[5.1] electron structure

Question 1

how many electrons can each shell (n) hold?

Answer 1

• n = 1 -> 2
• n = 2 -> 8
• n = 3 -> 18
• n = 4 -> 32

Question 2

what orbitals does each shell contain?

Answer 2

• n = 1 -> 1 × s orbital (1)
• n = 2 -> 1 × s, 3 × p orbitals (4)
• n = 3 -> 1 × s, 3 × p, 5 × d orbitals (9)
• n = 4 -> 1 × s, 3 × p, 5 × d, 7 × f orbitals (16)

Question 3

what is an orbital?

Answer 3

regions of space that electrons are most likely to be in

Question 4

what is an atomic orbital?

Answer 4

a region around the nucleus that can hold up to two electrons with opposite spins

Question 5

what is a sub-shell?

Answer 5

orbitals of the same type within a shell which are grouped together
- eg. shell 4 has 4s, 4p, 4d and 4f sub-shells present

Question 6

what are the maximum number of electrons in each sub-level?

Answer 6

s = 2
p = 6
d = 10
f = 14

Question 7

what are the shapes of s and p sub-levels?

Answer 7

s is spherical
p has a dumb-bell shape
(axis is z, y, x clockwise)

Question 8

how are orbitals filled? (including the aufbau principle and hund’s rule)

Answer 8

1) electrons enter the lowest energy orbital available (aufbau principle)
- electrons fill orbitals in order of increasing energy
2) electrons pair with opposite spins (half arrow heads going in opposite directions)
3) electrons prefer to occupy orbitals on their own, and only pair up when no empty orbitals of the same energy are available (hund’s rule)
- this prevents repulsion

Question 9

how are the 3d and 4s sub-shells different?

Answer 9

4s has a lower energy than 3d when drawing the diagrams

Question 10

what is the order in which orbitals fill?

Answer 10

1s, 2s, 2p, 3s, 3p, 4s, 3d, 4d, 4f

Question 11

how do you write down the electron configuration?

Answer 11

in shell order, rather than electron-filling order
- 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6

Question 12

how are ions formed? how are electrons lost in 4s and 3d?

Answer 12

in ions, the electrons in the highest energy levels are lost first, but when losing electrons, electrons are lost from 4s before 3d
- the energy levels are very close

Question 13

what are the 2 exceptions to the expected pattern of electronic configuration and what causes this?

Answer 13

Cr (24) and Cu (29)
- these exceptions stem from the 4s and 3d levels being very close in energy

Question 14

what is the electron configuration of Cr (24)?

Answer 14

1s2 2s2 2p6 3s2 3p6 3d5 4s1 (NOT 1s2 2s2 2p6 3s2 3p6 3d4 4s2)

Question 15

what is the electron configuration of Cu (29)?

Answer 15

1s2 2s2 2p6 3s2 4s1 3d10 (NOT 1s2 2s2 2p6 3s2 3p6 4s2 3d9)

Question 16

why are there slightly lower energy arrangements in Cr and Cu and what effect does this have?

Answer 16

• there is a slightly lower energy arrangement since the 4s and 3d energy levels are very close in energy
• the reduced e⁻-e⁻ repulsion in correct configuration makes up for the fact that one electron is in a slightly higher energy level