Electron Configurations

Purpose

• The properties of atoms are due to the properties of the electrons in the atom. For example, why does sodium like to lose one electron to make an ion while chlorine likes to gain one electron to make an ion. To understand this and much more chemical behavior, one needs to understand the electrons better. In this lesson, we will learn to assign electrons to specific energy levels and sublevels. At first, it may appear difficult, but toward the end of the lesson a simple trick using the periodic table will help you.

Electrons occupy different energy levels

• The electrons in an atom exist at different energy levels. • Each energy level holds up to 2n2 electrons where n represents the energy

level:– Energy level = 1, 2(1)2 = 2 electrons– Energy level = 2, 2(2)2 = 8 electrons– Energy level = 3, 2(3)2 = 18 electrons– And so on…technically to infinity

• Example: Si with 14 electrons– 2 electrons in energy level 1– 8 electrons in energy level 2– 4 electrons in energy level 3

• Note: While 18 will fit in energy level 3, silicon only has 14 electrons and 10 were already assigned leaving only 4 to partially fill the third energy level.

Ener

gy

1

2

345

∞

Pause and Practice

• How many electrons can fit into the 6th energy level for an atom?

Pause and Practice

• How many electrons can fit into the 6th energy level for an atom?– 2n2 electrons fit in an energy level– 2(6)2 = 72 electrons– Therefore, up to 72 electrons can fit into energy level 6 for any atom.

Energy levels can be broken down into separate sublevels and orbitals

• From the previous example with energy level 6, you may ask if all of the 72 electrons that can fit into energy level 6 occupy the same space around the atom. The answer is no. Each energy level can be further broken down into sublevels and orbitals. We will deal with sublevels now and save orbitals for the next lesson.

Every energy level (n) can be divided into n sublevels.

• Energy level 1 – 1 sublevel – called 1s• Energy level 2 – 2 sublevels called 2s and 2p• Energy level 3 – 3 sublevels called 3s, 3p, and 3d• Energy level 4 – 4 sublevels called 4s, 4p, 4d, and 4f• Energy level 5 … and so on technically to infinity.

Ener

gy

1

2

3

4

1s

2s

2p3s3p3d 4s

4p4d4f

Stop and notice some details

• A few things to notice– Each energy level (n) has n sublevels– The sublevels are assigned letters in a certain order

• s – the lowest energy sublevel for each level• p – second sublevel• d – third• f – fourth• Technically they go on, but we will stop at f.

– There is no 1p sublevel since energy level 1 can only have a single sublevel and the first one is labeled s. Similarly, there is no 2d or 3f.

– The energy of the 4s is slightly lower than the 3d. As you get to higher energy sublevels, there is some overlap between energy levels. We will learn how to use the periodic table to predict this.

Putting Electrons into sublevels

• We already learned that an energy level (n) can hold 2n2 electrons. We now know that energy levels are broken down into sublevels. We now need to know how many electrons can fit into each sublevel.

• Example: How many electrons fit in energy level 3 and how many in each sublevel?– 2(3)2 = 18 total electrons can fit in energy level 3

• 2 in 3s• 6 in 3p• 10 in 3d

Sublevel Maximum Number of e-

s 2

p 6

d 10

f 14

Summary

Energy Level Types of sublevels/ orbitals

Designations Electrons that they can hold

1 s 1s 2

2 sP

2s2p

26

3 sPd

3s3p3d

2610

4 spdf

4s4p4d4f

261014

Assigning electrons to orbitals according to the Aufbau principle

• The aufbau principle states that we fill lower energy sublevels before we place electrons into higher energy sublevels.– Earlier in the lesson we had an example with silicon and assigned 2

electrons to energy level 1, 8 to energy level 2, and 4 to energy level 3. We can now place the electrons into sublevels.

Ener

gy

1

2

3

1s

2s

2p3s3p3d

2 e-

8 e-

4 e-

2 e-

2 e-

2 e-

6 e-

2 e-

The Electron configuration shows how many electrons are in each sublevel

• We can now write what is called the electron configuration for silicon:– 1s22s22p63s23p2

Ener

gy

1

2

3

1s

2s

2p3s3p3d

2 e-

8 e-

4 e-

2 e-

2 e-

2 e-

6 e-

2 e-

1s2

Sublevel

Number of electrons in the sublevel

Stop and reflect

• By now you might be getting a little lost. Do not worry about assigning electrons to sublevels too much just yet as it will get much easier in a few slides. Just understand the big ideas.– Electrons do not all occupy the same space in an atom– Electrons occupy specific energy sublevels

The problem and the solution

• Problem:– We will run into a problem if we keep trying to assign electrons like we

did with silicon when we get to a certain number of electrons. The issue is that the 4s sublevel was actually a little lower in energy than the 3d sublevel. If we go further up in energy, we find that the 5s is similarly lower than the 4d and the 4f. In fact, 5s, 5p, and 6s are all lower in energy than the 4f. This would be difficult to try to remember.

• Solution:– We can use the periodic table to determine the order that the

sublevels fill.

Labeling the periodic table

• We will use the periodic table to remember the order that sublevels fill. Take a periodic table and do the following…

Label the first two columns in the first row starting with 1s as shown. Then the first two columns in the second row with 2s and

so on.

6

7

18

8A

17

7A

Period

1

2

3

1

1A

2

2A

4

4B

13

3A

12

2B

9

8B

10

8B

11

1B

4

5

15

5A

16

6A

14

4A

5

5B

3

3B

6

6B

7

7B

8

8B

1s

2s

3s

4s

5s

6s

7s

Label columns 13 – 18 starting with 2p as shown.

6

7

18

8A

17

7A

Period

1

2

3

1

1A

2

2A

4

4B

13

3A

12

2B

9

8B

10

8B

11

1B

4

5

15

5A

16

6A

14

4A

5

5B

3

3B

6

6B

7

7B

8

8B

1s

2s

3s

4s

5s

6s

7s

2p

3p

4p

5p

6p

7p

Label columns 3 – 10 starting with 3d as shown.

6

7

18

8A

17

7A

Period

1

2

3

1

1A

2

2A

4

4B

13

3A

12

2B

9

8B

10

8B

11

1B

4

5

15

5A

16

6A

14

4A

5

5B

3

3B

6

6B

7

7B

8

8B

1s

2s

3s

4s

5s

6s

7s

2p

3p

4p

5p

6p

7p

3d

4d

5d

6d

Label the two rows at the bottom of the periodic table starting with 4f as shown.

6

7

18

8A

17

7A

Period

1

2

3

1

1A

2

2A

4

4B

13

3A

12

2B

9

8B

10

8B

11

1B

4

5

15

5A

16

6A

14

4A

5

5B

3

3B

6

6B

7

7B

8

8B

1s

2s

3s

4s

5s

6s

7s

2p

3p

4p

5p

6p

7p

3d

4d

5d

6d

4f

5f

Summary

• Start the sublevel labeling in the following manner– 1s– 2p– 3d– 4f

• Notice how the number of columns in each block is the number of electrons that can fit in the sublevel.– Each s sublevel covers 2 columns, so they hold 2 electrons– Each p sublevel covers 6 columns s, so they hold 6 electrons– Each d sublevel covers 10 columns, so they hold 10 electrons– Each f sublevel covers 14 columns, so they hold 14 electrons

We can now use the periodic table to determine the filling order

• Go left to right in order of the elements:– 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 6s, 4f, 5d, 6p, 7s, 5f, 6d, 7p

6

7

18

8A

17

7A

Period

1

2

3

1

1A

2

2A

4

4B

13

3A

12

2B

9

8B

10

8B

11

1B

4

5

15

5A

16

6A

14

4A

5

5B

3

3B

6

6B

7

7B

8

8B

1s

2s

3s

4s

5s

6s

7s

2p

3p

4p

5p

6p

7p

3d

4d

5d

6d

4f

5f

Let’s look at an example

• What orbitals are occupied by the electrons in a bromine atom?– We have to get to atomic number 35. To get there, we go through:

• 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p

18

8A

17

7A

Period

1

2

3

1

1A

2

2A

4

4B

13

3A

12

2B

9

8B

10

8B

11

1B

4 Br

15

5A

16

6A

14

4A

5

5B

3

3B

6

6B

7

7B

8

8B

1s

2s

3s

4s

2p

3p

4p3d

Example Continued

• How many electrons are in each sublevel:– Count the number of elements that each sublevel passes through.

Stop at the designated element.– 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p5

– Note: Even though 4p can hold up to 6 electrons, bromine is the fifth element in the 4p block. Also, it only has 35 electrons and 30 of them occupied the other sublevels. This leaves 5 for the 4p.

18

8A

17

7A

Period

1

2

3

1

1A

2

2A

4

4B

13

3A

12

2B

9

8B

10

8B

11

1B

4 Br

15

5A

16

6A

14

4A

5

5B

3

3B

6

6B

7

7B

8

8B

1s

2s

3s

4s

2p

3p

4p3d

Pause and Practice

• Take a periodic table and label it 1s, 2s….• What is the electron configuration of 1s2 2s2… of the following atoms?

– S

– Zn

– Pb

Pause and Practice

• Take a periodic table and label it 1s, 2s….• What is the electron configuration of 1s2 2s2… of the following atoms?

– S• 1s22s22p63s23p4

– Zn• 1s22s22p63s23p64s23d10

– Pb• 1s22s22p63s23p64s23d104p65s24d105p66s24f145d106p2

• Note: Notice how each sublevel is filled to its capacity except the last one which may or may not be filled.

Try the exercises