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


  • 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”


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


  • 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 1s22s22px22py12pz1                        O+ 1s22s22px12py12pz1

S 1s22s22p63s23px23py13pz1                      S+   1s22s22p63s23px13py13pz1

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


  • 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