshell and then the number of electrons in it as an exponent.

Example 2:

Krypton has 36 electrons.

1s^2 2s^2 2p^6 3s^2 3p^6 4s^2 3d^10 4p^6

Note that 4p^6 is filled up. This is because krypton is a noble gas.

This is the visual way to display electron configuration. Note how each shell is filled with up and down arrows because they have opposite spins. Remember to fill each subshell with all up arrows and then go back and fill the down arrows.

Another way to display electron configuration is to use core notation.

The core-the set of electrons with the configuration of the nearest noble gas

The outer part of an atom- the electrons other than the core.


How:

1) Locate the closest noble gas to that element on the periodic table.
2) Cite the noble gas in [ ] and then write the rest of the configuration.

Example 3:

Nitrogen:
Has 7 electrons= 1s^2 2s^2 2p^3

What you do is backtrack and find the nearest noble gas. Go backward across the periodic table(left) until you run off the table and come out one level up on the right side.

In this case the nearest NG is helium.

Its atomic number is 2 so, [He].
Then eliminate the first 2 electrons from the configuration and add the rest.

2s^2 2p^3

[He] 2s^2 2p^3.


Exceptions
Instead of: Cr --> [Ar] 4s^23d^4
Cu--> [Ar] 4s^23d^9
In actuality:
Cr--> [Ar] 4s^13d^5
Cu--> [Ar] 4s^13d^10 

http://www.youtube.com/watch?v=YURReI6OJsg&feature=relmfu 

That should help!