Electron configuration and modern quantum mechanics

Question 1

Quantum Mechanics

Answer 1

• Quantum mechanics suggests that the principal shells (K,L,M and N) actually consist of energy sublevels or subshells (s,p,d and f) with slightly different energies.
• Electrons are visualised as moving in regions of space called orbitals located in the subshells.
• Orbitals may be empty or occupy a max of 2 electrons.

Question 2

SPDF Rules- number of subshells

Answer 2

• There are four subshells in total- s,p,d&f
• The first shell (K) has one subshell which is ‘s’.
• The second shell (L) has two subshells which are s and p.
• The third subshell (M) has three subshells which are spd.
• The fourth subshell (N) has four subshells which are spdf.

Question 3

SPDF Rules- number of orbitals in subshells

Answer 3

• The lowest energy level (K) within a shell consists of the s subshell containing 1 orbital called the s orbital.
• The second lowest energy level (L) within a shell consists of the p subshell containing 3 identical orbitals called the p orbitals.
• The third lowest energy level (M) within a shell consists of the d subshell containing a group of 5 identical orbitals called d orbitals.
• The fourth lowest energy level (N) within a shell consists of the f subshell containing a group of 7 identical orbitals called f orbitals.
• Electrons always tend to occupy the level of lowest energy.

Question 4

SPDF Rules- max number of electrons in subshells

Answer 4

s subshell- 2
p subshell- 6
d subshell- 10
f subshell- 14

*Each orbital can only occupy a maximum of 2 electrons thus we are multiplying the number of orbitals in a subshell by 2.

Question 5

The Pauli Exclusion

Answer 5

• The Pauli exclusion principal states that an orbital can contain only 0, 1 or 2 electrons.

Question 6

SPDF Rules- order of filling subshells

Answer 6

• 1s< 2s< 2p<3s<3p<4s<3d<4p<5s<4d

- Note that the 4s shell is lower in energy than the 3d and therefore must fill first

Question 7

How to write to electronic configuration using spdf

Question 8

Identifying an element using electronic configuration

Answer 8

You can identify the element by adding all the electrons that the subshells hold. This will give you a number equal to the proton number/atomic number. Note that this does not work for ions.

Question 9

Electronic configuration- groups and periods

Answer 9

Period- highest number in front represents the shell being filled therefore the period

Group- total number of electrons in highest shell number is the group. Don’t forget to add 10 to numbers greater than 2

Na = 1s2, 2s2, 2p6, 3s1 (period is 3, group is 1)

S = 1s2, 2s2, 2p6, 3s2, 3p4 (period is 3, group is 16)

Question 10

Electronic configuration- excited state

Answer 10

• When an atom absorbs energy and moves to an excited state from the ground state, an electron (usually the outermost electron) will be promoted to higher energy level.

1s2, 2s2, 2p5 becomes 1s2, 2s2, 2p4, 3s1

• Note that once the order of filling of subshells has been determined, the subshells are written in increasing numerical order, not the order of increasing energy.

So 1s2, 2s2, 2p6, 3s2, 3p6, 4s2, 3d3 becomes
1s2, 2s2, 2p6, 3s2, 3p6, 3d3, 4s2

Question 11

Chromium (24) exception

Answer 11

• 1s2, 2s2, 2p6, 3s2, 3p6, 4s2, 3d4 would be the assumed configuration, however the 4s subshell has 2 electrons while the 3d subshell has an extra empty orbital and the subshell ideally should be holding 5 electrons in total to make everything half full instead of partially full.
• Because of this, one electron from the 4s subshell will move to the empty 3d orbital, altering the configuration to 1s2, 2s2, 2p6, 3s2, 3p6, 3d5, 4s1 in numerical order not energy level.

Question 12

Copper (29) exception

Answer 12

• 1s2, 2s2, 2p6, 3s2, 3p6, 4s2, 3d9 would be the assumed configuration, however the 4s subshell has 2 electrons while the 3d subshell has an extra space for one electron and it ideally should be holding 10 electrons in total to make everything full instead of partially full.
• Because of this, one electron from the 4s subshell will move to the partially full 3d subshell, altering the configuration to 1s2, 2s2, 2p6, 3s2, 3p6, 3d10, 4s1 in numerical order not energy level. Note that is okay for the 4s subshell to have one electron because it is half full.

Question 13

Why chromium and copper are exceptions

Answer 13

• They are exceptions to the standard rule because by following the standard rule, chromium and copper atoms have only partially filled orbitals rather than half full or completely full ones. To satisfy this, one electron from each case moves to achieve a half full or full subshell, thus more stability.