This video demonstrates how to write electron
configurations and draw orbital diagrams for
main group elements.
To write an electron configuration you could
memorize the order in which orbitals are filled
according to their energy level, but a more
convenient method is to use the periodic table.
The periodic table is arranged in blocks,
each block represents an orbital, and each
space in the block counts as one electron.
The s block is the first two left-hand columns
of the periodic table and includes helium.
The p block is the last six columns on the
right hand side stating at boron.
The d block is the transition metals in the
middle of the periodic table, and the f block
is the lanthanide and actinide series.
To begin start at the top left hand corner
of the periodic table and work your way down
by reading across a row from left to right
and filling in the proper amount of electrons
for each orbital until you reach your element.
Use the number assigned to each row, 1 thru
7, as the value of the principle quantum number
n, when you arrive at the d and f blocks;
subtract one from the n value for the d orbitals,
and two from the n value for the f orbitals.
This periodic table has the electron configuration
for each row written along the left hand side
using the method just outlined.
Let’s use it to write the electron configuration
of a neutral bromine atom, a bromine atom
has 35 electrons.
Using the blocks in the periodic table we
can write the electron configuration of bromine
as: 1s22s22p63s23p64s23d104p5.
If we were writing the electron configuration
for the bromine anion, we would begin writing
the same configuration as the neutral bromine
atom except the number of electrons in the
p orbitals changes from five to six because
the bromine ion has 36 electrons.
To write a shorthand electronic configuration
use the chemical symbol of the previous noble
gas in square brackets, for bromine it’s
argon, then write out the orbitals that are
occupied by valence electrons.
This makes the shorthand electron configuration
for bromine [Ar]4s23d104p5.
The last example is an orbital diagram.
We are going to draw the orbital diagram for
oxygen.
A neutral oxygen atom has 8 electrons, so
the orbitals that will contain electrons are
the 1s, 2s and 2p.
The levels need to be arranged in order of
increasing energy, and the lowest energy level
orbitals are filled first.
Each filled orbital is assigned two electrons
of opposite spin according to the Pauli Exclusion
Principle.
So two of oxygen’s electrons go in the 1s
orbital, and two electrons go in the 2s orbital.
That leaves four electrons to fill the 2p
orbitals.
Remember Hund’s rule and place one electron
with the same spin orientation in each of
the three p orbitals before pairing the fourth
electron.
Now you know the basics of electron configurations
and orbital diagrams.
You should be able to write electron configurations
and draw orbital diagrams for all of the main
group elements including ions using this method.
