Atomic Structure: Electron Configuration - Shells & Orbitals

Question 1

Electron Shells / Energy Levels

Answer 1

• Electrons have fixed energies
• They move around the nucleus in shells or energy levels
• The main shells/energy levels are called principal energy levels and are given a number called the principal quantum number (n)
• The further a shell is from the nucleus, the higher its energy and the larger its principal quantum number

Question 2

Electron Sub-Shells

Answer 2

• The main shells are then split into sub-shells
• Different electron shells have different number of sub-shells, which each have a different energy
• The number of sub-shells in an electron shell is equal to its principal quantum number (electron shell 1, has 1 sub-shell; electron shell 2, has 2 sub-shells)
• Sub-shells can be s,p,d, or f sub-shells

Question 3

Electron Sub-Shell Orbitals

Answer 3

• The sub-shells have different numbers of orbitals
• An orbital is a region of a sub-level that contains a maximum of 2 electrons

• s

• Number of orbitals: 1
• Max number of electrons: 1x2 = 2

• p

• Number of orbitals: 3
• Max number of electrons: 3x2 = 6

• d

• Number of orbitals: 5
• Max number of electrons: 5x2 = 10

• f

• Number of orbitals: 6
• Max number of electrons: 7x2 = 14

Question 4

Total Number of Electrons in each Shell

Answer 4

• 1st shell

• Sub-shells: 1s
• Total number of electrons: 2

• 2nd shell

• Sub-shells: 2s, 2p
• Total number of electrons: 2 + 6 = 8

• 3rd shell

• Sub-shells: 3s, 3p, 3d
• Total number of electrons: 2 + 6 + 10 = 18

• 4th shell

• Sub-shells: 4s, 4p, 4d, 4f
• Total number of electrons: 2 + 6 + 10 + 14 = 32

Question 5

Aufbau Principal

Answer 5

• Electrons will enter the lowest energy sub-level available
• Energy levels are not entered until those below them are filled

Question 6

Order of Filling Orbitals

Answer 6

• The 4s orbital is filled before the 3d orbitals
• Orbitals are not always filled in numerical order because the principal energy levels get closer together/overlap as you get further from the nucleus

Question 7

Orbitals & Their Shapes

Answer 7

• Orbitals are regions of space that electrons are most likely to be in

• These shapes of these orbitals represent a volume of space in which there is a 95% probability of finding an electron
• Can hold a maximum of 2 electrons
• s orbitals: spherical
• p orbitals: dumb-bell shaped
• d orbitals: various shapes

Question 8

Spin Diagrams

Answer 8

• A spin diagram shows how the orbitals are filled
• Orbitals are represented by squares, and electrons by half-arrows pointing up or down
• When we have two electrons in the same orbital, these two electrons must have opposite spin

Question 9

Hund’s Rule

Answer 9

• Electrons occupy all empty orbitals within a sub-level before they start to form pairs in orbitals

Question 10

Electron Configuration of Transition Metal Elements

Answer 10

• Electrons fill the 4s sub-level BEFORE they fill the 3d sub-level
• However when the 4s sub-level is filled it becomes higher in energy than the 3d sub-level
• This means we would write the electronic configuration of scandium would be
• 1s2 2s2 2p6 3s2 3p6 3d1 4s2
• Ions - electrons are lost from the 4s sub-level BEFORE they are lost from the 3d-sub-level

Question 11

Exceptions to the Filling Rules

Answer 11

• Chromium

• 1s2 2s2 2p6 3s2 3p6 3d5 4s1
• One would expect the configuration of chromium atoms to end in 4s2 3d4
• For a more stable arrangement of lower energy, one of the 4s electrons is promoted into the 3d to give six unpaired electrons with lower repulsion

• Copper

• 1s2 2s2 2p6 3s2 3p6 3d10 4s1
• One would expect the configuration of copper atoms to end in 4s2 3d9
• To achieve a more stable arrangement of lower energy, one of the 4s electrons is promoted into the 3d

Question 12

Electronic Configuration of Ions

Answer 12

• The electronic configuration of ions can be deduced by simply adding or removing the appropriate number of electrons
• Electrons are lost from the 4s sub-level BEFORE they are lost from the 3d-sub-level

Question 13

Shorthand Electron Configurations

Answer 13

• Look for the closest noble gas with a lower Ar than your desired element
• Write that noble gas in brackets [Ne] and continue writing the rest of the electron configuration