orbital diagrams, electron configurations, & valence electrons

Orbital Diagrams,

Electron Configurations,

&

Valence Electrons

Bohr’s Model: electrons would exist in different rings around the nucleus just like the planets are in different orbits around the sun. This is sometimes called the planetary model of the atom

Bohr’s Atom

7 orbits correspond to 7 periods on Periodic Table.

Which Orbit has the lowest energy?

n = 1, the orbit closest to the nucleus.

Bohr’s model was replaced by the quantum mechanical model of the atom.

• This new model uses principle quantum levels that are similar to Bohr’s orbits, but are then divided into sublevels

The principle quantum level is a number from 1-7, with 1 being the lowest energy and 7 being the highest in energy.

The sublevels are s, p, d, and f

These numbers and letters correspond to the periods and the “blocks” on the periodic table.

• Each sublevel has a least one orbital.

• Each orbital can hold 2 electrons

The s sublevel only has 1 orbital, and each orbital holds 2 electrons

Shape: Sphere

Which matches the 2 elements in each period of the s block

The p sublevel has 3 orbitals, and each orbital holds 2 electrons, for a total of 6 electrons – Matching the 6 elements in each period of the p block

Shape: dumbbell or peanut

The d sublevel has 5 orbitals, and each orbital holds 2 electrons, for a total of 10 electrons – matching the 10 elements in each period of the d block

Shape: clover/double dumbbell

The f sublevel has 7 orbitals, and each orbital holds 2 electrons, for a total of 14 electrons – matching the 14 elements

Funky / flower

We put together the principle quantum number and sublevel letter to talk about a specific orbital

But not all sublevels are possible for

each energy level.

Principle Quantum Level Possible Sublevels

1 s2 s, p3 s, p, d4 – 7 s, p, d, f

1s2s 2p3s 3p 3d4s 4p 4d 4f

5s 5p 5d 5f6s 6p 6d (6f)

7s 7p (7d 7f)

The arrangement of electrons in an atom is called an orbital diagram or electron configuration.

1) There are three rules that we must follow when making an orbital diagram (OD) or an electron configuration (EC):

A) The aufbau principle says electrons must fill lower energy levels before electrons can fill higher energy levels.

This means 1s is filled before 2s, etc

B) The Pauli exclusion principle says that only two electrons can fill each orbital…and they must have opposite spins.

No Yes

C) Hund’s rule says electrons must spread out in the orbitals of each sublevel (p, d, or f) before they double up.

Yes No

□ □ □ □ □ □

2p 2p

2) If we use boxes to represent orbitals, then the following aufbau diagram shows all the possible places an electron could be:

Remember:

s has 1 orbital…..holds 2 electrons

p has 3 …..holds 6 electrons

d has 5 …..holds 10 electrons

f has 7 …..holds 14 electrons

Notice that the energy increases from bottom to top,

HighEnergy

LowEnergy

and some of the orbitals do not fill in the same number order as the others.

Fill from the bottom to the top, spreading out the electrons before doubling them up

□ □ □

□

□ □ □

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□ 1s

2s

3s

2p

3pHydrogenH

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□

□ □ □

□

□ 1s

2s

3s

2p

3pHeliumHe

CompletelyFilled

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□

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□

□ 1s

2s

3s

2p

3pLithiumLi

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□

□ □ □

□

□ 1s

2s

3s

2p

3pBerylliumBe

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□

□ □ □

□

□ 1s

2s

3s

2p

3pBoronB

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□

□ □ □

□

□ 1s

2s

3s

2p

3pCarbonC

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□

□ □ □

□

□ 1s

2s

3s

2p

3pNitrogenN

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□

□ □ □

□

□ 1s

2s

3s

2p

3pOxygenO

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□

□ □ □

□

□ 1s

2s

3s

2p

3pFluorineF

□ □ □

□

□ □ □

□

□ 1s

2s

3s

2p

3pNeonNe

CompletelyFilled

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□

□ □ □

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□ 1s

2s

3s

2p

3pSodiumNa

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□

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□ 1s

2s

3s

2p

3pMagnesiumMg

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□ 1s

2s

3s

2p

3pAluminumAl

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□

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□ 1s

2s

3s

2p

3pSiliconSi

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□

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□ 1s

2s

3s

2p

3pPhosphorusP

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□

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□ 1s

2s

3s

2p

3pSulfurS

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□

□ □ □

□

□ 1s

2s

3s

2p

3pChlorineCl

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□

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□ 1s

2s

3s

2p

3pArgonAr

CompletelyFilled

3) The correct order of filling is: 1s 2s 2p 3s 3p 4s 3d 4p 5s 4d 5p 6s 4f 5d 6p 7s 5f 6d 7p

Notice: s block fills in period 1 p block fills in period 2

d block fills in period 3: which is 1 behind the actual period f block fills in period 4: which is 2 behind the actual period

n = period #

s & p block fill at n

d block fills at n – 1

f block fills at n - 2

4) Orbital Diagram: Orbitals are sometimes shown as boxes in a

horizontal row

Remember: s = 1 orbital p = 3 orbitals d = 5 orbitals f = 7 orbitals

5) Arrows are used to represent the electrons, so if two arrows go in the same box, one points up and the other points down.

□ □ □

□

□ 1s

2s

2pNitrogenN

7 electrons

Becomes:

□ □ □ □ □1s 2s 2p

Cobalt (27 electrons – 27 arrows)

□ □ □□□ □ □□□ □ □□□□□1s 2s 2p 3s 3p 4s 3d

Cations and anions• * Ions: work the same way but remember that

electrons are negative. +2 means you lost two electrons, -2 means you gained two electrons.

Try Bromine

Bromine (35 electrons)

□ □ □□□ □ □□□ □ □□□□□ □□□1s 2s 2p 3s 3p 4s 3d 4p

Try Oxygen, O2- and Gallium on the back of your notes

Oxygen

□ □ □□□ □ □□□ □ □□□□□ □□□1s 2s 2p 3s 3p 4s 3d 4p

O-2

□ □ □□□ □ □□□ □ □□□□□ □□□1s 2s 2p 3s 3p 4s 3d 4p

Gallium

□ □ □□□ □ □□□ □ □□□□□ □□□1s 2s 2p 3s 3p 4s 3d 4p

6) Electron Configuration: Instead of drawing boxes and arrows, the number of electrons in each sublevel is turned into a superscript and is written with the quantum number and the sublevel letter.

A) If all the orbitals are filled, the entire sequence would be:

1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6

5s2 4d10 5p6 6s2 4f14 5d10 6p6 7s2

5f14 6d10 7p6

NitrogenN

□ □ □ □ □1s 2s 2p

Becomes:1s2 2s2 2p3

Cobalt

□ □ □□□ □ □□□ □ □□□□□1s 2s 2p 3s 3p 4s 3d

Becomes:

1s2 2s2 2p6 3s2 3p6 4s2 3d7

Try Bromine, Oxygen, O2-, Ca 2+

and Gallium on the back of your notes

Bromine (35 electrons)

□ □ □□□ □ □□□ □ □□□□□ □□□1s 2s 2p 3s 3p 4s 3d 4p

Becomes:1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p5

Electron configurations for:

Oxygen: 1s22s22p4

O2- : 1s22s22p6

Ca2+ : 1s22s22p63s23p6

Gallium: 1s22s22p63s23p64s23d104p1

7) Short cut: Noble gas configuration. Instead of writing out the entire electron configuration, we can use the previous noble gas to take the place of part of the electron configuration: must start with a noble gas

Example:

Magnesium: 1s22s22p63s2

Neon: 1s22s22p6

Noble Gas configuration:Magnesium: [Ne] 3s2

Example:

Polonium:1s22s22p63s23p64s23d104p65s24d105p66s24f145d106p4

Xenon:1s22s22p63s23p64s23d104p65s24d105p6

Polonium:[Xe] 6s24f145d106p4

Try Bromine, Oxygen, O2-, Ca 2+

and Gallium on the back of your notes

Noble gas configurations

Bromine [Ar] 4s2 3d10 4p5

Oxygen: [He] 2s22p4

O2-: [He] 2s22p6

Ca2+ : [Ne] 3s23p6

Gallium: [Ar] 4s23d104p1

8) Valence Electrons: Electrons in the outer-most orbital – which is the highest energy level.

Very important: electrons involved in chemical bonds – determine chemical properties of element

These electrons are called valence electrons.

Only the s and p block electrons are counted…so the number of valence electrons must be a number from 1 to 8

9) Electron Dot Diagrams (Lewis Dot Diagrams):

The number of valence electrons makes a big difference in how the element will bond, so to make it easy to predict, we draw electron dot diagrams.

A) In an electron dot diagram, we use the symbol of the element and dots to represent the number of valence electrons. The number of dots matches the group number on the Periodic Table.

B) Only s and p electrons with the highest quantum number count for dot diagrams, even if there are d and f electrons

Lithium = 1s22s1

So Li

Beryllium = 1s2 2s2

So Be

Boron = 1s2 2s22p1

So B

Carbon = [He] 2s22p2

So C

Nitrogen = [He] 2s22p3

So N

Oxygen = 1s2 2s22p4

So O

Fluorine = 1s2 2s22p5

So F

Neon = 1s22s22p6 or [Ne]

So Ne

All noble gases have full outer shells: 8 valence electrons (except He which is full at 2, since it only has the s orbital)

Magnesium = 1s22s22p63s2

So

Magnesium = 1s22s22p63s2

So

Mg

Mg

Polonium:

1s22s22p63s23p64s23d104p65s24d105p66s24f145d106p4

Polonium:

1s22s22p63s23p64s23d104p65s24d105p66s24f145d106p4

So Po

Po

Look at your Periodic Table Notice: the A group number = the number of valence electrons

1A

3A2A 4A 5A 6A 7A

8A

Look at your Periodic Table - Notice: The A group number = the number of valence electrons (except for He)