Electron Configuration

The electron configuration of an atom is a  notation which shows how electrons are distributed in orbits.  The format consists of a series of numbers, letters and superscripts.

To understand this notation we need to understand 3 concepts:

1.  Orbital - a place  around the nucleus where we are most likely to find the electron.  An orbital is like a sporty 2-seater car - it can hold no more than 2 electrons!

2.  Subshell - There are differently shaped orbitals.  Some orbitals are round or spherical.  Others are dumbbell shaped.  A subshell is just a way to organize the different orbitals.  Here is a table that assigns a letter to the different subshells.  It also tells us how many orbitals are in each subshell and the shape of the orbitals in that subshell.

 

 Subshells

Subshell # of Orbitals Orbital Shape
s 1 spherical
p 3 dumbbell
d 5 ***
f 7

***

Don't worry about the shapes of the d and f orbitals.

Important - In our definition of an orbital we said that an orbital holds no more than 2 electrons.  So, what's the maximum number of electrons that can occupy the "p" subhshell?  Well...I see from the table that the p subshell has 3 orbitals.  Each orbital can hold 2 electrons, so...

# electrons in p subshell = 3 orbitals  X  2 electrons/orbital

# electrons in p subshell = 6

 

3.  Shell - the shell represents the orbit around a nucleus.  The first shell (or orbit) is close to the nucleus.  The second shell is a little farther out from the nucleus.  We can think of the shells as the old Bohr orbits.   The energy of the orbits increase as we move away from the nucleus.

 Example:  An electron in the 1st Shell would have less energy than an electron in the 2nd Shell.

Ok, now that we understand the concept of orbitals, subshells, and shells we can look at the distribution of electrons in specific atoms.

Here is the electron configuration for the element Helium. 

1s2

Helium has an Atomic Number equal to 2.  We know that Helium has 2 protons in its nucleus and 2 electrons.  The electron configuration for Helium gives us the following information:

How do we know which orbitals, subshells, and shells to use when writing the electron configuration for an element?

That's easy.....we use something called an "Order of Filling Diagram". 

+

  elecfill.gif (5598 bytes)

 

Notice the "+" at the top of the diagram.  Think of the + as the positively charged nucleus at the center of the atom.

 

As you go down the diagram you are stepping into different orbits and getting farther from the nucleus.  For example...the "1" means you are in the 1st Shell.  The "2" is the 2nd Shell, etc.  Notice that the diagram above shows an atom with 7 Shells or orbits.

Also, as you change Shells (or orbits) you are changing energy.  The diagram shows you that energy INCREASES as you get farther from the nucleus.  An electron in the 3rd shell has more energy than an electron in the 1st shell.

In each Shell you see the various subshells.  For example, in the 2nd Shell there are 2 subshells:  an "s" subshell and a "p" subshell.

Each subshell also has a definite energy.  We can follow the order of subshell energy by following the diagonal arrows on the diagram.  Those arrows point in the direction of increasing energy.  Here are some examples:

The "1s" subshell has the lowest energy.

The "2s" subshell is the next in order of energy.

The "2p" subshell is the next in order of energy.

The "3s" subshell is the next in order of energy.

The "3p" subshell is the next in order of energy.

See how we are following the ARROWS.  Following the arrows allows us to arrange electrons in order of increasing energy.

Important - We write electron configurations by placing electrons in subshells in order of their energy.

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

Example

Write the electron configuration for Carbon.

Carbon has Atomic No. = 6

A Carbon atom has 6 electrons.

Recall that each subshell holds a definite number of orbitals and that each orbital holds no more than 2 electrons.

1s2 2s2 2p2

We started with lowest energy shell and placed 2 electrons into the "1s" subshell because the "s" subshell has only 1 orbital and thus only 2 electrons.

Next, we placed 2 electrons in the "2s" subshell.  It's higher in energy but still only holds 2 electrons.

Finally, we placed 2 electrons in the "2p" subshell.  A "2p" subshell has 3 orbitals and can hold up to 6 electrons, but we ran out of electrons for Carbon. 

Don't let orbitals, subshells, and shells confuse you.  Remember, it's a means of arranging the electrons in an atom in order of increasing energy.  Using the rules listed on this page and the Order of Filling diagram you can write the electron configuration for many elements.