The Electrons in The Outer Shell

The Chemistry of Art > 3. Electrons in the Atom > The Electrons in The Outer Shell >

The relationship between the number of electrons in the outer shell of an element and its electronegativity

Overview:

• If enough energy is absorbed by an atom, an electron can be completely removed, leaving behind a positively charged ion
  • This amount of energy is known as ionization energy

Official Definition:

“The amount of energy needed to remove the outermost electron from one mole of gaseous atoms or ions”

Trends:

Noble gases:

• The highest ionization energies belong to the noble gases
  • Indicates electron configurations are very stable
• First ionization energy decreases as you go down the noble gases group
  • The outer shell is in increasingly further from the pull of the nucleus
  • Increasingly shielded/repelled by inner electron shells

Group 1 alkali Metals:

• Require the least amount of ionization energy
  • One electron in their valence shell is less strongly held than the electrons of the closer inner shells
  • When this electron is removed, the resulting ion has a stable outer shell electron configuration

Across periods (Left to right):

• Ionisation energies (largely) increase across a period
  • As successive elements have one more proton than the last
  • Increased electrons are at a similar distance therefore there is a gradual increase in attractive force

Down groups

• Gradual decrease in ionization energy
  • Outer electrons are further from the nucleus
  • Number of electrons shielding/repelling increases

New content:

Irregularities in the above trends:

• Explained due to electron configurations and sub shells

e.g. Boron and Aluminum (Lower ionization energies than expected)

B  1s²2s²2p¹                                     B⁺ 1s²2s²

Al 1s²2s²2p⁶3s²3p¹                 Al⁺ 1s²2s²2p⁶3s²

Boron:

• The 2p electron is removed
  • The 2p subshell has a slightly higher energy than the 2s subshell
  • 2p well shielded by 2s electrons

e.g. Oxygen and Sulphur (Lower ionization than expected)

O 1s²2s²2p_x²2p_y¹2p_z¹                        O⁺ 1s²2s²2p_x¹2p_y12p_z¹

S 1s²2s²2p⁶3s²3p_x²3p_y¹3p_z¹                      S⁺   1s²2s²2p⁶3s²3p_x¹3p_y¹3p_z¹

Note: x,y and z denote the different orbitals in the p sub shells

Oxygen:

• The first three 2p electrons are in separate orbitals in accordance with Hund’s rule
• The additional electron must pair up with one of these
  • The proximity of the two electrons in one orbital results in greater electrostatic repulsion
  • Hence less energy is needed to ionize this electron from the orbital