## Electron Configurations

### More Detail About Electron Configurations

In Lesson 5 electron configurations were introduced, but you did not need to learn how to write electron configurations.  In this section we will go over the basics of electron configurations.

An electron configuration is, basically, an "address" that shows where the electrons can be found in an atom.  We use the wave-mechanical model when creating electron configurations.  Remember that the wave-mechanical model has atomic energy levels that are subdivided into "sublevels", which are further divided into "orbitals".

Examine the diagram below (found in example 3 in your workbook).

4f, has 7 orbitals called 4f and can have 14 electrons

4th            The 4th energy       4d, has 5 orbitals called 4d and can have 10 electrons

level has                4p, has 3 orbitals called 4p and can have  6 electrons

4 sublevels called:     4s, has 1 orbital  called 4s and can have  2 electrons

The 3rd energy      3d, has 5 orbitals called 3d and can have 10 electrons

level has                3p, has 3 orbitals called 3p and can have  6 electrons

3 sublevels called:   3s, has 1 orbital  called 3s and can have  2 electrons

The 2nd energy

level has                2p, has 3 orbitals called 2p and can have  6 electrons

2 sublevels called: 2s, has 1 orbital  called 2s and can have  2 electrons

The 1st energy

level has

1 sublevel  called: 1s, has 1 orbital  called 1s and can have  2 electrons

Just like in Bohr's model, the protons (and neutrons) are found in the nucleus of the atom.  The electrons are found in the space surrounding the nucleus.  Unlike Bohr's model, in the wave-mechanical model the electrons are found in orbitals, not orbits.  The farther the energy level is from the nucleus, the higher the energy and more electrons can fit into the larger space.  The first energy level, the level closest to the nucleus, has space for only one sublevel - called the 1s sublevel - which holds one orbital.  Each orbital, no matter which sublevel or energy level it is in, can hold a maximum of two electrons.  So the first energy level can hold two electrons.

If an atom has more than two electrons, then they will be found in higher energy levels.  In the second energy level there is more space than in the first energy level and we can see that there are two sublevels, called the 2s and 2p sublevels.  The 2s sublevel has just one orbital, which can hold two electrons, but the 2p sublevel has three orbitals, which can hold a total of six electrons (two electrons per orbital).

For atoms with just a few electrons, the order in which the electrons fill up the energy levels is exactly the way we would predict: the first two electrons fill up the 1s sublevel, the next two electrons go into the 2s sublevel, the next six go into 2p, and so on.  However, once we get to the third energy level, things get a little strange.  It turns out that it takes a little less energy for an electron to go into a 4s orbital than into a 3d orbital, and so the 4s fills up before electrons fill the 3d.  If you want to see a diagram of the energy levels and how they overlap, look back at example 18 in Lesson 5.  [Take a ruler or straightedge and lay it across the page so that it is perpendicular to the line labeled " Arbitrary Energy Scale."  Line it up so that it goes through the circle labeled 1s.  Now move the ruler up the page - keeping it perpendicular to the Energy Scale line - and look at the order in which it gets to each sublevel.  It should progress in the expected order (1s, then 2s, then 2p, etc) until just after you reach 3p.  The next sublevel you get to is 4s - before 3d!  Keep going until you get to the top of the diagram and notice any other "irregularities".]

### Writing Electron Configurations

When you write an electron configuration you write down where each electron can be found.  Remember that for a neutral atom, the number of electrons must be equal to the atomic number (the number of protons).  Electron configurations show the sublevel (1s, 2s, 2p, etc) and the number of electrons in each sublevel as a superscript after the sublevel.

Let's try the electron configuration of helium.  The atomic number of helium is 2, so a neutral atom of helium must have 2 electrons.  The electron configuration would be 1s2.

Why don't you try your hand at a few others?  Remember to fill each sublevel completely before moving on to the next sublevel.  (You can leave a sublevel partially full if you "run out" of electrons before you fill it up.)

Write out the electron configurations for:    Be, O, Ar, and Ca.

Be has 4 electrons, 1s22s2

O has 8 electrons, 1s22s22p4

Ar has 18 electrons, 1s22s22p63s23p6

Ca has 20 electrons, 1s22s22p63s23p64s2

Now you're ready to move on to the next section.

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