Introduction:
The electron configuration of an atom is the particular distribution of electrons among available shells. It is described by a notation that lists the subshell symbols, one after another. Each symbol has a subscript on the right giving the number of electrons in that subshell. For example, a configuration of the lithium atom (atomic number 3) with two electrons in the 1s subshell and one electron in the 2s subshell is written 1s22s1. sublevel orbital maximum # of electrons s 1
2
p
3
6 d 5
10
f
7
14

The notation for electron configuration gives the number of electrons in each subshell. The number of electrons in an atom of an element is given by the atomic number of that element.
On the left we have a diagram to show how the orbitals of a subshell are occupied by electrons. On the right there is a diagram for the filling order of electrons in a subshell.

Here are some examples that show how to use the filling order diagram to complete the electron configuration for a certain substance.

Element
# of Electrons in Element
Electron Configuration
He
2
1s2
Li
3
1s22s1
Be
4
1s22s2
O
8
1s22s22p4
Cl
17
1s22s22p63s23p5
K
19
1s22s22p63s23p64s1

The three rules for filling electrons in orbitals:
1. The Aufbau Principle
Each electron occupies the lowest energy orbital
All orbitals related to an energy level are of equal energy. (i.e. The three 2p orbitals are the same energy level).
2. Hund’s Rule
Single electrons with the same spin must occupy each equal-energy orbital before additional electrons with opposite spins can occupy the same orbitals.
3. Pauli Exclusion Principle
A maximum of two electrons may occupy a single orbital, but only if the electrons have opposite spins.
Exceptions to filling order are copper, chromium and some others- because half-filled shells are more stable, so an atom may borrow an electron from next s shell to half fill a lower energy level.

Orbital notation for potassium: ___ 1s ___2s ___ ___ ___2p ___3s ___ ___ ___3p ___4s

Noble gas notation for potassium: [Ar] 4s1

Electron Configuration and the Periodic Law:
With increasing atomic number, the electron configuration of the atoms displays a periodic variation. Because of this the elements show periodic variations of both physical and chemical behavior. The periodic law is a law stating that when the elements are arranged by atomic number, their physical and chemical properties vary periodically. In this simulation, we will look specifically at atomic radius.
The size of the electron cloud increases as the principal quantum number** increases. Therefore, as you look down the periodic table, the size of atoms in each group is going to increase. When you look across the periodic table, you see that all the atoms in each group have the same principal quantum number. However, for each element, the positive charge on the nucleus increases by one proton. This means that the outer electron cloud is pulled in a little tighter. One periodic property of atoms is that they tend to decrease in size from left to right across a period of the table. So, the atomic radii increases top to bottom and right to left on the periodic table.
**Each electron in an atom is described by four different quantum numbers. Three of these quantum numbers (n, l, and m) represent the three dimensions to space in which an electron could be found. The n quantum number relates to the size of the atomic orbital. n can have any positive integer value from 1 to 7. The smaller the value of n, the lower the energy level, the higher the value of n, the higher the energy level. The principal quantum number, n, also equals the number of subshells. n2 is the number of orbitals in each energy level and 2n2 is the number of electrons that can be held in each energy level.

Simulation: Electron Configuration gizmo from ExploreLearning.com

Purpose of this Simulation:
To:
1. Create the electron configuration of any element by filling