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**Unit 4: Electron Arrangement**

So, our old buddy Neils Bohr came up with the idea that electrons move in particular paths around the nucleus of an atom. Today, we call these paths **energy levels** or **electron shells. **Turns out there are seven possible energy levels or electron shells not all elements atoms are going to need all seven, but there are seven possible.

When we are talking about **energy levels** we **use numbers (1 -7) to identify them**.

When we are talking about **electron shells** we **use letters (K Q) to identify them**.

Energy level: 1 2 3 4 5 6 7

Electron shell: K L M N O P Q

The first energy level (a.k.a. K shell) is the one that is closest to the nucleus and has the lowest energy

The second energy level (a.k.a. L shell) is a bit further from the nucleus

The third energy level (a.k.a. M shell) is a little further out still and so on.

Each energy level (electron shell) has a different capacity of electrons they can hold different maximum numbers of electrons. To calculate their maximum capacities, we can use a very handy formula:

2n^{2 }

Where n = the number or energy level. Here s how it works: first, take the number of energy level and square it (multiply it by itself), then multiply that answer times 2.

Examples:

To find the maximum number of electrons in the K shell or first energy level

K = 1 so we re going to use the number 1 for our n. 1 X 1 = 1, 1 X 2 = 2. **The first energy level or the K shell can hold a maximum of 2 electrons.**

To find the maximum number of electrons in the L shell or second energy level

L = 2 so we re going to use the number 2 for our n. 2 X 2 = 4, 4 X 2 = 8. **The second energy level or the L shell can hold a maximum of 8 electrons. **

**M = 3**, 3 X 3 = 9, 9 X 2 = **18 N = 4**, 4 X 4 = 16, 16 X 2 =

**32**

O = 5, 5 X 5 = 25, 25 X 2 =

O = 5

**50**

P = 6, 6 X 6 = 36, 36 X 2 =

P = 6

**72**

Q = 7, 7 X 7 = 49, 49 X 2 =

Q = 7

**98**

Now, let s get into the chart on the following page, **Electron Configurations of the Elements**! Good grief, look at all those little numbers!

First, notice that on the left side of both columns there are symbols of the elements beginning with the simplest element in the whole universe, hydrogen. To the left of the elements symbols are numbers see them? For hydrogen, there s a 1 to the left, for helium, there s a 2, for lithium there s a 3, and so on. What do you figure those numbers represent? That s right, those are the atomic numbers! These atoms on this chart are all neutral so besides telling us how many protons each element s atoms have (the atomic number) that number also tells us how many electrons each element s atoms have. That s going to be very handy information for us.

Next, look across each column. There are seven different columns headed with a 1, then 2, then 3, and so on. What the heck are those numbers? Well, there are seven of them .hmmmmmm ..seven columns .seven energy levels! That s right each column represents an energy level (or electron shell). Under the numbers that head the columns, there are also some little letters (s, s p, s p d, and so on don t worry about those just yet we ll get to those soon.

OK, let s start with hydrogen. By looking just to the left of H we see a 1. That means that hydrogen has an atomic number of 1 (one proton) and because it is neutral it also has one electron. Where does that one electron go? It goes in the first energy level (there it is under the 1). Does that make sense? Sure. Remember, the first energy level is the one closest to the nucleus and it requires the least amount of energy to get an electron into it.

How about helium? There s a 2 to its symbol s left. 2 electrons. Where do they go? They both go into the first energy level.

How about lithium? There s a 3 to its left. 3 electrons. Where do they go? OK, keep an eye on this rascal! Remember that the first energy level can only hold 2 electrons. So here s what happens: the first two electrons go into the first energy level and fill it up no more room there! The third electron then goes into the second energy level. That s why there is a 2 under the 1 column and a 1 under the 2 column.

Beryllium has 4 electrons. The first two go into the first energy level and fill it up. The other two go into the second energy level.

Boron has 5 electrons. The first two go into the first energy level and fill it up. The other 3 go into the second energy level (for our purposes right now, add the 2 and the 1 under column 2).

How about carbon? It s got 6 electrons. The first 2 fill up the first energy level, the other 4 (2 + 2) go into the second energy level.

Now jump down to sodium. 11 electrons. 2 in the first energy level that fills it up, the next 8 go into the second energy level (2 + 6) remember that the second energy level has a maximum of 8, so the last electron has to go into the third energy level see it?

OK, now let’s use some of this information to figure out **series of elements**!

We will see series of elements also when we get into the Periodic Table in the next unit (those will be the horizontal rows on the Table) but guess what, the series of elements we discuss here will be exactly the same as those we find on the Table that s good news and it s all because of electron arrangement!

**The** **First Series **is made up of all the elements that have all of their electrons completely contained in the first energy level. Take a look at the Electron Configuration chart. There are only two elements that fit this criterion: **Hydrogen and Helium**

**The** **Second Series **is made of all the elements that have all of their electrons completely contained in the first **2** energy levels. Look at the chart: the second series **begins with Lithium (3) and it ends with Neon (10)**

**The** **Third Series** is made of all the elements that have all of their electrons completely contained in the first **3 **energy levels. **Begins with Sodium (11), ends with Argon (18)**

**The** **Fourth Series** is made of all the elements that have all of their electrons completely contained in the first **4 **energy levels. **Begins with Potassium (19), ends with Krypton (36)**

**The** **Fifth Series **is made of all the elements that have all of their electrons completely contained in the first **5 **energy levels. **Begins with Rubidium (37), ends with Xenon (54) Note: **take a look at number 46, Palladium. By a strict interpretation of the definition of the fifth series we couldn t include Pd because it has no electrons in the fifth energy level. But since it s got a bunch of fifth series elements above it and below it, we make an exception and leave it in the fifth series.

**The Sixth Series **is made of all the elements that have all of their electrons completely contained in the first **6 **energy levels. **Begins with Cesium (55), ends with Radon (86)**

**The Seventh Series **is made of all the elements that have all of their electrons completely contained in the first 7 energy levels. **Begins with Francium (87), ends with Oganesson (118). **Elements 113 – 118 were all discovered or synthesized by slamming nuclei of lighter elements together in nuclear accelerators in the early 2000s and officially named in 2012 and 2016, thereby completing the seventh series.

** **

** Now things are really going to get interesting!**Our friend Schr ger and his colleagues thought the Bohr s planetary model of the atom was a fine piece of work, but it wasn t complicated enough to reflect the true nature of the atom. In other words, the atom was more complicated than all that!

So the next step was to say that **within the energy levels (or electron shells) are sublevels (or orbitals). **We identify these little guys with little letters :

s, p, d, f

The s orbital has a spherical shape (take a look at the following diagram what tells you it s supposed to represent a sphere rather than a circle? Hmmmmmmmm. Let s see, there s an X axis, there s a Y axis those are all you would need for a two dimensional circle ..but wait, what s that crazy Z axis doing there? Representing the third dimension, that s what! The third dimension represents depth. You ve got three dimensions and so do atoms.

**The ****s orbital ****has only ****one orientation**. What s that mean? That means that there is only one way to put that sphere so that the point at which the X,Y, and Z axes intersect is the exact center of the sphere.

**The ****p orbital****, which has sort of an hourglass shape, has ****three orientations ****(px**: p for the shape and x because it s straddling the X axis**, py: **p for the shape and y because it s straddling the Y axis**, pz: **p for the shape and z because it s straddling the Z axis)

**The ****d orbital****has different shapes and ****five orientations**

**The ****f orbital****really has a bunch of different shapes **(I don t have them illustrated here but you ve got to trust me on this!) **and has ****seven orientations**

**To calculate the maximum number of electrons that can fit in a particular orbital we have to take a look at:**

**The Pauli** **Exclusion Rule!**

In 1925, Austrian physicist Wolfgang Pauli came up with his exclusion principle that is very handy in determining how many electrons each orbital can hold. It goes like this:

**Any orientation can hold a maximum of 2 electrons and if it does hold its maximum of 2 electrons, they electrons have to be spinning in opposite directions (think clock-wise and counter clock-wise)**

**So:**

**The ****s orbital****has 1 orientation and can hold a ****maximum of 2 electrons**

**The ****p orbital****has 3 orientations and can hold a ****maximum of 6 electrons**

**The ****d orbital****has 5 orientations and can hold a ****maximum of 10 electrons**

**The ****f orbital****has 7 orientations and can hold a ****maximum of 14 electrons**

It s always a good thing when two theories agree because if they don t you have to wonder which one is correct. Well, here s an aspect of the Planetary Model and an aspect of the Wave or Quantum Mechanical Model agree: **maximum numbers of electrons**.

**The K shell (or 1**^{st}** energy level) only contains an s orbital**

Maximum number of electrons in the K shell = 2, maximum number of electrons in an s orbital also = 2 (that s why the K shell can t hold any other orbitals they wouldn t fit!)

**The L shell (or 2**^{nd}**energy level) contains both s and p orbitals**

Maximum number of electrons in the L shell = 8. Maximum number of electrons in an s orbital = 2, maximum number of electrons in a p orbital = 6, 2 + 6 = 8

**Note what happens when you jump up to a higher shell or energy level: you start right back with another s orbital!**

**The M shell (or 3**^{rd}**energy level) contains s, p, and d orbitals**

Maximum number of electrons in the M shell = 18. Maximum number of electrons in an s orbital = 2, maximum number of electrons in a p orbital = 6, maximum number of electrons in a d = 10. 2 + 6 + 10 = 18

**All the other shells or energy levels will have s, p, d, and f orbitals**

Look at the Electron Configurations of the Elements chart again look under the 1 in the first column. What do you see: s

Look under the 2, what s there? s, p

Look under the 3: s, p, d

Look at the other columns: s, p, d, f (don t let that 7 column bother you it only has a s but they did that just to save space, it also would have s, p, d, f just like columns 4, 5, 6)

Actually, Schr ger and his colleagues said that whatever the number of energy level is, it will have that many different orbitals.

So the 4^{th} would have s, p, d, f

The 5^{th} would have s, p, d, f, g

The 6^{th} would have s, p, d, f, g, h

And the 7^{th} would have s, p, d, f, g, h, i

But, lucky for us, we are nowhere near the number of elements that we have to worry about the g, h, or i orbitals. Still, the fact that Schr ger has those in his model points out how far reaching it is he has already built into his model a capacity to deal with elements that haven t even been dreamed of yet!

**Electron Configuration Notation**

A neat short-hand way of indicating where electrons are not only in which shell or energy level, but also in which orbital, is called the Electron Configuration Notation. Here is how it works:

Hydrogen only has one electron. Its electron configuration notation would be written:

**1s**^{1}

The big 1 stands for the 1^{st}energy level, the s indicates the s orbital, and the superscript 1 tells us how many electrons we are dealing with.

Let s try Helium. 2 electrons. 1s^{2}

First energy level, s orbital, 2 electrons.Read it backwards, it makes sense that way too:

Helium has 2 electrons in the s orbital of the 1^{st} energy level

Lithium has 3 electrons. We always start at the beginning and that s the first energy level. It only has an s orbital and it only holds 2 electrons. It always fills up first. When it s full, we move to the 2^{nd} energy level and we always start over with another s orbital when we move to a higher energy level. So lithium s electron configuration notation is:

1s^{2}2s^{1}

Beryllium has 4 electrons. 1s^{2}2s^{2}

Boron has 5 electrons. We are going to stay in the 2^{nd} energy level we have filled up the s orbital there with 2 electrons so boron s last electron will go in the p orbital of the 2^{nd} energy level:

1s^{2}2s^{2}2p^{1 } or it can also be written 1s^{2}2s^{2}p^{1}

(I like to write that second 2 before the p just to reiterate that I m still in the 2^{nd} energy level, but the second way is very common)

Now that we re in the p orbital of the 2^{nd} energy level, we re just going to add an electron to the p until we fill it up with 6 electrons (and consequently fill up the 2^{nd}energy level with 8 electrons total. 2 in the s and 6 in the p.

So Carbon (with 6 electrons) would be 1s^{2}2s^{2}2p^{2}

Neon (with 10 electrons) would be 1s^{2}2s^{2}2p^{6}

Sodium (with 11 electrons) would be 1s^{2}2s^{2}2p^{6}3s^{1}since we filled up the 2^{nd}energy level with 8 electrons now we move on to the 3^{rd} energy level and start right back with an s orbital.

Let s try Aluminum with 13 electrons. 1s^{2}2s^{2}2p^{6}3s^{2}3p^{1}

How about Argon with 18 electrons? 1s^{2}2s^{2}2p^{6}3s^{2}3p^{6}

**Did you notice how the superscripts always add up to the original number of electrons? Makes sense, doesn t it? That s a good way to double check your work to be sure you have accounted for all electrons!**

Take a look at Potassium (19) on the Electron Configurations chart. It does something that you might not expect: it s got 2 electrons in the 1s, 2 electrons in the 2s, 6 electrons in the 2p, 2 electrons in the 3s, 6 electrons in the 3p (OK, so far so good) but then it puts its last electron in the 4s orbital! What the heck is going on? Why didn t the last electron go into the 3d? Here s the deal: an s orbital is a very low energy orbital (think of a door made out of a piece of paper; it would be very easy to walk right through!) a d orbital is a much higher energy orbital and it requires a lot more energy to get an electron into it (think of an oak door). So it actually requires less energy for potassium to get its last electron into the 4s (even though it s going into a higher energy level) than to get it into the d orbital of the 3^{rd}energy level. Calcium (20) does that too it puts its last 2 electrons in the 4s. We don t start backfilling the 3d until we get to Scandium (21). Check it out on the chart, go through a bunch of those rascals and see how they work!

One more kind of notation: **Electron Dot Notation**

Electron Dot Notation is a method used in Chemistry to indicate the number of electrons that are in an entire outer electron shell or energy level (not just the outer orbital but the whole outer shell). Outer shell electrons (also known as valence electrons) are very important in Chemistry because they have so much to do with determining the properties of elements we ll talk more about that in the next unit on the Periodic Table.

Here s how it works:

I m going to use an X as a generic element s symbol (use the specific element s symbol when you are working on a specific element s Electron Dot Notation.

Here’s a very cool site about the first image of an actual electron:

http://www.physorg.com/news122897584.html